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Project Description
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Results and Accomplishments from Prior Support
This section describes the state of the LIGO Laboratory at the close of 2002, summarizing the status of the construction, commissioning, operations, data analysis, collaborative research and support of the involved community, advanced detector R&D and efforts on education and outreach designed to produce broader impacts of LIGO. Further details are available in the references.
Overview
The Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors use laser interferometry to measure the distortions of space-time between free masses to directly detect passing gravitational-waves. The objective is to open the field of gravitational-wave astrophysics.
Scientists, engineers and staff at the California Institute of Technology (Caltech) and the Massachusetts Institute of Technology (MIT) are operating the LIGO detectors. Caltech has primary responsibility for the project under the terms of a Cooperative Agreement with the National Science Foundation (NSF). LIGO is a national facility for gravitational-wave research, providing opportunities for the broader scientific community to participate in detector development, observations, and data analysis. LIGO welcomes the participation of outside scientists in these endeavors.
The LIGO Scientific Collaboration (LSC) is the organization comprising the involved scientific community. This includes Caltech and MIT scientists and engineers responsible for data analysis, advanced R&D and the development of advanced subsystems for LIGO. This Collaboration is exploiting the initial detector and is pursuing the development of second-generation detectors. The LSC has its own management structure with shared participation in its governance, and corresponding obligations and privileges.
The initial LIGO comprises one three-interferometer detector system. The sites allow for expansion of the facility to a multiple-detector configuration and for upgrade and replacement of the existing detectors in order to reach the terrestrial limits of detector sensitivity and to fully exploit the promise of LIGO.
Observatory Operations and Detector Commissioning
The Detector and Engineering Groups and the Observatory staff focused on commissioning and operating the interferometers. We continue to work on reducing noise and improving duty cycle. We are refining the design and implementation of interferometer subsystems based on our growing operational experience.
At the close of the year we had achieved strain sensitivity better than has been achieved with any previous broadband detector for all three interferometers. This was the case over the entire gravitational wave band from 100Hz to several kHz. These improvements positioned the instruments for a very successful first Science Run (S1).
Hanford and Livingston Observatories
The sensitivity of the interferometers continued to improve in 2002. This improvement resulted from control subsystems being brought into operation, improvements in the performance of electronics and software subsystems, and tuning of the controls system.
We made steady progress improving sensitivity for both interferometers, achieving a best noise equivalent strain sensitivity better than 8(10-22/(Hz in the four-kilometer interferometer (H1) and 3x10-21/(Hz in the two-kilometer interferometer (H2), and 3(10-22/(Hz I at the Livingston four-kilometer interferometer.
Engineering runs 6, 7 and 8 were conducted to test the interferometers. During E7 the interferometers were operated in coincidence between the two Observatories, with GEO-600[1], and with ALLEGRO[2], the cryogenic resonant bar detector at Louisiana State University (LSU).
All interferometers participated in the first Science Run (S1) from August 23 to September 9, 2002, collecting nearly 100 hours of triple-coincidence data. GEO-600 and TAMA[3] also scheduled observing runs to coincide with S1.
At both Hanford and Livingston we completed the last phase of facilities construction, providing laboratory, office and meeting space. We hired additional staff required as we approach twenty-four-hour-per-day, seven-day-a-week operation.
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Figure 1. Strain Sensitivity Improvement for the Livingston Four-Kilometer Interferometer during 2002.
Engineering Runs
The seventh engineering run started on December 28, 2001, and data collection was completed January 15, 2002. The Livingston four-kilometer interferometer and the Hanford two-kilometer interferometer had a combined duty was about 40 percent. The overlap was primarily at night when the seismic conditions at Livingston were relatively quiet (this operational limitation is being addressed; see elsewhere). Combined observations with the bar detector at Louisiana State University (LSU) began on Wednesday, January 9. Concurrently, GEO operated a power-recycled Michelson.
We conducted an engineering run (E8) from June 8 to June 10, 2002 at the LIGO Hanford Observatory. The objective was to evaluate the Data Monitoring Tool (DMT) software developed by the LIGO Scientific Collaboration (LSC) in preparation for the first science run (S1). We tested sixteen monitoring programs under real operating conditions. Useful recommendations for needed modifications were provided to software authors. As a result, DMT programs ran reliably and usefully during S1.
DMT is an elaborate and useful suite of programs that continuously review the interferometer data streams, watch for various potential problems, and record summary information for later use concerning the three interferometers and their environments. DMT also supports interactive exploration of the data in the gravitational-wave channel and of non-linear noise processes. It supplies the non-strain-channel 'vetoes' used to improve the statistics of the processed strain data.
Science Runs
We have planned an initial progression of three science runs, interleaving interferometer development and improvement with increased scientific reach for each run. Important data analysis, and interferometer commissioning and development work are implemented between the scientific running periods. The three consecutive runs will provide a baseline for LIGO Data Analysis System (LDAS) development, detector modeling and diagnosis, as well as interferometer commissioning, modification, and revision. All three science runs are the joint responsibility of the Laboratory and the LSC.
We completed a very successful first science run (S1), ending on September 9, 2002. Analysis of the data from this run, and further interferometer commissioning and modification is under way. S1 was a showcase for a variety of new and improved DMT monitors. The LSC conducted an important November 2002 teleconference to summarize the status of the data analysis effort and define the path towards reporting results. Release of results for publication from the S1 analysis is planned for February 2003.
The experience gained during S1, including complete upper-limit operation and orientation of the LIGO-LSC scientific and operations staff, will help us to prepare for the second science run (S2), scheduled start in February 2003.
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Figure 2. Strain Sensitivity for the Hanford Four-Kilometer Interferometer
The S1 run included coincidence operation with the GEO[4] 600 interferometer and TAMA[5] 300 detector. This coincidence effort promises additional results and early experience running a network of detectors across the globe.
Further commissioning prior to S2 is nearing completion and is a staged ‘freeze’ to the configuration of the three interferometers will take place in late January 2003. The goal for S2 is at least an order of magnitude improvement in scientific reach, and we plan S2 to last approximately 8 weeks. With the presently realized improvements in strain sensitivity and duration, S2 will provide an opportunity to set very significant upper limits. We expect that S1 and S2 data will provide new publishable limits on the broadband gravitational wave flux, well beyond what has been previously reported.
Following S2 we plan additional commissioning including the important installation of the Livingston seismic pre-isolation system. S3, scheduled for late 2003, will mark the beginning of the first true search for gravitational waves with astrophysical significance.
Commissioning
At the close of the year we had achieved similar performance levels with all three instruments (H1 and H2 at Hanford, L1 at Livingston). We accomplished this by upgrading all hardware to the same revision and sharing commissioning experience through close collaboration and personnel exchanges between observatories.
The most significant changes were the installation of and completion of the commissioning of the digital suspension controllers. This second version of the electronics for pointing and actuating the test masses provides great flexibility in the design of (digital) filters to allow large actuation forces outside of the gravity-wave band to counter seismic motion, but very low noise operation in the target band. In addition, mechanical cross coupling in the suspensions can be 'inverted' to decouple length and angle motions.
In a related effort, the optical levers were refined and tuned. The optical levers provide an independent measure of optical alignment and are used to establish the initial alignment of the instruments in preparation for locking (brought into the linear control regime) . They can also be used to maintain the alignment during operation for those degrees of freedom, in anticipation of the completion of the wavefront sensor commissioning. The digital filtering capabilities of the suspension controllers improved the low-frequency performance of the optical levers and thus the overall interferometer sensitivity.
In the length control system, experience indicated that a change in control topology could provide a significant reduction in the appearance of frequency noise in the strain output. Once the instrument is locked an automated script transfers actuation away from the test masses and to the mode cleaner and laser systems. This also provided improvements in the low-frequency regime.
The improvements in the controls allow us to increase the light intensity on the main sensing photodiodes, reducing the photon shot noise at high frequencies. We moved from being dominated by electronics noise to seeing the quantum noise, as anticipated by the interferometer design.
At the Livingston Observatory, seismic noise in the 1-3Hz band arising from tree harvesting, traffic, and other human activity in the area surrounding the site continues to limit the operation of the interferometer to periods of low seismic noise, generally night time and weekends. Weather noise in the 0.1-0.3 Hz band has also interfered with operation at times. In response to these disturbances, several technical approaches have been initiated. We worked together with our LSC collaborators at Louisiana State University (LSU), to incorporate the fine actuators at the end masses into a feedback loop to reduce the seismic motion of the stacks between 1 and 10 Hz. These actuators remove the differential motion due to the tides and reduce the microseism. Using four longitudinal actuators at the corners of the support beams and two geophones as inertial sensors mounted on the beams, we reduced the Q's of the stack mode at 1.2 and 2.1 Hz by a factor of seven. These are the key modes for the excess test mass excursions driven by seismic motion at Livingston. This near-term solution to reduce excess seismic noise due to logging in the vicinity of the interferometer will be supplanted by the pre-isolator after S2 (see section 0 below.) In the interim, we can achieve higher duty cycle and greater sensitivity.
Investigations of Radio-Frequency Interference (RFI) and Electromagnetic Interference (EMI) in the detectors indicate that we have been suffering contamination from the switching-mode power supplies used as well as cross-coupling from digital electronics to low-level analog electronics. By modifying some power supplies (to linear models) and changing cabling and cabling configuration we successfully reduced the RFI/EMI in the subsystems selected. We have developed a comprehensive plan to address the contamination, which will be executed in stages starting after the end of S2.
Seismic Isolation Upgrade
We are developing a pre-isolator to address the excess seismic noise at Livingston. The technical solution is an early implementation of the external pre-isolator for Advanced LIGO. Our LSC Stanford University collaborators transferred their basic conceptual design for the hydraulic portion of the system to the LIGO-LSC collaboration for continued development. Prototypes for both the Hydraulic External Pre-Isolation (HEPI) and Electro-Magnetic External Pre-Isolator (MEPI) prototypes are complete, and testing and control law development is underway. We plan to complete the testing and to fabricate and install the pre-isolator at Livingston shortly after the S2.
We installed and tested Piezoelectric External Pre-Isolation (PEPI) in the End Test Mass (ETM) and Input Test Mass (ITM) chambers at Livingston as an interim approach to reducing the influence of seismic noise. We also finalized the design for coarse actuation/adjustment for the ITM chambers. This reduces the differential- and common-mode test mass actuator dynamic range requirements by a factor of 3, and allows a higher duty cycle for the instrument.
Data and Computing Group
Simulation and Modeling
We improved the second generation LIGO simulation package based on the end-to-end (e2e) time domain simulation engine to incorporate more realistic hardware and servo system models with the latest detector designs. Notably important recent additions include the Wave Front Sensor (WFS) and the Alignment Control System using WFS signals, and the common mode servo. Our ability to simulate LIGO end-to-end is nearly complete.
Sample sensitivity curves simulated by the model and the LIGO sensitivity measured during S1 are shown in Figure 3.
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Figure 3. Actual and Simulated Sensitivity Noise for S1.
While additional adjustment of the input parameters of the simulation in Figure 3 is needed, the good agreement demonstrates the relative completenss of this model. Note that the simulated noise curve is calculated from simulated time-series data (ground noise, input photon shot noise, electronics noise, etc.), enabling simulation ‘experiments’ of changes in the isolation system, control law tuning, etc., without disturbing the actual interferometer.
We have improved the time domain simulation code to support the more demanding functionalities of the simulation code. Recent improvements include a more accurate treatment of the response of the Core Optics Component to the frequency noise and improvements of the Modal Model. Other improvements include an interface, which makes it possible to run the simulation under different conditions, analogous to changing the hardware or software setup during LIGO operation, without human intervention. These changes make it even easier for non-expert to use and extract more useful information.
A new object-oriented field/optics model is being developed. The new scheme makes it possible to easily implement realistic features like wedge angles or internal reflections in substrates.
Commissioning Modeling
Interferometer locking at the Livingston site is being compromised due to seismic noise induced by passing trains and on-going logging activity near by. The effects of these seismic noise sources on the interferometer performance are being modeled to support the required design modifications in the seismic isolation system.
Now that the wave front sensing is fully implemented in the simulation, we have begun a collaboration to understand the complex response of the real LIGO wave front sensing. We have modified the simulation program interface to make it easier for the non-expert to use it and analyze the response.
We investigated the effect of heating of the input test mass (ITM) after a long time in lock at a high input power using the Han2k package (the first generation LIGO simulation model used for the lock acquisition design). The study showed that the model can cover a satisfactorily wide range of ITM deformation.
Alfi5—Graphical Front-end
Alfi5, a new version of alfi completely rewritten using JAVA, was stable from the first day of its release. Refinements have improved stability and reliability. All features in the previous versions written in C++ are now fully implemented in the new version. We added many new features to make this “e2e editor” much more useful and convenient. Examples include copy and paste capability, and the easy arrangement of port locations. When alfi was written in C++, it was distributed together with the simulation code. Now the JAVA version is distributed as a separate compact package. This makes it easy to distribute new versions in a timely manner.
Near Term Plans
Now that the simulation package is almost completed, the focus will move to the application simulation to assist in the noise reduction effort for all three interferometers. We are working towards establishing an effective simulation effort at each LIGO site. Three major development areas are planned: support of mirror imperfections, implementation of a refined field/optics model in the e2e simulation engine framework, and speed improvements of the simulation code. Alfi5 development will continue. Although basic features have already been incorporated, further refinements are needed to improve productivity.
LIGO Data Analysis System (LDAS)
LDAS Software Development
LDAS software development continued through 2002. Preparations for engineering run E7, and for the first science run (S1) drove our effort. We applied lessons learned during E7 to our current LDAS release (0.5.0), which was used during S1. Intense use of LDAS during E7 and S1 enhanced our understanding of the user and usage modes. LDAS systems are now being used at all Laboratory sites as well as at several LIGO Scientific Collaboration (LSC) Institutions (University of Wisconsin at Milwaukee, Penn State University, University of Texas at Brownsville, and Australian National University). LDAS 0.5.0 uses the new international standard (FRAME 6) for frame data storage.
We used LDAS version 0.5.0 during S1, and subsequently for all upper limits analyses. We achieved significant improvements in the system performance, grid safety, and memory management. LDAS 0.5.0 is the first release to have a diskCacheAPI that can manage raw data on the large RAID[6] storage systems now on-line at Hanford and Livingston This release performed reliably, to the 99 percent level, on S1, up from the approximately 90 percent level for the release used during E7. LDAS 0.5.0 also produces reduced frame data files, which are easier to share. Experience with the more than 10 terabytes of data collected during E7 has underscored the fact that we will need this capability for longer data runs.
We added a new frame CPP library to LDAS. We made significant improvements in the processing efficiency and enhanced the interface protocol used by the search codes to support raw sequences in data exchanges. Thorough testing and the sharing of pre-release frame files with the Virgo[7] project (Italian-French laser interferometer collaboration) have allowed us to significantly improve the reliability of the code, and assure compliance with the specification.
The LIGO Scientific Collaboration (LSC) upper limits groups used LDAS online during the science run S1, and nearly 230,000 jobs were submitted to the Hanford, Livingston, and MIT systems during the run. This is roughly twice the number of jobs submitted during E7. These jobs inserted over 7,000,000 rows into the databases. This represents roughly the same number of triggers and events as were produced during E7 even though the burst group chose not to enter their event candidates into the database during this run.
LIGO data collected during E7 and S1 was successfully transferred from the remote observatories to the Caltech archive and from there, using secure mechanisms based on GriPhyN (see Section 0) tools, to the LSC Tier 2 Computational centers where members of the LSC are conducting a significant proportion of the LIGO science.
LDAS Hardware
The primary activity was upgrading the Storage Area Network (SAN) and the compute clusters at the Hanford and Livingston Observatories in preparation for the first science run (S1). The production analysis system at Caltech is fully operational with all of the servers integrated with Beowulf clusters. In addition to an initial 16-node Beowulf cluster, we installed a 17-terabyte-disk farm holding all of the S1 data. All of the LDAS servers in five of the six LIGO Laboratory run LDAS systems have been upgraded to Gigabit Ethernet networks. The Gigabit Ethernet network was also replicated at the Observatories where it connects the main buildings housing data acquisition equipment to the new facilities housing the LDAS equipment.
SunFire880 servers were integrated into the existing LDAS systems to operate as the main data servers. The large disk storage systems at the observatories were moved to the new servers.
All engineering and science run data generated by the LIGO Laboratory have been archived at Caltech in the LIGO data archive running HPSS[8]. The current archive[9] contains 54 terabytes.
After a thorough review and with consultation from experts in the field, we decided to replace HPSS platform with SAM-QFS[10]. The SAM-QFS archiving platform offers a number of important enhancements relative to HPSS namely: simplicity, reliability, ability to move media between systems without data replication, sufficiently low licensing fees to allow use at the Observatories as well as Caltech, disaster recovery, metadata performance, and minimization of the number of vendors supporting LDAS. Initial testing and disaster recovery experiments have gone well. A demonstration run during S1 verified SAM-QFS performance as well as its capability to archive and retrieve the 17 terabytes of S1 data without corruption. We are in the process of negotiations with Sun for the licensing fees.
We are adding a grid interface currently installed at Caltech to the LIGO LDAS systems at Hanford and Livingston Observatories, and at MIT. We are expanding the existing LIGO compute clusters at Caltech, Hanford, Livingston, and MIT with additional nodes to establish sufficient processing capability for scientific analysis. A procurement of up to1000 central processing units is anticipated during the first half of the next fiscal year. A trade study is currently under way to choose between the currently used Intel hardware and newer technology that offers significant cost or performance advantages.
We are also planning to expand our data storage capability at all LIGO sites. We have been evaluating the feasibility of using Large Disc Storage IDE technology for non-critical path components. The IDE tape storage alternative is a factor of 10 less expensive than the comparable SCSI product. IDE disc technology, though less expensive, might even surpass SCSI disc storage capability. We have purchased 10 terabytes of IDE disc storage to support a technology demonstration.
Grid Computing and Related Research
LIGO is making strides towards performing scientifically significant data analysis using Grid resources. As part of the collaboration between the LIGO and the GriPhyN[11] projects, this past year we have focused on a specific LIGO problem: the gravitational-wave periodic source (“GW pulsar”) search. The data needed to conduct the search spans a significant period of time (~4 months, 2×1011 points). A source would appear on the frequency-time image as a wavering line, whose frequency might be 1 kHz, but modulated by a few parts in 106 over a day and a few parts in 104 over a year. In addition, if the source exhibits any secular variations due to slowing down of its rotational period, these will be encoded in the data as well. We successfully ran more than 50 pulsar searches, collecting useful statistics on the performance of the system for future improvements[12]
We have successfully integrated LIGO's existing data analysis with a Grid environment interface. The LIGO Data Analysis System (LDAS) can perform a wide range of sophisticated and computationally intensive data analysis. We are developing an infrastructure in which LDAS can be accessed as a Grid resource and to also enable LDAS to schedule its jobs on the Grid.
General Computing
We have completed and implemented the General Computing Policy. An accompanying computing and IT security plan was also developed and adopted. A baseline security audit was conducted at all four LIGO sites. A number of issues were discovered and addressed.
We are developing a schedule for additional audits during FY 2003. Security and related services have a high priority. Additional network security hardware will be installed at all locations.
We converted the Hanford Observatory network connection to the DOE ESnet through PNNL[13] from a T1 line to 10 Megabit ethernet-over-fiber. This required purchasing and installing media converters at both ends of the connection, PNNL and Hanford. We are preparing to move from the ethernet network to an OC3 network connection through ESnet. This upgrade is scheduled to take place during the first half of FY 2003.
A new web server was added that is devoted exclusively to LIGO Scientific Collaboration (LSC) web sites. We acquired the domain name, , for this server through a charitable gift.
Campus Research Facilities
40-Meter Laboratory
LIGO operates a 40-Meter prototype gravitational-wave interferometer on the Caltech campus. To prototype the Advanced LIGO optical configuration and controls, and study its performance, a fully instrumented suspended-mass interferometer is needed. The 40-Meter facility is dedicated to this task.
A Conceptual Design Review for the 40-Meter Dual Recycling project was held in October 2001. At that time, detailed conceptual designs were presented (with accompanying documentation) for the overall project, the tentative optical configuration and control scheme, the optical layout, all sensing table instrumentation, core suspended optics, mechanical suspensions, digital suspension controllers, and auxiliary optics (stray light control, initial alignment system, optical levers, video monitoring, etc), laboratory infrastructure and vacuum systems, environmental monitoring, data acquisition, computing and networking. Progress on key components was reviewed. The schedule of milestones was presented and discussed. The review committee was satisfied with the design and the progress, and a few specific concerns were addressed.
As of January 2003, we are on schedule. In particular, the following components and subsystems are implemented:
o The infrastructure has been brought to specifications. The laboratory building has been expanded and upgraded, all electronics racks needed for the full interferometer controls have been installed, and all optical tables and optical enclosures have been installed. Vacuum equipment (pumps, gauges, RGAs) have been upgraded.
o The existing vacuum envelope has been augmented with a new output optic chamber with seismic stack, a 13 meter mode cleaner beam tube, a small chamber and seismic stack for the end mode cleaner suspended optic
o A commercial active seismic pre-isolation system (STACIS) was installed on all four test mass chambers and is now in continuous use.
o An Initial-LIGO pre-stabilized laser (PSL) was installed in spring 2001. It has been fully commissioned and characterized, and is in continual use.
o The optics and suspensions for the 13-meter mode cleaner were produced,and in April 2002, the three suspended optics for the mode cleaner were hung, tested, and installed into the vacuum envelope.
o The characterization of the mode cleaner performance, and its interaction with the pre-stabilized laser system, occupied much of fall 2002. By the end of December 2002, the noise performance of the system met specifications.
o All of the core optics and suspensions for the main dual recycled interferometer (including spares) were produced August 2002. Three core optics (the beamsplitter, ITMx, and ITMy) were suspended and damped in September 2002.
Thus, there has been considerable progress in the fabrication and commissioning of a full dual-recycled interferometer with LIGO-engineered controls. To complete the fabrication phase, the remaining optics and suspensions must be installed, and some additional sensing and control electronics must be designed and fabricated. These systems will be installed, and the process of commissioning them begun, by summer 2003. First experiments in dual recycled configuration response, lock acquisition, and control are planned for 3Q 2003, and are expected to take at least a year. We expect that LSC members, as well as students, will participate in this most interesting phase of the project.
MIT Facilities (LASTI)
The LIGO Advanced System Test Interferometer (LASTI) facility is designed to develop and test advanced and improved LIGO subsystems at full mechanical scale, without disrupting or delaying scientific operations at the observatory sites. Located in a purpose-built high bay laboratory on MIT's northwest campus, LASTI comprises a suite of vacuum chambers and beam tubes (on a much-reduced 16m baseline), seismic isolation supports, lasers, and electronic and computing infrastructure closely replicating those found at the Livingston and Hanford LIGO observatories.
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Figure 4: The LASTI vacuum envelope. The system consist of three ‘HAM’ auxiliary optics chambers, one ‘BSC’ testmass chamber, connecting tube, and pumping system. The photo is looking along one arm; taken before the seismic isolation support piers and cleanrooms were installed.
During fiscal 2002, LASTI was primarily dedicated to accelerated development of seismic pre-isolators for the Livingston Observatory, which has been impacted by excess environmental noise due to nearby human activity. Prior to discovery of this phenomenon, LASTI had been slated to receive prototypes of the fully active seismic isolation systems planned for advanced LIGO. To accommodate testing the initial LIGO pre-isolator retrofit, we instead rapidly installed an initial LIGO isolation stack into our BSC (Basic Symmetric Chamber) early in the year. Another chamber, one of our three HAM (Horizontal Axis Modules), already carried an initial LIGO isolation stack used in prior work commissioning our laser stabilization system (see below).
We systematically characterized these initial LIGO stacks to establish similarity with the Livingston instantiations and to explore off-diagonal modes not previously measured. These measurements allowed development and confirmation of a dynamic numerical model which predicts the reactances and transmissibilities of these fairly complex "payloads" for external forces [Coyne et al., 2002; Hytec Inc., 1998].
Two variants of an external isolator were prepared for testing, differing principally in their force actuator technology. The HEPI (Hydraulic External Pre-Isolator) system is based on laminar-flow hydraulic differential pistons, originally developed by Stanford University [DeBra et al, 2000]. Eight prototype actuators and a regulated hydraulic supply system were engineered by a collaboration of engineers and scientists from Stanford, LIGO Livingston, Caltech and MIT and installed near the end of 2002 on the LASTI BSC chamber. This system is currently undergoing actuation trials.
A second external actuator variant, dubbed MEPI (Magnetic External Pre-Isolator), was simultaneously developed at MIT. This alternative may potentially afford lower complexity and cost than the hydraulic system, at the expense of somewhat lower stroke and force capability. A suite of eight MEPI force actuators was fitted to one of the LASTI HAM chambers in the third quarter of 2002 and is undergoing closed-loop control tests. The two actuator variants share a common physical mounting, designed for direct compatibility with existing LIGO structural interfaces. Selection of one of the variants is expected in the first quarter of 2003, to be followed by intensive development to refine, test and replicate the design for installation at the LIGO LIvingston observatory.
During 2002 the LASTI prestabilized laser also served as the development platform for an improved Pre-Stabilized Laser (PSL) frequency/phase control system. The fielded laser stabilization electronics operate reliably, but have routinely failed to achieve unity-gain bandwidths in excess of 200 kHz. This has limited the performance of successive phase and frequency loops, which depend hierarchically on this initial stabilizer to determine their own bandwidths. Using the LASTI PSL as a trial platform, modified electronics were developed and tested; a bandwidth exceeding 1 MHz was achieved along with significant improvements in robustness and reliability. These developments are now being engineered into an upgrade for the observatory laser systems, slated for installation in the second quarter of 2003 [McKenzie, Rollins, Ottaway & Zucker, 2003 (in preparation)].
Finally, during 2002 LASTI scientists wrapped up characterization of a full quadruple-pendulum Advanced LIGO suspension mockup. This prototype, with metallic dummy masses and wires in place of the eventual sapphire and silica mirrors and fibers, was assembled by collaborating scientists from University of Glasgow, Caltech and MIT in the LASTI high bay and tested to validate dynamical and control models. Several iterations to both the mechanical system and its simulation brought them into sufficient agreement to develop a refined design for the advanced mirror suspensions. This design is now in fabrication at Caltech, and a first article is scheduled for delivery to LASTI in the first quarter of 2003.
Research and Development toward Advanced LIGO
This year we initiated or continued a broad range of research and development to support the Advanced LIGO concept. This effort is very strongly collaborative, and the highlights of the progress in 2002 described below are often the result of collaborations with other institutions in the LIGO Scientific Collaboration (LSC).
Seismic Isolation
The seismic isolation team focused on the pre-isolator development for initial LIGO described above. This advance implementation of the pre-isolator is, in addition to an important near-term aid for the Livingston interferometer, also a significant step forward for the Adv LIGO seismic isolation system. A photograph of the hydraulic-actuator variant is shown in Figure 5.
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Figure 5: Hydraulic pre-isolator (HEPI) vertical isolator. On the left, the vertical actuator is shown; differential pressure in the bellows exerts force on the septum in the middle, which is carried to the load via the pyramidal flex joint at the top. On the right, as installed at the MIT LASTI testbed.
A second-generation active isolation system prototype was designed by the LSC team and fabricated by the LIGO Laboratory. It is currently being commissioned at the Stanford Engineering Test facility. A computer rendering of the design, and a photograph of the prototype, are shown in FIGURES 5 AND 6 OF THE PROJECT BOOK. This technology demonstrator will be used to (a) inform the development of the full-scale LASTI seismic systems for the HAM and BSC chambers, which will be developed this coming fiscal year and (b) serve as a controls test bed for the active isolation systems. Initial testing of the demonstrator show that a key measure of intrinsic mechanical alignment, the coupling from a requested horizontal actuation to an accidental tilt of the platform, is very low, which will ease the low-frequency controls design. Other measurements indicate that the first internal mechanical resonance, which will limit the maximum control loop bandwidth, is roughly 200 Hz, compatible with the design goal of 50 Hz for the loop bandwidth.
Testing and control law development will continue on this system during 2003. A request for bid for the next generation prototype is in preparation and will be issued in early 2003, enabling the delivery to LASTI in late 2003.
Suspensions
An all-metal test mass quadruple suspension prototype was developed at the University of Glasgow GEO lab and then sent to MIT for testing. All of the solid body modes were identified, and the model for the suspensions developed at Caltech, Stanford, and Glasgow was refined. Further trade studies on the lengths and masses were made based on the updated model. A challenge in the design is to damp the solid-body modes of the suspension without introducing excess noise in the gravitational-wave band (10 Hz and higher). Several approaches are being followed: using passive eddy-current damping, development of a miniaturized interferometric sensor, and an approach using a split feedback system has been developed in the VIRGO (French-Italian gravitational wave detector) Project.
An analysis of the thermal noise of tapered fused silica fibers at Caltech showed that this is an attractive alternative to ribbons for ease of fabrication and ultimate thermal noise performance.Some first samples have been fabricated for tests. Development of ribbons continued at Glasgow as the baseline design. Refinement of the attachment technique of the fused silica suspension fibers to the masses, using hydroxy-catalysis bonding, to sapphire (for the test mass) and high-density glasses (candidate for the penultimate mass) was made with good success.
We have completed the design and fabrication of the first prototype auxiliary optics suspensions; see FIGURE 10 IN PROJECT BOOK – OR TRANSFER HERE AND DROP THERE?. This suspension design carries the mode-cleaner optics, and will first be exercised at Caltech to check the solid body modes and damping characteristics, and then transferred to the MIT LASTI facility to look at installation and control issues.
A significant step in 2002 was the installation of the complete set of triple-pendulum fused-silica fiber suspensions in the GEO-600 interferometer by the GEO project. The Advanced LIGO suspension design is directly derived from the GEO-600 design, and the test of fabrication, installation, and now ultimately performance of the working design will be invaluable for refining the Advanced LIGO design.
Optics
We made significant progress in producing and characterizing sapphire as the preferred test mass material for Advanced LIGO. Our industrial partner fabricated full-sized boules (see Figure 6) which will now be polished to allow a more complete characterization.
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Figure 6: Sapphire substrate pathfinder. This piece, fabricated by Crystal Systems, Inc., is the full size of an Advanced LIGO test mass, 32 cm dia, 40kg mass. (Courtesy Insaco)
To address absorption of the substrate of the 1-micron laser light, annealing processes were refined in collaboration with Stanford, resulting in promising reductions. Industrial partners successfully pursued approaches to compensating for inhomogeneity. The notion is to polish a surface, which has features complementing the defects in transmission, on the anti-reflection side of the optic using two different approaches. One (Goodrich) involves a small rotary abrasive tip and an x-y table; the other (CSIRO) uses an ion-milling technique. Both can bring the net optical path seen by the light to an acceptable level. In parallel, manufacturers were able to produce material with improved homogeneity.
The Thermal Noise Interferometer (TNI) research at Caltech produced its first preliminary results with fused silica test masses, and noise hunting and reduction is underway.
Optical Coatings
One important measure of the optical coatings is the optical absorption. Acceptable (sub-ppm) losses have been demonstrated this year with conventional coatings by several vendors.
We pursued a strong LSC/LIGO Laboratory program this year to identify the magnitude and source of coating mechanical losses, and to improve the model of the coating thermal noise. The mechanical losses lead to thermal noise; the coating is an important contributor due to the geometry of the test mass, coating, and laser beam, There is a limited choice of materials and of processes which lead to both low mechanical and low optical losses. We are executing a program to identify the source of loss and to explore alternative coating materials and processes which meet the combined optical and mechanical requirements.
Significant progress has been made: We were able to demonstrate the source of the mechanical losses in the coating. The high-index tantalum material, rather than the low-index material or interfaces, is responsible. We are now pursuing alternative coating materials with several vendors with incremental progress in reducing losses.
Thermal Compensation
The initial prototyping of the two schemes for thermal compensation concluded this year and resulted in the PhD thesis of a LIGO student . The lens formed in the substrates due to the absorption of the laser light in the substrate make the interferometer sensitive to the power level. Including a thermal compensation system allows the interferometer to be used with a wide range of input powers, allowing e.g., better low frequency sensitivity with a reduction in the power. It also allows a trade to be made with the material properties of the substate; this is useful for sapphire, and necessary in the fallback case of fused silica.
The basic approach for compensation is to add a complementary additional heat source, so that the sum of the laser and compensation heating leads to a uniform optical path. In one technique, a circular heater adds heat to the edge of the optic. In this way, the scattering effect of lensing can be reduced (in experiments and models, which show excellent agreement) by more than a factor of 50; see Figure 7, upper plots. This is a very effective approach for the case of uniform absorption, which is expected to dominate.
[pic]
Figure 7: Thermal compensation demonstration results. Top left:, the contour map for a uniform absorption of a Gaussian beam. Top right: The residual deformation after compensation with a ring heater. Bottom left: the distortion due to a ‘point absorber’ (mimicked by a small probe laser beam); Bottom right: the map after compensation with a scanned compensation beam.
In the second approach, a scanning laser beam is played on the substrate and the dwell time and/or intensity can be modulated to deposit heat in a pattern optimized to compensate for a specific defect, for example a volume of higher thermal absorption. As shown in the lower plots in Figure 7, an additional suppression of a factor of 8 can be achieved for this example of point defect.
Pre-stabilized Laser
The program to develop 200 W laser sources continued at Adelaide, Stanford, and Hannover. During this year, each group has built up a prototype of their approach to making the high-power head: an injection-locked end-pumped rod design from Hannover, an injection-locked stable-unstable slab in Adelaide, and a slab amplifier at Stanford. The near-term goal is to make a selection based on a set of criteria developed at Hannover, one of which is to produce 100W by February 2003. Greater than 90 W have been produced in several designs, although not in the final configurations; see Figure 8 for an example output curve for a linear resonator using the Hannover approach.
[pic]
Figure 8: Laser Zentrum Hannover early prototype of a high-power laser head in a linear cavity configuration (at left). The final configuration is a ring-resonator. At right, the power output of the system as a function of the pump light input power; the system approached the initial goal of 100 W.
Input Optics
Progress on the challenges in the Input Optics subsystem was made by the University of Florida and their collaborators. A novel Faraday Isolator design was developed which uses a pair of crystals in a compensation technique to deliver high isolation at high powers. In this design shown in Figure 9, two 22.5° Faraday rotators and a reciprocal quartz polarization rotator placed between them replace the traditional single crystal 45° Faraday rotator. In such a configuration, polarization distortions that a beam experiences while passing the first rotator, will be compensated in the second. Tests to the maximum power available are encouraging.
[pic]
Figure 9: Top, compensated Faraday Isolator design; Bottom, isolation for a conventional and the compensated designs.
We reviewed the Input Optics Design Requirements prepared by the group at the University of Florida, and the group was given approval to proceed to the preliminary design.
Systems and Interferometer Sensing and Control
We refined the baseline design and conducted a System Design Requirements Review. A number of subsystem requirements and trade studies were concluded. We initiated a study of the data readout approach for the signal-recycled interferometer. The preliminary result is that the DC readout (in contrast to the traditional RF-modulation technique) appears to take advantage of the coupling that exists in a signal-recycled interferometer between the shot-noise fluctuations and the photon pressure on the test masses. We have also been working with industry to develop a low noise Digital-to-Analog Converter (DAC). Test results on the first prototypes should be available before the end of this year.
LIGO Scientific Collaboration (LSC)
The LIGO Scientific Collaboration[14] (LSC) is the means for organizing technical and scientific research in LIGO. Its mission is to insure equal scientific opportunity for individual participants and institutions by organizing research, publications, and all other scientific activities.
It includes scientists from the LIGO Laboratory as well as collaborating institutions. The organization is separate from the LIGO Laboratory, with its own leadership and governance, but reports to the Laboratory Directorate for final approval of its research program, technical projects, observational physics publications, and talks announcing new observations and physics results.
The March 2002 LSC meeting was held at the Livingston Observatory. In conjunction, Louisiana State University (LSU) hosted a symposium honoring Bill Hamilton. Numerical relativists with an interest in collaborating with LIGO and LISA made presentations and participated throughout the LSC meeting. LIGO-LSC and the numerical relativists initiated plans for useful activities that will support LIGO observational programs and guide theoretical research.
The eleventh meeting of the LIGO Scientific Collaboration (LSC) was held at the Hanford Observatory August 19-23, 2002. The significance of the upcoming science run and the organization of the subsequent data analysis effort were discussed. The schedule for Advanced LIGO was also presented.
The first science run (S1) included active participation from not only LIGO Laboratory scientists and staff but also in numerous ways from the broader community of scientists composing the LSC. LSC scientists contribute to real-time monitoring of the interferometer data for detector diagnostics and conduct analyses of the data.
The LSC scientists, as members of “upper limits” groups, pioneer the analysis of LIGO data in the search for gravitational waves. Several of these collaborating groups perform real-time searches using computers at the observatories. These searches are useful in providing rapid feedback to the control room on any instrumental pathology that might mimic a true gravitational-wave source.
Keeping the LIGO interferometers running smoothly and continuously requires a cadre of skilled operators at each site, working in teams on rotating shifts. The operators must bring the interferometers into lock, tune the alignment and gains to optimize sensitivity, and try to preserve those optimum conditions. Beyond operating the interferometers, the quality of the science data must be assured. This requires a parallel implementation of scientific monitoring shifts.
LSC scientists have been staffing the scientific monitoring shifts. The scientists or “scimons” focus on ensuring that the interferometer data is of the highest quality. Since scimons start with diverse specialties and backgrounds, formal training was instituted for bringing new participants up to speed. An expert scimon is paired with a “trainee,” a pattern that began back in November 2000. As a result, the pool of scimon experts has steadily increased. This will be essential to support the anticipated periods of steady state, multi-month data runs.
More than 180 eight-hour shifts were staffed by scientists from Caltech, Carleton College, the University of Florida, Hanford Observatory, Livingston Observatory, Louisiana Tech., the University of Southeastern Louisiana, Loyola University, Louisiana State University, the University of Michigan, Massachusetts Institute of Technology, the University of Oregon, Pennsylvania State University, the University of Rochester, Syracuse University, the University of Texas at Brownsville, Washington State University–Pullman, and the University of Wisconsin-Milwaukee.
Just prior to the start of S1, several of the upper limits groups verified that an emulated gravitational-wave signal could be detected in the data. They simulated the response of the LIGO mirrors to a variety of gravitational waveforms, allowing downstream confirmation that the signal did indeed appear as expected.
We have scheduled the next LSC meeting at Livingston, March 17-20, 2003. At this meeting, results from the S1 data analysis will be discussed; the pre-stabilized laser head downselect will be presented; and a preliminary assessment of sapphire/fused silica test mass downselect data will be made.
Astrophysics and Data Analysis
The work on astrophysical data analysis is an LSC activity with a strong Laboratory contribution. The present effort is organized into four groups, with the objective of setting interesting upper limits on the flux from short-term burst sources, stochastic sources, binary inspiral ‘chirps’, and for continuous-wave sources.
Searches for Un-modeled (burst) sources
The LSC Bursts Working Group (BWG) pursues the search for gravitational wave bursts in LIGO. The group has more than 40 members from LIGO and the LIGO Scientific Collaboration (LSC) The goal of the BWG is to look for short transients (lasting less than one second) of gravitational radiation of unknown waveform. These include burst signals from supernovae and black hole mergers for which the physics and computational implications are complex enough that make any analytical calculation of the expected waveforms extremely difficult. The detailed knowledge of a signal waveform would have allowed the use of matched filtering which is the optimal detection technique; this is something that falls outside the goals of the BWG and the Inspiral Working Group rigorously pursues it. Only general considerations regarding the duration and the requirement for the signal to have significant strain amplitude in LIGO's sensitive frequency band are made and general time-only domain and time-frequency domain search techniques are employed. These aspects of search strategy make the search for bursts with LIGO open to any unanticipated source of gravitational radiation that falls under the general time-frequency considerations, an issue that should not be neglected in these early stages of gravitational wave astronomy.
An additional focus of the BWG is to look for correlations of gravitational wave bursts with γ-ray bursts (GRBs). A number of GRB progenitors are plausible gravitational wave burst emitters and a comparison of the correlation function of the LIGO detectors immediately before a GRB (“on source”) and at random times (“off source”) may statistically establish their association[15].
Finally, the BWG plans to integrate the GEO and LIGO data in a single analysis making the most out of a multi-detector coincidence analysis.
The LIGO S1 run reflects an integrated ~96 hours of coincidence observation with the three LIGO detectors. The gravitational wave channel, AS_Q, was analyzed within the LDAS (LIGO Data Analysis System) environment after it was whitened. Three main astrophysical search algorithms were employed for the detection of bursts; these are referred to as Event Trigger Generators (ETGs) and, technically, they are Dynamical Shared Objects (DSOs) running within LDAS.
The “SLOPE” ETG is a time-domain algorithm (commissioned by Ed Daw and inspired by Arnaud et al.[16] that fits the AS_Q time series to a least-squares line and selects candidate events based on the value of the slope. The algorithm reports the start time and significance (value of the slope) of the excursion.
The “TFCLUSTERS” ETG is a time-frequency method (developed and commissioned by Julien Sylvestre[17]; gr-qc/0210043, to appear in Phys. Rev. D) relying on successive spectrograms taken every 0.125 seconds, which are then thresholded to identify time-frequency tiles (“pixels”) with statistically significant excess of power. These pixels are then clustered and the total power, central frequency, bandwidth, start time and duration of the formed cluster are reported.
A third method, “POWER[18]” uses like “TFCLUSERS” Fast Fourier Transforms to calculate power spectra of the raw time series for any given start time and duration. The algorithm then compares the power in the data over every user-defined time-frequency tile to the statistical distribution of noise power. A bursts signal is detected if the excess of power is greater than expected from the statistical fluctuations of the noise. Same bursts features like in the “TFCLUSTERS” are reported by the algorithm.
For each search method the candidate bursts and their features were stored in the LDAS database.
At the same time, a set of interferometer and environmental channels (with insignificant or null coupling to gravitational radiation) were analyzed within the DMT (Data Monitoring Tool) for the identification of transients of non-astrophysical origin, i.e., glitches attributed to the instrument and its environment. Following some high-pass filtering (typically 30Hz cut-off), glitch-finding methods based on time-over-threshold (both in absolute -ADC counts- and relative -sigma- sense) methods were invoked in identifying the start time, duration and significance of non-astrophysical transients. These defined the so-called veto triggers and their temporal coincidence with gravitational wave event triggers resulted in excluding the latter from further consideration. The use of vetoes in the burst analysis pipeline was thus able to reject a fraction of the candidate events at the cost of loss of detector lifetime.
Requiring their temporal coincidence in the three LIGO interferometers attained further reduction of the remaining candidate events. A transient signal of astrophysical origin is expected to yield time-correlated triggers in the three LIGO detectors subject only to the propagation time between the sites and any dispersion introduced in establishing the burst time via the search algorithms. Further correlation in burst duration, frequency band and amplitude as well as that of the raw time series themselves between the sites is to be expected.
So far we have employed only the temporal coincidence of burst triggers across the three LIGO interferometers as well as the frequency band matching for the ETGs performing a time-frequency analysis. The exploitation of the full power of the multi-detector search for bursts is expected to take place in the near future.
A central element in analyzing the S1 data has been the definition of an ~10% of the S1 coincidence data as the “playground” set. In order to avoid any statistical biases in the search for bursts, we have allowed any tuning involved in setting the search algorithm and veto parameters to be performed only on this set. Once this was done, the analysis was applied to the remaining ~90% of the S1 coincidence data.
The bursts analysis pipeline we have just described was applied to the S1 triple-coincidence data and an upper bound on the rate of events observed by the three detectors was established using the unified approach on setting upper limits of Feldman and Cousins[19]. A full set of Monte Carlo simulations was also used to inject gaussian and sine-gaussian signals of variable strength, width and frequency content onto the real interferometer time series. This allowed us to establish the efficiency of the entire bursts analysis pipeline to selected ad hoc benchmark bursts. We were thus able to translate the bound on the event rate to an exclusion plot of fluxes versus strength of gravitational wave bursts originating from fixed strength sources positioned on a fixed sphere centered on earth. These plots will be released as soon as the bursts analysis is internally reviewed and approved by the entire LIGO Scientific Collaboration (LSC). A richer interpretation invoking astrophysics motivated signal waveforms as well as source depth and angular distributions are currently being worked on and will be reported in the near future.
Search for a stochastic gravitational wave background
Stochastic backgrounds are signals produced by many weak incoherent sources. They are non-deterministic and can only be characterized statistically. Such signals can arise from early-universe processes (analogous to the electromagnetic CBR) and from present-day phenomena. They give rise to a (probably stationary and Gaussian) signal which is correlated between the two detectors. It will have the same spectrum in each detector, and is differentiated from detector noise by its inter-detector correlation, which depends in a known way on the signal spectrum and the detector separation and orientation. The greatest risk is that similar correlations may be produced by the (electromagnetic) environment.
Stochastic signals are expected to be quite weak compared to the intrinsic noise of an individual LIGO interferometer; consequently, detecting or placing a limit on a stochastic gravitational wave signal will require long observation periods over a bandwidth a few times the inverse light travel time between the interferometers.
Activities of the Stochastic Upper Limit group have centered around the analysis of two data collection runs, the E7 and the S1 science run. During these runs, the LIGO Hanford and LIGO Livingston Observatories recorded coincident data suitable for analysis for stochastic gravitational wave sources. More detailed information is available in the group's E7[20] and S1[21] reports. However, vetted science results are not yet available from S1 analysis.
GEO600 also took coincident data with LIGO detectors during the E7 and S1 runs; however, GEO/LIGO correlations are not reported on here. Although a GEO-LIGO correlation will not improve the upper limit by much due to the small overlap between the GEO and LIGO interferometers, it will provide insight about any inter-continental cross-correlated environmental noise. The ALLEGRO[22] resonant bar detector took data in three different orientations during E7. Analysis of these data will not be reported on here.
The search for a stochastic background of gravitational radiation in the E7 and S1 data employs the standard optimally filtered cross-correlation technique. We have summarized this procedure in a LIGO technical document[23].
For the S1 data analysis, 7.5 hours of triple coincidence (L1-H1-H2) data were set aside for stochastic upper-limit playground analyses. These data were purposely chosen to be scattered throughout S1, and to represent "typical" instrument performance (both bad and good). All investigations that could bias our final upper-limit on the stochastic background signal strength were initially performed on the playground data.
In addition, simulated stochastic background signals were injected into two 1024-sec stretches of post-S1 data in the Hanford 2-km and Livingston-4km interferometers. The hardware injections allowed us to test the full data analysis pipeline-from mirror movement to upper-limit values-for large SNR signals where we knew (a priori) the expected results.
In an actual search for a stochastic background signal, we work with discretely sampled data broken up into segments T = 90s in length. Within the LIGO data analysis system (LDAS), we request the gravitational wave data in 15-minute chunks (each 15-minute chunk representing a single job), originally sampled at 16384 Hz. We then down-sample the data to 1024 Hz for LHO-LLO correlations (2048 Hz for H1-H2 correlations), and estimate power spectra for each detector, which are used in the calculation of the optimal filter for a stochastic background with Ωgw(f) = const := Ω0.
Within the stochastic search code, we calculate instrument response functions, which are valid for the particular job we are analyzing. We then split the data into 10 (90-second) segments, each of which is windowed in the time domain, zero-padded to twice its length, and discrete Fourier-transformed. A value of the optimally filtered cross-correlation statistic is calculated for each T = 90-sec segment, while the theoretical variance is calculated only once for the whole 15-minute job. For each 15-minute job, we calculate the sample mean, and sample standard deviation of the 10 cross-correlation statistic values. Finally, we then form a weighted average to obtain a point estimate of the stochastic background signal strength. Frequentist methods are used to convert the composite cross-correlation measurements to limits on Ωgw(f) x h1002.
As an example of the expected 90% confidence level upper-limits for a pair of LIGO interferometers, we consider H2-L1. An upper-limit was calculated using, Eq. (10) [ref. T020166], solving for Ωgw(f) = const in the 40Hz - 265Hz frequency range, with SNR set to 1.28 (for 90% confidence), and typical S1 power spectra substituted for P1f) and P2(f). The observation time (corresponding to the amount of clean, coincident H2-L1 S1 locked data) was 100h, resulting in an expected upper limit of Ωgw(f) x h1002 < 15. This serves to set the scale of expected performance for the LIGO interferometers during the S1 run.
Simulated stochastic background signals with Ωgw(f) = const were injected into two 1024-sec stretches of post-S1 data in the LHO and LLO interferometers (so-called hardware injections). Ωgw(f) x h1002 = 24414 and 3906 for these two injections, corresponding to SNRs of roughly 10 and 5 in a 15-minute observation. The effect of these two hardware injections on the power spectral densities of the interferometers is evident as excess noise in the injected frequency band.
Averages of the point estimates of Ωgw(f) x h1002 as produced by the stochastic DSO analysis software for the times of the hardware injections fall within one or two standard errors of the injected point estimate, giving us confidence that the full data analysis pipeline is working as expected.
In addition to performing the hardware injections, we are able to inject via software simulated stochastic background signals into the data. Functionality exists within LAL to simulate stochastic signals (with power law dependence Ωgw(f) = fα) for the LHO and LLO interferometers, convolved with the appropriate instrument response functions. Results of the stochastic DSO analysis of software injections were consistent with that of the hardware injections, up to an overall sign. It is interesting to note that this comparison of hardware and software injections first discovered an overall sign difference between the interferometer transfer functions of LHO and LLO, which was subsequently measured and confirmed at the sites.
Production analyses are ready to be performed. Preliminary analyses were based on a static amplitude calibration for the gravitational wave channel, one that ignored the fluctuations in optical gain in the differential-arm control loop. These calibration data are now available for all three LIGO interferometers, so that production analyses can commence.
We are also studying a hierarchical approach in a collaboration with IUCAA, Pune, India. We have defined and prototyped an improved hierarchical scheme in searching for inspiraling binaries. The earlier hierarchical scheme developed by Mohanty and Dhurandhar used the two masses of the constituent stars for implementing the hierarchy. This scheme is being currently implemented into LAL in order to make the algorithm available for the next phase of LIGO science runs. The improved scheme extends the hierarchy to yet another parameter, namely, the time-of-arrival, so that the hierarchy is now in three parameters. This procedure is expected to reduce the cost by a factor of 4 or 5 over the Mohanty-Dhurandhar search and by a factor of about a 100 over the flat search. The algorithm and its prototyping results have been discussed in several conference proceedings[24],[25] and is pending publication[26].
Search for Binary Inspiral signals
The Inspiral Upper Limit Working Group is focussed on the search for gravitational-wave “chirps” emitted by compact binary systems as the bodies spiral ever closer to one another and ultimately coalesce. The group’s charter is to extract astrophysically significant results (presumably upper limits rather than detections) from early LIGO data, collected while the detectors have modest sensitivity. So far, the group has focused on low-mass systems, including binary neutron star systems (in which each body is expected to have a mass of ~1.4 solar masses).
We search for gravitational wave signals from binary inspirals in the LIGO data using matched filtering. This method uses linear filters constructed from the expected waveforms which are computed using post-Newtonian methods described in section 2. The waveforms used to generate the filters in this analysis are the stationary phase approximation to the Fourier transform of the second post-Newtonian order. We neglect spin effects and and use the restricted approximation described in. This gives a two parameter family of waveforms; the parameters being the masses of the objects in the binary system. We further restrict the binary systems to circular orbits. These waveforms are known as 2pN inspiral chirps.
The group began its work in earnest in early 2002, using data from the E7 engineering run to develop data analysis procedures and to explore ways of using environmental and instrumental auxiliary channels to identify transient disturbances in the detectors, in order to “veto” false gravitational-wave candidates. One of the key concepts introduced was to set aside a small fraction of the data as a “playground” in which analysis cuts and veto conditions can be studied and tuned freely; then, after freezing all details of the analysis, the final result can be extracted from the remainder of the data without fear of human bias. The main gravitational-wave inspiral search uses the standard technique of optimal Wiener filtering with a bank of inspiral templates covering the mass parameter space of interest, utilizing code from the LIGO Algorithm Library (LAL) and using the job control, data conditioning, parallel processing, and database functionality provided by the LIGO Data Analysis System (LDAS). In addition, an exploratory analysis was done using the Fast Chirp Transform (FCT) algorithm running within LDAS.
In September, the focus of the group naturally shifted to the greatly improved data from the S1 science run, in which binary-neutron-star inspiral signals are detectable to a greater distance than ever before.
Matched filtering requires a good set of template waveforms that accurately predicts the possible signals. Ideally the signal from a binary inspiral would be computed from an exact two-body solution to the Einstein equations for the general relativistic gravitational field. However, the exact two-body solution is not known and one must use some kind of approximation. We use the restricted second-post-Newtonian (2PN) Taylor-series approximation to waveforms for nonspinning compact (point) objects in quasi-circular orbits. Restricted 2PN means that the amplitude is computed only to leading order in GM/rc2 (where M is the total mass and r is the orbital separation in harmonic coordinates), while the phase evolution of the waveform is computed to two orders in GM/rc2 beyond leading order. The waveform phase can be approximated, in this case by a Taylor series, in either the time domain or the frequency domain. Since the difference between these two approximations can give us some idea of the errors due to the failure of the post-Newtonian approximation in the late stages of inspiral, we use both:
The template bank and the actual templates used to filter the data are constructed using the frequency-domain approximation, and the injected waveforms used to test the efficiency of the analysis use the time-domain approximation.
As an additional pathology check, we compute the overlap of each template — put into the frequency domain with a discrete FFT — with a stationary-phase approximation of itself in the frequency domain. Since the stationary phase approximation is known to go bad only when the post-Newtonian expansion goes bad, this is an independent and tighter check that sidesteps issues of time-domain vs. frequency-domain approximations. We find that the overlap is below 90% for binaries of M > 2.5Mo with the H2 noise curve and much better for the others. The physical reason for this is that the frequency of the last circular orbit, about when the merger waveform begins, is in the interferometer’s sensitive band for such high masses.
The IUL detector characterization sub-group initially considered all control channels and all PEM channels as possible sources for vetoes. The approach adopted for S1 built upon experience developed during the E7 analysis. Numerous software tools were used to examine the data; DMT programs, home made MATLAB scripts, and examination by eye using DTT. In the E7 analysis there was much hope that a smoking gun would be found among the PEMs. Virtually all of the accelerometer, seismometer, microphone, voltage line monitor and magnetometer channels were examined. After a careful search, however, we found that the inspiral-template-based L1:LSC-AS I veto (the interferometer antisymmetric output signal, demodulated at 90 degrees to the strain channel) does the best job with moderate deadtime.
The group is now in the process of refining the analysis, especially in the areas of applying appropriate auxiliary-channel vetoes and modeling the spatial distribution of sources in the Milky Way, so that the efficiency of the search can be calculated accurately. The group expects to publish at least one result in 2003 based on the S1 data.
Search for Periodic signals
The primary astrophysical candidates for periodic emission of gravitational waves are spinning neutron stars, either isolated or in binary systems. Continuous gravitational wave are emitted from these candidates when there are asymmetries due to either rotation about a nonsymmetry axis, precession, or stellar pulsations. A subset of these objects are observed in the electromagnetic spectrum, for example as pulsars or in x-ray binary systems. A further subset of these objects spin fast enough to put their potential gravitational wave emission frequency into the LIGO and GEO band. (For the simplest case the gravitational wave frequency is emitted at twice the spin frequency). However, there should be many more neutron stars than those observed, and there is always the possibility of an unknown class of periodic sources. Thus, both targeted and untargeted searches are warranted. Targeted searches include known pulsars, for which the position, spin frequency, and spin evolution are known, and low-mass x-ray binaries, for which the position is known, but a search over a limited frequency band and orbital parameters is needed. Targeted searches could also include a targeted set of positions on the sky (such as that of a globular cluster or the galactic center) for which a search over the other signal parameters is needed. Untargeted searches involve a search over many sky positions and intrinsic source parameters. Note that in addition to intrinsic source evolution, the changing velocity and orientation of the detector relative to the source induces amplitude and phase modulations into the data. And since periodic signals are expected to be weak, long observation times are required for detection. An untargeted search is very interesting, but doing so using coherent techniques (i.e., tracking the phase of the signal, such as in matched filtering) is computationally expensive in terms of the required CPU cycles. Incoherent and hierarchical methods must be used to make the untargeted searches feasible. Note that all incoherent and hierarchical searches invoke coherent methods as part of their strategies. Targeted searches using coherent techniques are computationally affordable and relatively easy to implement compared with untargeted searches. Targeted searches are of interests in their own right, but also much of the code developed for a targeted search can be used in an untargeted search as well, making this an important first step in that direction. The groups proposal for the initial analysis of LIGO and GEO data was crystallized by August of 2001.
Much of the coding, testing, and the first analysis of science data took place during the period covered by this report. In the last year PULG has consider four types of searches. Each search is described briefly below.
Time Domain Searches for Signals From Known Isolated Pulsars
The time domain analysis is based around the idea of heterodyning—unwinding the expected phase of the pulsar signal by multiplying (mixing) the data with a complex function of the form exp(-2\pi ift), where f is the expected frequency of the gravitational wave signal. The procedure is done in several steps of heterodyning, filtering, and down-sampling of the data. This reduces the volume of data by factors of about 10^6, which is of great practical importance in data management. If carried out with the right pulsar timing parameters, the only time-varying quantity remaining in the signal left in the data is the antenna pattern of the interferometer, which varies on timescales of a day. The procedure should, by the central limit theorem, give the noise a near-gaussian probability density. This is checked very carefully, and if satisfied, the probability of the data for sets of signal parameters is found using chi-squared. Standard Baysian statistical techniques are used to assign posterior probabilities to the parameters. This code has been validated by using fake data and injections of fake signals into real data. This code has been used in the preliminary analysis of LIGO and GEO S1 data.
Frequency Domain Searches For Signals From Known Isolated Pulsars
The frequency domain analysis is being implemented both as a stand-alone code and as a DSO under LDAS. Both the stand-alone code and a stand-alone version of the DSO have also
been used as a test-bed for distributed grid computing under the GriPhyN project the DSO in conjunction with LDAS. Both realizations use match filtering to coherently sum the data to extract the signal, correcting for phase and amplitude modulation. The output of the matched filtering code is the optimal statistic, defined as the F-statistic; F is derived using the principle of maximum likelihood. The scheme involves two steps. Step 1 is to split the observation time into shorter segments and generate a Short-time Fourier Transform (SFTs) for each segment using ordinary FFT routines. Step 2 is to input the SFT data for a narrow frequency band of interest from all the SFTs that cover the observation time and to send this data into the LAL functions that calculate the F-statistic. The distribution of F is found by a Monte Carlo simulation that injects signals into the real data, and classical statistical analysis is used to find confidence intervals or upper limits on the signal amplitude. (The estimated parameters that maximize F can also be computed; this has not yet been implemented).Some Monte Carlo simulations have been completed. This code has been used in the preliminary analysis of LIGO and GEO S1 data.
Blind Unbiased Incoherent Searches
An “unbiased” all-sky search for sources of periodic gravitational radiation is under way.
It is unbiased in the sense that few assumptions are made about the nature of the sources. In particular, no attempt is made to track the phase of such a source over extended time intervals. The technique used is based on incoherent averaging of one-sided power spectral density estimates, that is, on averaged periodograms. Incoherent averaging is generally less sensitive to weak sources than is coherent integration, but its reduced computational load permits a search over the entire sky, and the phase-insensitivity of power averaging makes the technique robust against uncertainties in source parameters. A statistically independent determination of background noise level is found using nearby (but not quite neighboring) bands to a given narrow frequency search range. Regions of the spectrum with sharp features, e.g., near a 60 Hz harmonic will be excluded from consideration or handled (in the long term) with a more sophisticated algorithm. The effects of the modulations will be determined empirically and parameterized, as detection efficiency corrections, using Monte Carlo software signal injections.
This search makes use of the SFTs generated for search 2 above. This search method has been used in the preliminary analysis of LIGO and GEO S1 data.
Targeted Search for a Signal from the Pulsar in the Binary System ScoX1
The accretion of hot material onto the surface of a neutron star was suggested over 20 years ago as a possible mechanism to generate quasi-monochromatic gravitational waves. In this scenario the induced quadrupole moment is directly related to the accretion rate (which can be copious) allowing the gravitational energy reservoir to be continuously replenished: gravitational radiation balances the torque due to accretion.
Such scenario has attracted considerable new interest in the past few years and has been fully revitalized by the launch of the Rossi X-ray Timing Explorer, designed for precision timing of accreting NS’s. The observational evidence that Low Mass X-ray Binaries (LMXBs) - binary systems where a compact object accretes material from a low mass companion — in our Galaxy are clustered around a rotation frequency ~ 300 Hz, led Bildsten to propose a mechanism to explain this behavior. The fundamental idea is that continuous emission of GW’s radiates away the angular momentum that is transferred to the NS by the infalling material. The fact that the rate of angular momentum loss through GW’s scales as f^5, provides a very natural explanation for the clustering of rotation frequency of several sources. The physical process responsible for producing a net quadrupole moment is the change of composition in the NS crust, which in turn is produced by the temperature gradient caused by the in-falling hot material.
Recently, Ushomirsky et al. have posed this initial idea on more solid theoretical grounds.
More recently Wagoner has argued that LMXBs could reach a stable equilibrium state by emitting GW through r-modes, allowing GWs from Sco X-1 and other LMXBs to be potentially detectable by advanced LIGO. If one of these mechanism does operate, LMXBs are extremely interesting candidate sources for Earth-based detectors. Several systems would be detectable by advanced LIGO, if the detector sensitivity is tuned, through narrow-banding, around the emission frequency. In particular, Sco X-1, the most luminous X-ray source in the sky, might be marginally detectable by “initial” LIGO, and GEO600 (the latter in narrow-band configuration), where an integration time of approximately 2 years would be required.
The implementation of the data analysis scheme follows the frequency domain search for known pulsars. Code has been developed which generalizes LALDemod in order to take into account the orbital motion of the source. This code represents the core of the search together with a function designed to place filters in the space defined by the orbital parameters. At present the code implementation allows us to search only for binary systems in circular orbits. Some effort is on-going to generalize the codes to handle the more general case of binaries with nonzero eccentricity.
Outreach
During 2002, the LIGO Laboratory has worked with Jill Andrews (Caltech Assistant to the Provost for Educational Outreach) and NSF program director Beverly Berger to plan an enhancement of existing LIGO Laboratory efforts with a cohesive, comprehensive, scalable education and outreach effort. LIGO has already conducted an impressive range of local educational outreach activities at both its Observatories.
To leverage previous successful NSF-funded education and outreach (E&O) program experience, each Observatory Head is recruiting local educators and community leaders who form a “Local Educators’ Network” (LEN). Focus groups and, as necessary, a more permanent advisory group will be recruited from the larger group of LEN participants. All existing or planned activities will undergo assessment in the context of relevance and feasibility by Observatory Heads with their LEN focus groups. The goal is to develop and maintain long-term, interactive partnerships to inspire, excite and motivate a broad spectrum of learners through inquiry, exploration and experience in science and engineering research. With collective input from our LEN focus groups (which we expect to underscore outcomes from similar efforts based at Caltech and other universities) LIGO Observatory Heads will implement a plan that features a balanced set of LIGO-related educational activities, programs and products with broad impact in formal education, informal education, and public learning venues.
In November 2002, LIGO Managers and Jill Andrews presented updated plans to Henry Blount, NSF Multidisciplinary Activities. We are submitting a proposal to the MPS Internships in Public Science Education (NSF 01-39) for continued and supplemental support of existing and/or new programs in three main areas. These are:
1. Formal Education: Internships in Public Science Education. We plan to seek funds in order to continue hosting science educators at each Observatory. These educator interns work with LIGO researchers in developing resource materials and products that both capitalize on LIGO science and satisfy the needs of local educators.
2. Informal Education: To reach broader audiences, we will seek supplemental resources and form partnerships in the local communities to create museum-quality exhibits in each Observatory. We will work to reach teachers and students who are unable to visit the Observatories by pursuing the resources necessary to create a “Mobile Science Unit,” and will seek out community youth programs already in place to enhance their programs with our people and products.
3. Public Outreach: We are planning products such as educational videos for Television, radio public service announcements or “spots,” and a more interactive Website.
Educational Outreach – Hanford
LIGO Hanford Observatory has contributed to the expansion of high-school science education by directly involving students in LIGO research. This year approximately 70 students from Gladstone High School (in northwest Oregon) worked on LIGO-related projects throughout the academic year. On May 28, 2002, students from grades 9-12 described their contributions to LIGO research to a packed audience of community members at Gladstone High School. (See story at - Article_1 for additional details.)
In the summer, a high-school teacher and a middle-school teacher held visiting appointments at the observatory, helping us to develop in-classroom and informal educational resources. This work has been disseminated using the "teachers corner" web pages ( at LIGO Hanford Observatory.
A special emphasis was put on science and math lesson plans in both high-school and middle-school versions, that not only include plans, activities and worksheets, but also the web pages () highlight lesson-plan alignment to the state education standards for Oregon and Washington.
The observatory hosted five undergraduate research students through the REU/SURF program this summer. Eric Adelberger (University of Washington) gave the LIGO Public Lecture this summer, entitled "How Many Dimensions Are There to the Universe?,” to an enthusiastic audience of approximately 225 people, ranging in age from pre-teens to retirees.
Approximately 600 visitors toured the observatory this year.
We have formed a Local Educators Network to advise us on future outreach effort. This group consists of teachers and education professionals, members associated with museums and other informal education activities, and people actively working with Native American and Hispanic groups.
Educational Outreach -- Livingston
We continue to be involved in a wide range of educational outreach activities aimed at communicating to the public what we do in LIGO. Approximately 2000 students and teachers visit the LIGO site each year as part of school sponsored field trips, and about 1000 adult visitors also tour the site as part of community and professional groups and as informal participants in weekly tours for the general public.
We have implemented a summer Research Experiences for Teachers (RET) program that provides opportunities for teachers to participate in the research activities at Livingston and simultaneously to develop materials and plans that they can take back to their home schools.
Our Research Experiences for Undergraduates (REU) program continues to grow. This year we hosted seventeen students participating in Caltech’s “SURF” program or similar REU or summer programs at LSC member institutions.
We have also hosted several regional workshops for teachers in order to enhance our interactions with K-12 educators in the region.
LIGO Laboratory and Southern Louisiana University (SLU) exchanged a Memorandum of Understanding (MOU). LIGO effort at SLU includes improvements of the LIGO end-to-end model (e2e), and the use of e2e simulation in the education outreach. Prof. Yoshida and his students have been measuring seismic spectra at the LIGO Livingston Observatory, vital information for the LIGO simulation. They are analyzing the measured data using the e2e simulation package to extract more fundamental data for e2e use and to understand the noise due to the beam jitter caused by mirror motions before the recycling mirror.
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