What's new in LIGO

David Shoemaker, LIGO-MIT dhs-at-ligo.mit.edu
In an important sense, LIGO has recently turned a corner in its history: It has moved from the commissioning to the observation phase.

Since the last MOG, a number of technical issues have been addressed in all three interferometers. Increases in the laser input power, tuning of the system which compensates for the thermally-induced focusing in optics, work on reducing scattered light paths and acoustic excitation of optic motion, and control-law optimizations are among the specific efforts. This has both improved the strain sensitivity as well as increased the duty cycle of operation of the instruments.

The result is that the two 4km interferometers exceed the performance promised in the 1995 LIGO Science Requirements Document of a sensitivity of $10^{-21}$ in strain for a 100 Hz bandwidth, with the 2km interferometer also functioning well given its shorter length. The LIGO Scientific Collaboration had given its agreement to proceeding with the definitive S5 science run at the August LSC meeting, and the NSF Annual Review of LIGO held in November 2005 also confirmed that the target sensitivity was achieved.

The S5 science run, underway since mid-November 2005, is intended to collect one year of integrated coincidence data between the two LIGO Observatories. We plan to take breaks in observation from time to time to implement small improvements, and repair any equipment that breaks down during the run. Some observation time is lost to maintenance, and the first stage of construction of an Outreach center at Livingston will impact the day-time duty cycle of that instrument for the beginning of the run. All factors taken into account, we plan to run for about 1.5 years to accumulate these data.

Online (near real time) data analysis tools are characterizing the data on-the-fly, helping the staff optimize the instruments and recognizing quickly any problems that need to be addressed. The four basic searches, for signals with the character of bursts, a stochastic background, periodic or quasi-periodic, and binary inspiral signatures, are being applied to the data, and the LSC plans to keep the analysis process active continuously throughout the run.

Analysis continues on the previous science runs, with better upper limits on a variety of sources established and new techniques exercised which will be employed also in the S5 analysis. Papers have appeared or accepted on searches for periodic sources (``First all-sky upper limits from LIGO on the strength of periodic gravitational waves using the hough transform'') and on burst searches `triggered' by GRB signals (``Search for gravitational waves associated with the gamma ray burst GRB030329 using the LIGO detectors''). A variety of other publications is in preparation; searching on gr-qc for `the LIGO Scientific Collaboration' is an effective way to stay up-to-date.

Advanced LIGO has also made strides forward. The characterization of the mirror suspensions and of the seismic isolation systems has progressed, and full-scale prototypes of the suspensions and seismic isolation will converge for integrated testing at the MIT LASTI facility in the coming months. The 40m interferometer test bed at Caltech has successfully demonstrated the length control scheme for the Advanced LIGO signal- and power-recycled Fabry-Perot Michelson configuration. Extensive modeling has helped our understanding of thermal compensation, possible parametric excitation of mirror modes, and the requirements to be placed on the mirror figure. Four of the actual to-be-installed fused silica test masses, contributed by the UK, have been delivered and will go through a pathfinding process to identify polishing and coating techniques.

Advanced LIGO has also appeared in the recent 2007 budget materials from the OMB and the NSF as indicated for an FY2008 start. Although the official decision is still in the future, this is a strong indication of the support from the NSF and the interest in the government to support this field, and an affirmation of the LSC's very successful effort to advance the astrophysics and the instrument science to the point where all agree that this is timely. A baseline review will be held in late May 2006 to confirm the cost, schedule, risk handling, and technical plans, and we hope to be very busy with preparing for the start of the project from that point onward.

Last but not least: In the last MOG, we mentioned that the LIGO Laboratory was involved in a search for a new director. Jay Marx, formerly of LBNL, has accepted the position of Director, and we welcome him warmly to the Lab and the field.

Jorge Pullin 2006-02-28