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Preparing to Search for Gravitational Wave Bursts

Preparing to Search for Gravitational Wave Bursts

- Contributed by Erik Katsavounidis


One type of gravitational radiation LIGO is expecting to detect is that attributed to bursting episodes in the universe. In preparation of LIGO's upcoming first science run, and armed with the data collected when all three LIGO detectors ran simultaneously for the first time during the January 2002 engineering run (E7), we will shortly begin pursuing the end-to-end analysis exercise of that data in search for sources of burst-type gravitational radiation. The LIGO Data Analysis System (LDAS) at MIT will be the primary venue where this exercise will take place.

Bursts of Gravitational Waves: Burst sources emit gravitational radiation lasting just a few cycles within the characteristic frequency band of LIGO, and they generally come with little or no indication of their waveforms. This last aspect has become, to good extent, part of the "bursts" definition at least in the context of the initial LIGO searches (unmodeled bursts). Among such anticipated sources are supernovae explosions, as long as they are asymmetric. Depending on the degree of asymmetry during the explosion, a fraction of the total gravitational binding energy is emitted in the form of gravitational waves. It will be hard to miss such a star collapse if it were to happen somewhere in our galaxy (a collapse emitting 10-3 of the total energy into gravitational waves will result to an rms strain at the level of 10-18 in LIGO's band). Unfortunately, such an event has a mean rate of the order of one in 35 years, thus in order to be able to see a few events in a year's worth of observation time we should be able either to survey a large volume of the universe or equivalently maintain high sensitivity. The search for bursts may achieve such high sensitivity by implementing correlation methods between the burst triggers coming from the three LIGO detectors (in the so-called internal or self-trigger mode) as well as by exploiting the triggers coming from the gamma-ray burst and neutrino detectors (external trigger mode).

LDAS at MIT: Since August 2001, the LIGO Data Analysis System (LDAS) has been installed and running at the LIGO Laboratory at MIT awaiting the moment to start looking at interferometer data of significant sensitivity. The LDAS-MIT configuration is comprised of 16 PC's each with 1.1GHz AMD Athlon processor and 1GB RAM. This clustered configuration of PC's running linux is often referred to as a "beowulf" cluster. The system also includes a number of multi-processor PC's serving as "master" and data-conditioning nodes in the cluster, as well as two quad-processor SUNTM units that act as the frame (data) and database servers. In its current configuration, the LDAS-MIT system offers a bit over 1TB of disk storage while it is equipped with a stand-alone AIT-2 tape unit. Here is a recent picture of the MIT cluster, celebrating its ninth month:

The LDAS cluster at MIT.

LDAS's primary role is to run astrophysical searches, i.e., the algorithms optimized to look for specific astrophysical signatures in the gravitational wave channel of the interferometers. At the same time, and outside the LDAS cluster, another SUNTM unit (a Blade 1000) is used for performing analyses of the enviromental, auxiliary and gravitational wave channels for diagnostic, detector characterization and veto purposes. This is the Data Monitoring Tool (DMT) software environment that may read frame data from--as well as write database entries to--the LDAS side.

Burst Search at LDAS-MIT: The Bursts Working Group of the LIGO Scientific Collaboration has undertaken the task of implementing the end-to-end Bursts analysis pipeline. The group's main challenge is to prepare a pipeline able to detect an unmodeled, possibly very weak signal, to distinguish it from detector noise and to interpret the result in an astrophysically meaningful way. There are several pieces that make up this challenge, some highlights of which may be itemized in the following.

About 15 members of the Bursts Working Group will come together at MIT in the first week of May to see this pipeline fully exercised and validate its readiness. All the details in each of these steps are currently worked out by a small group of LSC scientists. In order to avoid biases for either rejection or detection of a possible gravitational wave signal in the analysis of E7, data were split into two sets: one representing the "playground" set and the other the "signal" set. The "playground" set (roughly 10% of the total multi-detector data) is currently available through LDAS-MIT and is being used by code developers to understand the best way of choosing detection thresholds, as well as to define the vetoing strategy that will maximize the astrophysical reach. An integral part in this whole exercise is the simulation of the entire analysis pipeline that will allow the determination of efficiencies to some benchmark-type of burst signals over a range of frequencies, durations (bandwidth) and gravitational wave strain strengths.

It's an exciting moment for the burst search with the LIGO detectors, and a moment of intense activity across the LIGO Scientific Collaboration's Bursts Working Group in the fine-tuning and preparation of its pipeline for the imminent science run. The E7 analysis reflects a milestone in this effort. Expect the unexpected and stay tuned!

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