Gamma-ray bursts (GRBs) are intense flashes of gamma rays that are observed to be isotropically distributed over the sky. They are observed directly by gamma-ray and x-ray satellites in the Interplanetary Network (IPN) such as HETE, Swift, Konus-Wind, INTEGRAL, and Fermi.
Short GRBs, with a duration of less than 2 seconds, are thought to originate primarily in the merger of a neutron star with another compact object, such as a neutron star or black hole. Models suggest that the neutron star and compact companion in otherwise stable orbit lose energy to gravitational waves and spiral into each other. If the speed of gravitational radiation equals the speed of light as expected, then for an observer in the cone of the beamed outflow, the gravitational-wave inspiral signal will arrive a few seconds before the electromagnetic signal from internal shocks, the interaction of outgoing matter shells at different velocities.
Compact binary mergers are anticipated to generate strong gravitational waves in the sensitive frequency band of Earth-based gravitational-wave detectors. The detection of gravitational waves associated with a short GRB would provide direct evidence that the progenitor is indeed a compact binary; with such a detection, it would be possible to measure the binary component masses and spins, constrain neutron star equations of state, test general relativity in the strong-field regime, and measure calibration-free luminosity distance, which is a measurement of the Hubble expansion and dark energy.
This paper reports on a search for gravitational-wave inspiral signals associated with the short GRBs that occurred during the fifth science run (S5) of LIGO and the first science run (VSR1) of Virgo. X-ray and gamma-ray instruments identified a total of 212 GRBs during the S5 and VSR1 runs, however the search focused on 22 GRBs that were either short (less than 2 seconds) or have spectral features hinting at compact merger as the progenitor.
The search for gravitational-wave signals was performed within an on-source segment of 6 seconds around each trigger time for each GRB of interest. Because gravitational waves associated with a GRB are only believed to occur in the on-source segment, off-source trials (6-second chunks of data collected minutes before or after the GRB occurred) were used to estimate the distribution of background due to the accidental coincidences of noise triggers. The off-source trials were reanalyzed with simulated signals added to the data to test the response of the search; these are called injection trials.
No evidence was found for a gravitational-wave signal in coincidence with any GRB in the sample. The loudest observed candidates in each GRB's on-source segment are consistent with expectations from off-source trials. The approach of Feldman & Cousins (1998) was used to compute exclusion distances, regions in distance where gravitational-wave events would, with a given confidence, have produced results inconsistent with the observations.