LOOC UP: SEEKING OPTICAL COUNTERPARTS TO GRAVITATIONAL WAVE SIGNALS

Abstract

Far-reaching investigations into astrophysics, precision measurements of cosmological parameters, and tests of fundamental physics are expected to be enabled through observations of gravitational wave (GW) signals. The gravitational waveform traces the bulk motion of matter in distant sources, and so observations of GWs would reveal the details of interesting astrophysical mechanisms that are inaccessible by any other means. Currently, the most sensitive GW instruments in the world are the Laser Interferometer Gravitational Wave Observatory (LIGO) and the Virgo observatory. These kilometer scale observatories make use of sensitive optics to record miniscule changes in the space-time metric induced by GW signals, and are located at three sites in the United States and Europe.

Some models for sources of observable, transient GW signals predict that an electromagnetic (EM) counterpart will accompany the GW signal. These counterparts would be identifiable as transients which fade over the course of hours or days. Finding the EM counterpart to a GW signal would present significant benefits. The EM counterpart could confirm the astrophysical origin of the signal, and would also enhance the scientific investigations possible with the observation.

This work describes the first prompt search for EM counterparts to GW event candidates. For the search, carried out between December 2009 and October 2010, event candidates from the LIGO/Virgo network were identified with latencies of only a few minutes. The sky position of each potential source was estimated, and was delivered to a collection of radio, optical, and x-ray telescopes with roughly thirty minutes of latency. The telescopes then observed the estimated source position, in an attempt to discover an EM counterpart. The low-latency GW data analysis, the methods for estimating source positions, and the observing strategy that guides telescopes based on GW data are all novel features of the search. The ability of the LIGO/Virgo network to correctly localize sources on the sky was studied using a Monte Carlo simulation. These developments lay the groundwork for similar searches in the future with the next-generation GW detectors Advanced LIGO and Advanced Virgo.