Searching for high-energy gamma-ray counterparts to Gravitational Wave sources with Fermi-LAT: a needle in a haystack

Giacomo Vianello, Nicola Omodei, James Chiang

Published 2016-07-06Version 1

At least a fraction of Gravitational Wave (GW) progenitors are expected to emit an electromagnetic (EM) signal in the form of a short gamma-ray burst (sGRB). The discovery of such a transient EM counterpart is challenging because the LIGO/VIRGO localization region is much larger (several hundreds of square degrees) than the field of view of X-ray, optical and radio telescopes. The Fermi Large Area Telescope (LAT) has a wide field of view ($\sim 2.4$ sr), and detects $\sim 2-3$ sGRBs per year above 100 MeV. It can detect them not only during the short prompt phase, but also during their long-lasting high-energy afterglow phase. If other wide-field high-energy instruments such as Fermi-GBM, Swift-BAT or INTEGRAL-ISGRI cannot detect or localize with enough precision an EM counterpart during the prompt phase, the LAT can potentially pinpoint it with $\lesssim 10$ arcmin accuracy during the afterglow phase. This routinely happens with gamma-ray bursts. Moreover, the LAT will cover the entire localization region within hours of any triggers during normal operations, allowing upper limits to be evaluated. We illustrate two new ad hoc methods to search for EM counterparts with the LAT and to measure upper limits, and their application to the GW candidate LVT151012.