Modeling the afterglow of the possible Fermi-GBM event associated with GW150914

Abstract

We model the possible afterglow of the Fermi GBM event associated with LIGO detection GW150914, under the assumption that the gamma-ray are produced by a short GRB-like relativistic outflow. We model GW150914-GBM as both a weak, on-axis short GRB and normal short GRB seen far off axis. Given the large uncertainty in the position of GW150914, we determine that the best chance of finding the afterglow is with ASKAP or possibly the MWA, with the flux from an off-axis short GRB reaching 0.2 - 4 mJy (0.12 - 16 mJy) at 150 MHz (863.5 MHz) by 1 - 12 months after the initial event. At low frequencies, the source would evolve from a hard to soft spectrum over several months. The radio afterglow would be detectable for several months to years after it peaks, meaning the afterglow may still be detectable and increasing in brightness NOW (mid-July 2016). With a localization from the MWA or ASKAP, the afterglow would be detectable at higher radio frequencies with the ATCA and in X-rays with Chandra or XMM.