Abstract
As the first detection of Gravitation Wave (GW) event arising from the coalescence of two stellar-mass Black Holes (BH) was announced by LIGO, a new era for astronomy began. Searches for electromagnetic (EM) counterparts of GW events is of fundamental importance, as they increase the confidence in the GW detection and help characterize the parameters of the merger. The Fermi gamma-ray space telescope has the best sensitivity to simultaneously observe a large fraction of the sky from 10 keV to more than 300 GeV, providing the unique capability of rapidly covering the entire probability region from a LIGO candidate.Here we present observations by the Fermi Gamma-Ray BurstMonitor (GBM) [1] and by the Large Area Telescope (LAT) [2] of the LIGO Gravitational Wave event GW150914, which has been associated to the merger of two stellar-mass BHs. We report the presence of a weak transient event in GBM data, close in time to the LIGO one. We discuss the characteristics of this GBM transient, which are consistent with a weak short GRB arriving at a large angle to the direction in which Fermi was pointing. Furthermore, we report LAT upper limits (ULs) for GW150914, and we present the strategy for follow-up observations of GW events with the LAT.
Highlights
As the first detection of Gravitation Wave (GW) event arising from the coalescence of two stellar-mass Black Holes (BH) was announced by LIGO, a new era for astronomy began
The triggers closest in time were from two events on the same day which are consistent with particle precipitation in or near the spacecraft, at 04:09:23 UT on entering the South Atlantic Anomaly (SAA) and at 14:21:34 UT when Fermi was at high geomagnetic latitude, nearly 6 hr before and 4.5 hr after the GW event, respectively
The first undirected search is based on analysis of the Gamma-Ray Burst Monitor (GBM) Continuous Time-tagged Event (CTTE) data over four energy bands and 10 timescales, and it reported no candidates above the detection threshold on the day of the GW event
Summary
On September 16th, 2015 the LIGO and Virgo collaborations reported that a candidate event had been identified in data recorded on September 14th [3]. The first undirected search is based on analysis of the GBM Continuous Time-tagged Event (CTTE) data over four energy bands and 10 timescales, and it reported no candidates above the detection threshold on the day of the GW event. The directed search of the GBM data is seeded with the time and (optionally) the sky location of any LIGO/Virgo candidate event. We searched 30 s of GBM data before and after the LIGO coalescence time for a plausible counterpart and found two candidate events. We explore whether the GBM data for GW150914-GBM suggest an astrophysical origin and, if so, whether the source is consistent with GW150914 or can be attributed to other causes
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