Fermi GBM Observations of LIGO Gravitational Wave event GW150914

TL;DR

Fermi GBM detected a weak gamma-ray transient shortly after the GW150914 event, suggesting a possible electromagnetic counterpart to a black hole merger, which challenges existing expectations about such phenomena.

Contribution

This study reports the first potential gamma-ray counterpart to a gravitational wave event from a stellar black hole merger, expanding the understanding of electromagnetic signals associated with such events.

Findings

Detected a transient 0.4s after GW150914 with 2.9σ significance

Transient's spectrum and duration are consistent with a weak short Gamma-Ray Burst

Luminosity estimated at approximately 1.8×10^{49} erg/s

Abstract

With an instantaneous view of 70% of the sky, the Fermi Gamma-ray Burst Monitor (GBM) is an excellent partner in the search for electromagnetic counterparts to gravitational wave (GW) events. GBM observations at the time of the Laser Interferometer Gravitational-wave Observatory (LIGO) event GW150914 reveal the presence of a weak transient above 50 keV, 0.4~s after the GW event, with a false alarm probability of 0.0022 (2.9$\sigma$). This weak transient lasting 1 s was not detected by any other instrument and does not appear connected with other previously known astrophysical, solar, terrestrial, or magnetospheric activity. Its localization is ill-constrained but consistent with the direction of GW150914. The duration and spectrum of the transient event are consistent with a weak short Gamma-Ray Burst arriving at a large angle to the direction in which Fermi was pointing, where the GBM detector response is not optimal. If the GBM transient is associated with GW150914, this electromagnetic signal from a stellar mass black hole binary merger is unexpected. We calculate a luminosity in hard X-ray emission between 1~keV and 10~MeV of $1.8^{+1.5}_{-1.0} \times 10^{49}$~erg~s$^{-1}$. Future joint observations of GW events by LIGO/Virgo and Fermi GBM could reveal whether the weak transient reported here is a plausible counterpart to GW150914 or a chance coincidence, and will further probe the connection between compact binary mergers and short Gamma-Ray Bursts.