Fermi GBM Observations of LIGO Gravitational Wave event GW150914

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.

• The paper examines a potential electromagnetic counterpart detected 0.4 seconds after GW150914, marked by a weak gamma-ray transient with a false alarm probability of 0.0022.
• It employs Fermi GBM’s extensive 70% sky coverage and spectral analysis to characterize a short-duration burst with energies above 50 keV.
• The findings suggest that if the transient is linked to GW150914, black hole mergers might emit unexpected EM signals, prompting further multimessenger investigations.

Fermi GBM Observations of LIGO Gravitational Wave Event GW150914

This paper presents an analysis of the Fermi Gamma-ray Burst Monitor (GBM) observations taken during the first confirmed detection of a gravitational wave event, GW150914, by the LIGO collaboration. The objective was to ascertain whether any coincident electromagnetic (EM) signals can be associated with the gravitational wave (GW) event, thus shedding light on the possible electromagnetic counterparts to GW sources and validating models that predict such phenomena.

Summary of Findings

Fermi's GBM system boasts a substantial instantaneous sky coverage of 70%, making it a potent ally in uncovering EM counterparts of GW sources. During the detection of GW150914, GBM observed an intriguing weak transient event above 50 keV, occurring approximately 0.4 seconds post-GW event with an assessed false alarm probability of 0.0022 (2.9σ). Characteristically, the transient's features—a brief duration of about one second and energy spectrum—suggest it aligns with a short gamma-ray burst (GRB) detected at a considerable angular deviation relative to the detector orientation. Notably, the detection stands isolated, with no corroboration from any other instrument nor association with known atmospheric or solar phenomena.

Despite its ambiguous localization, analysis implies that the transient can be situationally consistent with the GW150914 progenitor direction. The spectrum and duration mimic typical short-duration GRBs, albeit notably arriving at a large angle from the detector's optimal sensitivity direction. Should the Fermi GBM transient be a counterpart to the GW150914, this entails an unexpected optical signal phenomenon produced by a black hole merger, positioning this detection in an unforeseen radiative context.

Implications and Future Directions

The implications of this work hold significance primarily in the field of multimessenger astrophysics. If future events replicate this kind of signal concurrence, it may necessitate a revision of the theoretical architecture surrounding black hole mergers concerning their electromagnetic emissions. Furthermore, such joint observations could improve localization accuracy, enabling follow-up observations to better identify possible host galaxies.

Numerically, the calculated luminosity of the hard X-ray emission spanned the range from 1 keV to 10 MeV at $1.8^{+1.5}_{-1.0} \times 10^{49}$ erg/sec, presenting a conservative estimate for similar future events in a homologous context. The Fermi GBM's role as a corroborative partner to LIGO/Virgo is critically exemplified in this research, stimulating efforts to refine joint detection frameworks and pipelines for prospective Advanced LIGO and Virgo operations.

Practically, the results from this study catalyze significant speculation regarding alternate astrophysical emission mechanisms from compact binary mergers and contribute to identifying optimal approaches for future EM observation campaigns in GRB-GW coordinate efforts. The paper suggests that further compilation and analysis of multi-wavelength data will be pivotal, particularly as detector sensitivities and alert systems advance.

Conclusively, whilst the association between GW150914 and the Fermi GBM transient remains ambiguous, pursuing these lines of inquiry furthers the potential to demystify phenomena surrounding stellar mass black hole mergers and their prospective optical emissions. As observational technology and methodologies progress, the scope for transparent insight into these spectacular cosmic events expands, emphasizing the importance of integrative discipleship in deciphering the universe's latent secrets.