On the GBM event seen 0.4 sec after GW 150914

TL;DR

This study reanalyzes the GBM data around GW 150914, concluding the 0.4 s transient is likely a background fluctuation, and emphasizes the importance of proper statistical modeling in faint short GRB detection.

Contribution

It introduces a statistical correction method for faint GRB fluence estimation, clarifying the nature of the GW 150914 associated transient and improving analysis accuracy.

Findings

The 0.4 s GBM event is consistent with background noise.

Proper statistical modeling reduces overestimation of faint GRB fluences.

Faint short GRBs with low significance are confirmed as real sources after correction.

Abstract

In view of the recent report by Connaughton we analyse continuous TTE data of Fermi-GBM around the time of the gravitational wave event GW 150914. We find that after proper accounting for low count statistics, the GBM transient event at 0.4 s after GW 150914 is likely not due to an astrophysical source, but consistent with a background fluctuation, removing the tension between the INTEGRAL/ACS non-detection and GBM. Additionally, reanalysis of other short GRBs shows that without proper statistical modeling the fluence of faint events is over-predicted, as verified for some joint GBM-ACS detections of short GRBs. We detail the statistical procedure to correct these biases. As a result, faint short GRBs, verified by ACS detections, with significances in the broad-band light curve even smaller than that of the GBM-GW150914 event are recovered as proper non-zero source, while the GBM-GW150914 event is consistent with zero fluence.