Abstract
Abstract The weak transient detected by the Fermi Gamma-ray Burst Monitor (GBM) 0.4 s after GW150914 has generated much speculation regarding its possible association with the black hole binary merger. Investigation of the GBM data by Connaughton et al. revealed a source location consistent with GW150914 and a spectrum consistent with a weak, short gamma-ray burst. Greiner et al. present an alternative technique for fitting background-limited data in the low-count regime, and call into question the spectral analysis and the significance of the detection of GW150914-GBM presented in Connaughton et al. The spectral analysis of Connaughton et al. is not subject to the limitations of the low-count regime noted by Greiner et al. We find Greiner et al. used an inconsistent source position and did not follow the steps taken in Connaughton et al. to mitigate the statistical shortcomings of their software when analyzing this weak event. We use the approach of Greiner et al. to verify that our original spectral analysis is not biased. The detection significance of GW150914-GBM is established empirically, with a false-alarm rate (FAR) of Hz. A post-trials false-alarm probability (FAP) of ( ) of this transient being associated with GW150914 is based on the proximity in time to the gravitational-wave event of a transient with that FAR. The FAR and the FAP are unaffected by the spectral analysis that is the focus of Greiner et al.
Highlights
With the detection by the Laser Interferometer Gravitationalwave Observatory (LIGO; Aasi et al 2015; Abbott et al 2016a, 2016c, 2017) of two highly significant gravitational-wave (GW) events and one additional probable GW event during their O1 science run came the search for possible electromagnetic counterparts
The pitfalls of using rmfit to analyze low-count data are known —in the routine calculation by the Gamma-ray Burst Monitor (GBM) team of the duration of a GRB, where spectral fits are performed over successive short time intervals, 8-channel data are used in preference to 128-channel data, a practice we follow in VC+16
No further transients were uncovered connected with the other GW and high-confidence GW candidate detected by LIGO during O1, either by GBM (Racusin et al 2017) or by other instruments taking part in the follow-up campaign
Summary
With the detection by the Laser Interferometer Gravitationalwave Observatory (LIGO; Aasi et al 2015; Abbott et al 2016a, 2016c, 2017) of two highly significant gravitational-wave (GW) events and one additional probable GW event during their O1 science run came the search for possible electromagnetic counterparts. Details of the targeted offline search deployed during O1 are provided in Blackburn et al (2015) and VC+16 It can be summarized as a search over the whole sky, coherently combining the data from all 14 GBM detectors (NaI and BGO) to test the statistical preference for a source above background. We explored further the localization of the gamma-ray transient using the process employed by the GBM team in regular operations This process informed the development of the targeted search, which retains common features such as its use of spectral templates and grid searches for likely source arrival directions. The post-trials significance of 2.9s reflects an empirical measurement of how likely it is that a transient of the signal size and consistency with a point source indicated by the likelihood and associated FAR (whether background or astrophysical) occurs by chance so close in time to a GW event. We do this without challenging the premise of JG+16 that there exist more suitable statistical approaches to the spectral analysis of weak transients than that used in rmfit
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