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GCN Circular 25406

Subject
Fermi GBM-190816: A subthreshold GRB candidate potentially associated with a subthreshold LIGO/Virgo compact binary merger candidate
Date
2019-08-20T05:23:25Z (5 years ago)
From
Adam Goldstein at Fermi-GBM, USRA <adam.michael.goldstein@gmail.com>
The LIGO Scientific Collaboration, the Virgo Collaboration and the Fermi
GBM team report:

In routine Fermi GBM follow-up analysis of subthreshold GW triggers from
LIGO/Virgo, a potential short gamma-ray burst counterpart GBM-190816 was
identified.

Offline analysis of data from LIGO Livingston Observatory (L1) and Virgo
Observatory (V1) identified a possible compact binary merger candidate at
2019-08-16 21:22:13.027 UTC (GPS time: 1250025751.027). The LIGO Hanford
Observatory was not collecting low-noise data at the time. The candidate
was found by the PyCBC Live [1], MBTAOnline [2], and GstLAL [3] analysis
pipelines.

The GBM Targeted Search [4,5,6], a sensitive and coherent search for
subthreshold GRB-like signals, was run from +/-30 s around the GW candidate
and identified a candidate gamma-ray signal starting at 21:22:14.563 UTC,
1.5 s after the GW trigger time. GBM-190816 is approximately 0.1 s in
duration and was identified with the hard spectral template [6]. The
offset, duration, and spectral properties are consistent with a short GRB
origin, while the observed properties are inconsistent with other
astrophysical or terrestrial transients that GBM observes. The False Alarm
Rate (FAR) for the GBM Targeted Search detection statistic is 1.2E-4 Hz.

Neither the GW trigger nor the potential short GRB are significant enough
to report on their own merit. However, these events are of interest because
of their potential association. Investigation on the data quality of the
gravitational-wave event is ongoing. At this time we cannot reliably
estimate the FAR of the gravitational-wave event. Analysis is ongoing in
establishing the FAR, data-quality and overall potential astrophysical
nature of the event.

The skymap available at this moment is obtained primarily by combining the
localization from L1-V1 using BAYESTAR [7] with the Fermi-GBM localization;
the 90% error area corresponds to 5855 sq. deg. while the 50% error area is
1257 sq. deg. This skymap is available through the OpenLVEM wiki page at

https://wiki.gw-astronomy.org/OpenLVEM/FermiGBM-LVC

From a preliminary inspection of the GW analysis, if the signal is
astrophysical, the lighter compact object may have a mass < 3 solar masses.

 [1] Nitz et al. PRD 98, 024050 (2018)

 [2] Adams et al. CQG 33, 175012 (2016)

 [3] Messick et al. PRD 95, 042001 (2017)

 [4] Blackburn et al. 2015, ApJS 217, 8

 [5] Goldstein et al. arXiv:1612.02395

 [6] Goldstein et al. arXiv:1903.12597

 [7] Singer & Price PRD 93, 024013 (2016)
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