TITLE: GCN CIRCULAR NUMBER: 25406 SUBJECT: Fermi GBM-190816: A subthreshold GRB candidate potentially associated with a subthreshold LIGO/Virgo compact binary merger candidate DATE: 19/08/20 05:23:25 GMT FROM: Adam Goldstein at Fermi-GBM, USRA 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)