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GCN Circular 34054

Fermi GBM-230619: A soft subthreshold candidate potentially associated with a subthreshold LIGO/Virgo compact binary merger candidate
2023-06-22T01:03:24Z (a year ago)
Joshua Wood at NASA/MSFC <>
J. Wood (NASA), O.J. Roberts (USRA), E. Burns (LSU), P. Veres (UAH), A. Tohuvavohu (U Toronto) et al. report on behalf of the Fermi-GBM Team:

In routine Fermi GBM follow-up analysis of subthreshold GW triggers from
LIGO/Virgo, a potential soft gamma-ray transient GBM-230619 was identified
close in time to S230619bd.

Online analysis of data from LIGO Livingston (L1) and Hanford (H1) Observatories
identified a possible subthreshold compact binary merger candidate S230619bd at
2023-06-19 19:25:06.4 UTC (GPS time: 1371237924.4). The candidate
was found by the SPIIR [1] [2] analysis pipeline with classification probabilities
of 81% terrestrial origin, 18% BNS, and <1% NSBH.

The GBM Targeted Search [3,4,5], 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 19:25:22.3 UTC,
15.9 s after the GW trigger time. GBM-230619 is approximately 4.1 s in
duration and was identified with the soft spectral template [5]. Its localization
is consistent with the GW localization, corresponding to an 89% chance of
coming from the same source. The offset, duration, location and spectral properties are
suggestive of a galactic source class and are inconsistent with expectations
from a short GRB with an on-axis jet. However, we cannot rule out off-axis GRB
emission although we acknowledge it should be challenging to detect it at the purported
GW distance of 526+/-142 Mpc. The False Alarm Rate (FAR) for the
GBM Targeted Search detection statistic is 3.2e-5 Hz.

Neither the GW candidate nor the potential gamma-ray transient are significant enough
to report on their own merit. However, these events are of interest because
of their potential association. Follow-up is encouraged to determine the nature of the
gamma-ray transient. Investigation on the data quality of the
gravitational-wave event is ongoing.

The joint skymap available at this moment is obtained by combining the
localization from L1-H1 using BAYESTAR [6] with the Fermi-GBM localization;
the 90% error area corresponds to 587 sq. deg. while the 50% error area is
145 sq. deg. This skymap is available via Zenodo:

Swift was passing through the South Atlantic Anomaly, with detectors disabled, at the time of this trigger.

Fermi and Swift expect to automatically disseminate joint alerts that meet the IGWN significance threshold, through GCN Kafka, within the next few weeks.

 [1] Hooper et al. 2012, Phys. Rev. D, 86, 024012.
 [2] Chu, Q. 2017, Ph.D. Thesis, The University of Western Australia.
 [3] Blackburn et al. 2015, ApJS 217, 8
 [4] Goldstein et al. arXiv:1612.02395
 [5] Goldstein et al. arXiv:1903.12597
 [6] Singer & Price PRD 93, 024013 (2016)

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