TITLE: GCN CIRCULAR NUMBER: 34054 SUBJECT: Fermi GBM-230619: A soft subthreshold candidate potentially associated with a subthreshold LIGO/Virgo compact binary merger candidate DATE: 23/06/22 01:03:24 GMT FROM: 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: https://gcc02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fzenodo.org%2Frecord%2F8067512&data=05%7C01%7Ccirculars%40gcn.nasa.gov%7C9b379d6300f14f9a7dbe08db72bc68de%7C7005d45845be48ae8140d43da96dd17b%7C0%7C0%7C638229925920569415%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=8%2F3wCrwzbZyrwhT9LlIcKU2pNLElP5AGUQT%2Fnn6et0c%3D&reserved=0 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)