{
  "submitter": "Joshua Wood at NASA/MSFC <joshua.r.wood@nasa.gov>",
  "bibcode": "2023GCN.34054....1W",
  "circularId": 34054,
  "createdOn": 1687395804551,
  "body": "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:\n\nIn routine Fermi GBM follow-up analysis of subthreshold GW triggers from\nLIGO/Virgo, a potential soft gamma-ray transient GBM-230619 was identified\nclose in time to S230619bd.\n\nOnline analysis of data from LIGO Livingston (L1) and Hanford (H1) Observatories\nidentified a possible subthreshold compact binary merger candidate S230619bd at\n2023-06-19 19:25:06.4 UTC (GPS time: 1371237924.4). The candidate\nwas found by the SPIIR [1] [2] analysis pipeline with classification probabilities\nof 81% terrestrial origin, 18% BNS, and <1% NSBH.\n\nThe GBM Targeted Search [3,4,5], a sensitive and coherent search for\nsubthreshold GRB-like signals, was run from +/-30 s around the GW candidate\nand identified a candidate gamma-ray signal starting at 19:25:22.3 UTC,\n15.9 s after the GW trigger time. GBM-230619 is approximately 4.1 s in\nduration and was identified with the soft spectral template [5]. Its localization\nis consistent with the GW localization, corresponding to an 89% chance of\ncoming from the same source. The offset, duration, location and spectral properties are\nsuggestive of a galactic source class and are inconsistent with expectations\nfrom a short GRB with an on-axis jet. However, we cannot rule out off-axis GRB\nemission although we acknowledge it should be challenging to detect it at the purported\nGW distance of 526+/-142 Mpc. The False Alarm Rate (FAR) for the\nGBM Targeted Search detection statistic is 3.2e-5 Hz.\n\nNeither the GW candidate nor the potential gamma-ray transient are significant enough\nto report on their own merit. However, these events are of interest because\nof their potential association. Follow-up is encouraged to determine the nature of the\ngamma-ray transient. Investigation on the data quality of the\ngravitational-wave event is ongoing.\n\nThe joint skymap available at this moment is obtained by combining the\nlocalization from L1-H1 using BAYESTAR [6] with the Fermi-GBM localization;\nthe 90% error area corresponds to 587 sq. deg. while the 50% error area is\n145 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\n\nSwift was passing through the South Atlantic Anomaly, with detectors disabled, at the time of this trigger.\n\nFermi and Swift expect to automatically disseminate joint alerts that meet the IGWN significance threshold, through GCN Kafka, within the next few weeks.\n\n [1] Hooper et al. 2012, Phys. Rev. D, 86, 024012.\n [2] Chu, Q. 2017, Ph.D. Thesis, The University of Western Australia.\n [3] Blackburn et al. 2015, ApJS 217, 8\n [4] Goldstein et al. arXiv:1612.02395\n [5] Goldstein et al. arXiv:1903.12597\n [6] Singer & Price PRD 93, 024013 (2016)\n\n\n",
  "subject": "Fermi GBM-230619: A soft subthreshold candidate potentially associated with a subthreshold LIGO/Virgo compact binary merger candidate"
}