TITLE: GCN CIRCULAR NUMBER: 42746 SUBJECT: LIGO/Virgo/KAGRA S251112cm: Upper limits from Glowbug gamma-ray observations DATE: 25/11/18 22:20:54 GMT FROM: C.C. Cheung at Naval Research Lab <Teddy.Cheung@nrl.navy.mil> M. Kerr, C.C. Cheung, R. Woolf, J.E. Grove (NRL), A. Goldstein (USRA), C.A. Wilson-Hodge, D. Kocevski (MSFC), and M.S. Briggs (UAH) report: The compact binary merger candidate S251112cm (event time, T0 = 2025-11-12T15:19:22.360 UTC) was identified in LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data (GCN 42690). The Glowbug gamma-ray instrument [1,2,3] observed much of the sky to which a high probability for the candidate origin was assigned. Glowbug has an all-sky field-of-view limited only by earth occultation and large-scale structures on the International Space Station (ISS). At T0, the Glowbug boresight was pointed towards R.A., Dec. = 92, 22 deg. We examined three high-probability regions in the contours with positions R.A., Dec. = 346, -13 deg; 173, 43 deg; and 149, 6.5 deg, which were at zenith angles of 110 deg., 68 deg., and 57 deg., respectively, all nominally above the limb of the earth. Positions at lower zenith angles, however, are more likely to be obstructed by ISS structure. Using 50-2000 keV data and two representative (normal and hard spectrum) GRB templates from [4], we searched for transient gamma-ray signals using maximum likelihood methods and found no plausible counterpart up to 30s before or after T0. We determined 3-sigma upper limits on the flux for a GRB at the three trial positions by selecting data centered on T0 and integrating the posterior probability to determine the flux beyond which the tail probability is 0.27%. For various timescales and the two GRB spectral templates, the range of limits (over the three positions) in units of 1e-7 erg/cm2/s are: Timescale Normal Hard ------------------------------------ 0.128 s: 1.4-2.8 3.0-6.0 1.024 s: 0.8-1.0 1.5-2.0 8.192 s: 0.2-0.4 0.4-1.0 These results do not account for scattering or occultation by structures on the ISS. We do not report results for an assumed soft spectrum due to a likely increase in the low-energy threshold of Glowbug due to accumulated radiation exposure. The Glowbug sky coverage is complementary to that obtained by Fermi/GBM (GCN 42655). Glowbug is a NASA-funded technology demonstrator for sensitive, low-cost gamma-ray transient telescopes developed, built, and operated by the U.S. Naval Research Laboratory (NRL) with support from the University of Alabama in Huntsville, USRA, and NASA MSFC. It was launched on 2023 March 15 aboard the Department of Defense Space Test Program’s STP-H9 to the ISS and operated until 2024 April when it was put in safe storage on orbit. Glowbug was removed from storage and resumed operation on 2025 September 12. [1] Grove, J.E. et al. 2020, Proc. Yamada Conf. LXXI, arXiv:2009.11959 [2] Woolf, R.S. et al. 2022, Proc. SPIE, 12181, id. 121811O [3] Woolf, R.S. et al. 2024, Proc. SPIE, 13151, id. 1315108 [4] Goldstein, A. et al. 2020, ApJ 895, 40, arXiv:1909.03006 Distribution Statement A: Approved for public release. Distribution is unlimited. View this GCN Circular online at https://gcn.nasa.gov/circulars/42746. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...