TITLE: GCN CIRCULAR NUMBER: 42784 SUBJECT: GRB 251118A: Glowbug gamma-ray detection DATE: 25/11/20 20:44:33 GMT FROM: C.C. Cheung at Naval Research Lab <Teddy.Cheung@nrl.navy.mil> C.C. Cheung, R. Woolf, M. Kerr, J.E. Grove (NRL), A. Goldstein (USRA), C.A. Wilson-Hodge, D. Kocevski (MSFC), and M.S. Briggs (UAH) report: The Glowbug gamma-ray telescope [1,2,3], operating on the International Space Station, reports the detection of GRB 251118A, which was also detected by Fermi/GBM (GCN 42743, 42745, 42783), Fermi/LAT (GCN 42752), NuSTAR (GCN 42774), and GECAM-B (GCN 42779). Using an adaptive window with a resolution of 32-ms, the burst onset is determined to be 2025-11-18 20:31:43.57 with a duration of 47.1 s and a total significance of over 360 sigma. The light curve comprises two peaks at ~T0+1.5s and +37s, matching the two brightest peaks in the Fermi/GBM and GECAM-B lightcurves. The initial faint emission at the GBM trigger time (20:31:30) may be present in the Glowbug light curve at low significance. The GRB was detected despite the elevated background due to Glowbug being in and exiting the Southern polar region at the time of the burst. Note that data throughout the burst interval suffered from deadtime in various detectors. The analysis results presented here are preliminary and use a response function that lacks a detailed characterization of the surrounding passive structure of the ISS. 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 Distribution Statement A: Approved for public release. Distribution is unlimited. View this GCN Circular online at https://gcn.nasa.gov/circulars/42784. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...