TITLE: GCN CIRCULAR NUMBER: 42771 SUBJECT: GRB 251118C: Glowbug gamma-ray detection of the fast X-ray transient EP251118A DATE: 25/11/19 20:54:02 GMT FROM: richard.s.woolf.civ@us.navy.mil R. Woolf, C.C. Cheung, 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 251118C, a possible gamma-ray counterpart of EP251118A (GCN 42749), also detected by Konus-Wind (GCN 42765), SVOM/GRM (GCN 42766), and Swift/BAT (GCN 42770). Using an adaptive window with a resolution of 32-ms, the burst onset is determined to be 2025-11-18 16:44:45.264 with a duration of 8.2 s and a total significance of about 23.8 sigma. The light curve comprises a single primary peak. Note that data from ~T0+8s to ~T0+12s suffered from deadtime in various detectors. The best-fit localization is RA, Decl. (J2000, deg) = 125.0, 14.2 with a radius of 10.5 deg (95% confidence), with a highly uncertain systematic uncertainty. The Glowbug position for GRB 251118C is consistent with that reported for the fast X-ray transient EP251118A (GCN 42749). 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/42771. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...