TITLE: GCN CIRCULAR NUMBER: 35988 SUBJECT: GRB 240328C: Glowbug gamma-ray detection DATE: 24/03/28 20:59:17 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], operating on the International Space Station, reports the detection of GRB 240328C.

Using an adaptive window with a resolution of 32-ms, the burst onset is determined to be 2024-03-28 00:37:54.024 with a duration of 4.1 s and a total significance of about 16.9 sigma. The light curve comprises a single peak.

Using a standard power-law function with an exponential high-energy cutoff [3] to model the emission over this duration results in a poorly constrained power-law index and a cutoff energy ("Epeak") of 213 keV. The modeled 10-10000 keV fluence is 5.3e-07 erg/cm^2.

The best-fit localization is RA, Decl. (J2000, deg) = 165.0, 30.6 with a radius of 27.8 deg (95% confidence), with a highly uncertain systematic uncertainty.

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. The detector comprises 12 large-area (15 cm x 15 cm) CsI:Tl panels covering the surface of a half cube, and two hexagonal (5-cm diameter, 10-cm length) CLLB scintillators, giving it a large field of view (instantaneous FoV ~2/3 sky) over a wide energy band of 50 keV to >2 MeV.

[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] Goldstein, A. et al. 2020, ApJ 895, 40, arXiv :1909.03006

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