TITLE:   GCN CIRCULAR
NUMBER:  35307
SUBJECT: GRB 231208A: Glowbug gamma-ray detection
DATE:    23/12/08 16:59:19 GMT
FROM:    C.C. Cheung at Naval Research Lab <Teddy.Cheung@nrl.navy.mil>

C.C. Cheung, M. Kerr, J. E. Grove, R. Woolf (NRL), A. Goldstein (USRA), C.A. Wilson-Hodge (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 231208A, which was also detected by Astrosat/CZTI (GCN 35306).

Using an adaptive window with a resolution of 32-ms, the burst onset is determined to be 2023-12-08 02:29:43.384 with a duration of 5.1 s and a total significance of about 38 sigma.  The light curve comprises two primary peaks at ~T0+2s and ~T0+3s.

Using a standard power-law function with an exponential high-energy cutoff [3] to model the emission over this duration results in a photon index dN/dE~E^x of x=2.5 and a cutoff energy ("Epeak") of 136 keV.  The modeled 10-10000 keV fluence is 9.1e-07 erg/cm^2.

The best-fit localization is RA, Decl. (J2000, deg) = 339.7, 21.2 with a radius of 14.8 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

Distribution Statement A: Approved for public release.  Distribution is unlimited.