TITLE: GCN CIRCULAR NUMBER: 39781 SUBJECT: GRB250317D: EIRSAT-1 GMOD Detection DATE: 25/03/19 14:00:39 GMT FROM: Caimin McKenna at University College Dublin caimin.mckenna@ucdconnect.ie

C. McKenna, D. Murphy, C. de Barra, A. Ulyanov, P. McDermott, G. Finneran, M. Doyle, R. Dunwoody, J. Mangan, G. Corcoran, L. Cotter, A. Empey, J. Fisher, F. Gibson Kiely, J. Thompson, D. McKeown, A. Martin-Carrillo, L. Hanlon, S. McBreen, on behalf of the EIRSAT-1 team:

EIRSAT-1 reports the detection of the long gamma-ray burst GRB250317D by the Gamma-ray Module (GMOD) instrument, which also triggered Fermi GBM (Fermi Trigger 763933627 / bn250317824, retrieved from Fermi GBM burst catalog [von Kienlin, A. et al. 2020, Gruber, D. et al. 2014, von Kienlin, A. et al. 2014, and Bhat, P. et al. 2016])

The GMOD detection was made starting at 2025-03-17 19:47:53.5 UTC.

The GMOD light-curve for GRB250317D with 1.2s binning shows a bright pulse starting at 19:47:53.5 UTC, also visible on the Fermi light-curve, followed by a less significant second pulse of one bin width. The pulses seen earlier than this time in the Fermi light-curve cannot be distinguished against the background in the GMOD light-curve.

The spacecraft location at the time of detection was 17.359 N, 35.838 E, at an altitude of 414.30 km.

The GMOD light curve for this event can be found here: https://grb.eirsat1.ie/250317D/250317D_LC_onboard_preliminary.png

EIRSAT-1 is Ireland’s first satellite (Doyle et al. Proceedings of the 4th SSEA, 2022). It is a 2U CubeSat and carries onboard a number of experiments including the Gamma-Ray Module (GMOD), a novel, compact, gamma-ray detector (Murphy et al, Experimental Astronomy, 53, 961–990, 2022). GMOD consists of a 25 mm × 25 mm × 40 mm Cerium Bromide scintillator coupled to SiPMs and is designed to detect gamma-ray bursts in the ~ 60 keV - 1.5 MeV range. EIRSAT-1 was developed in University College Dublin with support from ESA’s Fly Your Satellite! programme and was launched on 1st December 2023.

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