TITLE: GCN CIRCULAR NUMBER: 40255 SUBJECT: GRB 250424A: Swift-BAT refined analysis DATE: 25/04/26 22:48:38 GMT FROM: Rahul Gupta at NASA GSFC <rahul.gupta@nasa.gov> C. B. Markwardt (GSFC), S. D. Barthelmy (GSFC), S. B. Cenko (GSFC), R. Gupta (GSFC), H. A. Krimm (NSF), S. Laha (GSFC/UMBC), A. Y. Lien (U Tampa), M. J. Moss (GSFC), D. M. Palmer (LANL), T. Parsotan (GSFC), D. Sadaula (GSFC/UMBC), T. Sakamoto (AGU) (i.e. the Swift-BAT team): Using the data set from T-239 to T+963 sec from the recent telemetry downlink, we report further analysis of BAT GRB 250424A (trigger #1306404) (Cenko, et al., GCN Circ. 40224). The BAT ground-calculated position is RA, Dec = 217.528, -35.025 deg which is RA(J2000) = 14h 30m 06.7s Dec(J2000) = -35d 01' 30.6" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 39%. The mask-weighted BAT light curve of the burst (began during a slew) shows a faint precursor emission followed by a bright main pulse. T90 (15-350 keV) is 19.03 +- 1.06 sec (estimated error including systematics), with the T90 starting at T0-20.76 sec due to the slew-delayed T0. The time-averaged spectrum from T-38.75 to T+262.67 sec is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.54 +- 0.13, and Epeak of 106.5 +- 33.7 keV (chi squared 47.05 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 3.2 +- 0.1 x 10^-05 erg/cm2 and the 1-sec peak flux measured from T-15.38 sec in the 15-150 keV band is 47.0 +- 1.3 ph/cm2/sec. A fit to a simple power law gives a photon index of 1.79 +- 0.03 (chi squared 58.54 for 57 d.o.f.). All the quoted errors are at the 90% confidence level. The results of the batgrbproduct analysis are available at https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1306404 View this GCN Circular online at https://gcn.nasa.gov/circulars/40255. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...