TITLE: GCN CIRCULAR NUMBER: 41423 SUBJECT: GRB 250818A: Swift-BAT refined analysis DATE: 25/08/19 01:42:00 GMT FROM: Amy yarleen@gmail.com

S. Laha (GSFC/UMBC), S. D. Barthelmy (GSFC), S. B. Cenko (GSFC), R. Gupta (GSFC), H. A. Krimm (NSF), A. Y. Lien (U Tampa), C. B. Markwardt (GSFC), 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-721 to T+482 sec from the recent telemetry downlink, we report further analysis of BAT GRB 250818A (trigger #1343270) (Cenko et al., GCN Circ. 41403). The BAT ground-calculated position is RA, Dec = 213.816, -58.010 deg which is RA(J2000) = 14h 15m 15.8s Dec(J2000) = -58d 00' 36.5" with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 44%.

The mask-weighted light curve shows a multi-peaked structure that starts at ~T-15 s and ends at ~T+140 s. The main peak occurs at ~T+78 s. T90 (15-350 keV) is 106.86 +- 3.79 sec (estimated error including systematics).

The time-averaged spectrum from T-15.92 to T+141.43 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.85 +- 0.04. The fluence in the 15-150 keV band is 1.2 +- 0.03 x 10^-05 erg/cm2. The 1-sec peak photon flux measured from T+77.63 sec in the 15-150 keV band is 5.5 +- 0.3 ph/cm2/sec. 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/1343270/

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