TITLE: GCN CIRCULAR NUMBER: 22574 SUBJECT: GRB 180331A: Swift-BAT refined analysis DATE: 18/03/31 16:01:42 GMT FROM: Amy Lien at GSFC T. Sakamoto (AGU), S. D. Barthelmy (GSFC), J. R. Cummings (CPI), H. A. Krimm (NSF/USRA), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC), D. M. Palmer (LANL), M. Stamatikos (OSU), T. N. Ukwatta (LANL) (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 180331A (trigger #820347) (Markwardt et al., GCN Circ. 22568). The BAT ground-calculated position is RA, Dec = 66.005, 13.386 deg which is RA(J2000) = 04h 24m 01.1s Dec(J2000) = +13d 23' 09.2" with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment). The partial coding was 81%. The mask-weighted light curve shows a weak pulse that starts at ~T0, peaks at ~T+9 s, and ends at ~T+50 s. T90 (15-350 keV) is 48.0 +- 22.6 sec (estimated error including systematics). The time-averaged spectrum from T+0.58 to T+48.58 sec is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.11 +- 1.04, and Epeak of 66.1 +- 28.3 keV (chi squared 57.70 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 6.1 +- 1.0 x 10^-7 erg/cm2 and the 1-sec peak flux measured from T+8.08 sec in the 15-150 keV band is 0.5 +- 0.1 ph/cm2/sec. A fit to a simple power law gives a photon index of 1.46 +- 0.19 (chi squared 64.59 for 57 d.o.f.). All the quoted errors are at the 90% confidence level. The results of the batgrbproduct analysis are available at http://gcn.gsfc.nasa.gov/notices_s/820347/BA/