TITLE: GCN CIRCULAR NUMBER: 20967 SUBJECT: GRB 170331A: Swift-BAT refined analysis DATE: 17/04/01 06:15:10 GMT FROM: Amy Lien at GSFC C. B. Markwardt (GSFC), S. D. Barthelmy (GSFC), J. R. Cummings (CPI), N. Gehrels (GSFC), H. A. Krimm (NSF/USRA), A. Y. Lien (GSFC/UMBC), A. Melandri (INAF-OAB), D. M. Palmer (LANL), T. Sakamoto (AGU), 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 170331A (trigger #744791) (Melandri et al., GCN Circ. 20946). The BAT ground-calculated position is RA, Dec = 323.778, -24.406 deg which is RA(J2000) = 21h 35m 06.8s Dec(J2000) = -24d 24' 22.4" with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 18%. The mask-weighted light curve shows a weak pulse that starts at ~T-3 s, peaks at ~T+1 s, and ends at ~T+17 s. T90 (15-350 keV) is 17.57 +- 2.57 sec (estimated error including systematics). The time-averaged spectrum from T-2.78 to T+17.17 sec is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.37 +- 1.19, and Epeak of 48.2 +- 12.7 keV (chi squared 61.51 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 9.7 +- 1.6 x 10^-7 erg/cm2 and the 1-sec peak flux measured from T+0.42 sec in the 15-150 keV band is 1.2 +- 0.4 ph/cm2/sec. A fit to a simple power law gives a photon index of 1.86 +- 0.21 (chi squared 68.07 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/744791/BA/