TITLE: GCN CIRCULAR NUMBER: 20293 SUBJECT: GRB 161218A: Swift-BAT refined analysis DATE: 16/12/19 13:48:29 GMT FROM: Takanori Sakamoto at AGU C. B. Markwardt (GSFC), S. D. Barthelmy (GSFC), J. R. Cummings (CPI), A. D'Ai (INAF-IASFPA), N. Gehrels (GSFC),H. A. Krimm (NSF/USRA), A. Y. Lien (GSFC/UMBC), 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-352 to T+963 sec from the recent telemetry downlink, we report further analysis of BAT GRB 161218A (trigger #727288) (D'Ai, et al., GCN Circ. 20283). The BAT ground-calculated position is RA, Dec = 245.250, -4.113 deg which is RA(J2000) = 16h 21m 00.1s Dec(J2000) = -04d 06' 46.3" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 60%. The mask-weighted light curve shows a bright FRED-like peak starting from T-2 sec, peaking at T+1 sec and ending at T+14 sec. A possible extended emission is evident in the 10-sec binning light curve lasting until T+80 sec. T90 (15-350 keV) is 7.1 +- 0.6 sec (estimated error including systematics). The time-averaged spectrum from T-1.4 to T+14.5 sec is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.56 +- 0.16, and Epeak of 117 +- 13 keV (chi squared 38.16 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 6.7 +- 0.1 x 10^-6 erg/cm2 and the 1-sec peak flux measured from T+0.27 sec in the 15-150 keV band is 17.6 +- 0.5 ph/cm2/sec. A fit to a simple power law gives a photon index of 1.29 +- 0.03 (chi squared 111.22 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/727288/BA/