TITLE: GCN CIRCULAR NUMBER: 29266 SUBJECT: GRB 210104B: Swift-BAT refined analysis DATE: 21/01/05 23:27:10 GMT FROM: Amy Lien at GSFC T. Sakamoto (AGU), S. D. Barthelmy (GSFC), H. A. Krimm (NSF), S. Laha (GSFC/UMBC), 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 210104B (trigger #1015919) (Laha et al., GCN Circ. 29243). The BAT ground-calculated position is RA, Dec = 53.561, 37.951 deg which is RA(J2000) = 03h 34m 14.7s Dec(J2000) = +37d 57' 05.1" with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 59%. The mask-weighted light curve shows a double peaks structure that starts at ~T0 and ends at ~T+23 s. The two peaks occurs at ~T0 and ~T+11 s, respectively. T90 (15-350 keV) is 19.94 +- 2.33 sec (estimated error including systematics). The time-averaged spectrum from T-0.15 to T+22.76 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.83 +- 0.14. The fluence in the 15-150 keV band is 1.2 +- 0.1 x 10^-6 erg/cm2. The 1-sec peak photon flux measured from T+10.83 sec in the 15-150 keV band is 1.9 +- 0.3 ph/cm2/sec. 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/1015919/BA/