TITLE: GCN CIRCULAR NUMBER: 15508 SUBJECT: GRB 131120A, Swift-BAT refined analysis DATE: 13/11/22 01:12:54 GMT FROM: Takanori Sakamoto at AGU A. Y. Lien (NASA/UMBC), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), J. R. Cummings (GSFC/UMBC), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC), D. M. Palmer (LANL), T. Sakamoto (AGU), M. Stamatikos (OSU), J. Tueller (GSFC), T. N. Ukwatta (MSU) (i.e. the Swift-BAT team): Using the data set from T-250 to T+963 sec from the recent telemetry downlink, we report further analysis of BAT GRB 131120A (trigger #578227) (Maselli, et al., GCN Circ. 15505). The BAT ground-calculated position is RA, Dec = 278.937, -12.026 deg which is RA(J2000) = 18h 35m 44.8s Dec(J2000) = -12d 01' 34.8" with an uncertainty of 3.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 50%. The mask-weighted light curve shows two weak peaks at T-20 sec and T+50 sec. The emission starts at T-100 sec, and ends at T+80 sec. T90 (15-350 keV) is 131 +- 20 sec (estimated error including systematics). The time-averaged spectrum from T-71.1 to T+64.3 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.94 +- 0.50. The fluence in the 15-150 keV band is 5.7 +- 1.5 x 10^-7 erg/cm2. The 1-sec peak photon flux measured from T+49.40 sec in the 15-150 keV band is 0.5 +- 0.2 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/578227/BA/ We note that the fluence ratio in a simple power-law fit between the 25-50 keV band and the 50-100 keV band is 1.92. This fluence ratio is larger than 1.32 which can be achieved in the Band function of alpha=-1.0, beta=-2.5, and Epeak=30 keV. Thus, preliminary analysis shows that Epeak of the burst is very likely around or below 30 keV. Therefore the burst can be classified as an X-ray flash (e.g. Sakamoto et al. 2008, ApJ, 679, 570).