TITLE: GCN CIRCULAR NUMBER: 6642 SUBJECT: GRB 070721A, Swift-BAT refined analysis DATE: 07/07/21 14:07:31 GMT FROM: Scott Barthelmy at NASA/GSFC T. Sakamoto (GSFC/ORAU), L. Barbier (GSFC), S. D. Barthelmy (GSFC), J. Cummings (GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC), H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), D. Palmer (LANL), A. Parsons (GSFC), G. Sato (GSFC/ISAS), M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC), T. Ukwatta (GWU), H. Ziaeepour (UCL-MSSL) (i.e. the Swift-BAT team): Using the data set from T-239 to T+352 sec from the recent telemetry downlink, we report further analysis of BAT GRB 070721A (trigger #285653) (Ziaeepour, et al., GCN Circ. 6639). The BAT ground-calculated position is RA, Dec = 3.144, -28.530 deg which is RA(J2000) = 0h 12m 34.5s Dec(J2000) = -28d 31' 47" with an uncertainty of 2.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 72%. The mask-weighted lightcurve has a single peak starting at ~T-4 sec, peaking at ~T+1 sec, and ending at ~T+8 sec. T90 (15-350 keV) is 3.4 +- 0.2 sec (estimated error including systematics). The time-averaged spectrum from T-0.1 to T+3.7 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.46 +- 0.41. The fluence in the 15-150 keV band is 7.1 +- 1.8 x 10^-8 erg/cm2. The 1-sec peak photon flux measured from T+0.12 sec in the 15-150 keV band is 0.7 +- 0.1 ph/cm2/sec. All the quoted errors are at the 90% confidence level. 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 1.38. 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.