TITLE: GCN CIRCULAR NUMBER: 13836 SUBJECT: Swift Trigger 535026 is GRB 121001A: Swift-BAT refined analysis DATE: 12/10/02 01:14:20 GMT FROM: Craig Markwardt at NASA/GSFC T. Sakamoto (AGU), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), J. R. Cummings (GSFC/UMBC), V. D'Elia (ASDC), E. E. Fenimore (LANL), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC), D. M. Palmer (LANL), G. Sato (ISAS), M. Stamatikos (OSU), J. Tueller (GSFC), T. N. Ukwatta (MSU) (i.e. the Swift-BAT team): Using the data set from T-60 to T+243 sec from the recent telemetry downlink, we report that Swift trigger #535026 (D'Elia, et al., GCN Circ. #13831) is consistent with being a GRB. We follow the designation of previous authors for the trigger as GRB 121001A (Zheng et al. GCN Circ. #13832; Andreev et al. GCN Circ. #13833; Gorbovskoy et al. GCN Circ. #13834). Based on the analysis below, the burst has strong emission up to 100 keV, and a FRED-like light curve profile with duration longer than 100sec. Neither of these is consistent with a neutron star thermonuclear X-ray burst (which would be blackbody emission with temperature kT ~ few keV). The soft emission we reported in GCN Circ. #535026 is associated with an X-ray burst from HETE J1900.1-2455, another active X-ray binary in the BAT field of view. Thus, we conclude that trigger #535026 is consistent with a GRB interpretation. We report further BAT analysis of GRB 121001A (trigger #535026). The BAT ground-calculated position is RA, Dec = 276.029, -5.667 deg which is RA(J2000) = 18h 24m 06.9s Dec(J2000) = -05d 40' 01.0" with an uncertainty of 2.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%. The light curve shows a FRED-like profile, starting near T-25sec, rising to maximum at T-15sec, and gradually decaying to background near T+130sec. T90 (15-350 keV) is 147 +- 24 sec (estimated error including systematics). The time-averaged spectrum from T-30 to T+143 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.34 +- 0.15. The fluence in the 15-150 keV band is 1.7 +- 0.2 x 10^-6 erg/cm2. The 1-sec peak photon flux measured from T+3.27 sec in the 15-150 keV band is 0.5 +- 0.1 ph/cm2/sec. All the quoted errors are at the 90% confidence level. The BAT results for this circular represent a limited time window. The results may be revised if additional downlinked data show more emission. The results of the batgrbproduct analysis are available at http://gcn.gsfc.nasa.gov/notices_s/535026/BA/