TITLE: GCN CIRCULAR NUMBER: 9888 SUBJECT: GRB 090904A: Swift-BAT refined analysis DATE: 09/09/04 13:16:39 GMT FROM: Scott Barthelmy at NASA/GSFC D. M. Palmer (LANL), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), J. R. Cummings (GSFC/UMBC), E. E. Fenimore (LANL), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC/UMD), M. Perri (ASDC), T. Sakamoto (GSFC/UMBC), G. Sato (ISAS), M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC), T. N. Ukwatta (GWU) (i.e. the Swift-BAT team): Using the data set from T-60 to T+243 sec from the recent telemetry downlink, we report further analysis of BAT GRB 090904A (trigger #361830) (Perri, et al., GCN Circ. 9879). The BAT ground-calculated position is RA, Dec = 100.855, 50.235 deg, which is RA(J2000) = 06h 43m 25.2s Dec(J2000) = +50d 14' 06.9" with an uncertainty of 3.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 75%. The mask-weighted light curve shows low level emission starting at ~T-40 sec, with a small peak at ~T+5 sec. Then there was a larger peak at ~T+50 sec, and then the largest peak (FRED-like) starting at ~T+123 sec, peaking at ~T+128 sec. Riding on that tail is a peak ~T+173. The event is over in the BAT energy band at ~T+240 sec. T90 (15-350 keV) is 122 +- 10 sec (estimated error including systematics). The time-averaged spectrum from T+29.3 to T+186.3 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.01 +- 0.10. The fluence in the 15-150 keV band is 3.0 +- 0.2 x 10^-6 erg/cm2. The 1-sec peak photon flux measured from T+127.07 sec in the 15-150 keV band is 1.9 +- 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/361830/BA/