TITLE: GCN CIRCULAR NUMBER: 8749 SUBJECT: GRB 081228, Swift-BAT refined analysis DATE: 08/12/28 17:04:20 GMT FROM: Hans Krimm at NASA-GSFC C. B. Markwardt (GSFC/UMD), 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), K. McLean (GSFC/UMD), K. L. Page (U Leicester), D. M. Palmer (LANL), A. M. Parsons (GSFC), 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-239 to T+963 sec from the recent telemetry downlink, we report further analysis of Swift GRB 081228 (trigger #338338) (Page, et al., GCN Circ. 8742). The BAT ground-calculated position is RA, Dec = 39.477, 30.833 deg which is RA(J2000) = 02h 37m 54.4s Dec(J2000) = +30d 49' 57.3" with an uncertainty of 2.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 85%. The mask-weighted light curve begins with a possible faint soft 20-second precursor beginning at T-180 sec. The main emission consists of a single peak beginning at T-0.5 sec and lasting ~2.5 seconds. T90 (15-350 keV) is 3.0 +- 1.4 sec (estimated error including systematics). An observing constraint caused the Swift spacecraft to slew away from the burst location at ~T+250 seconds, so no information about the burst is available after ~T+300 sec, when the burst was no longer in the field of view. The time-averaged spectrum from T-0.1 to T+2.9 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.10 +- 0.31. The fluence in the 15-150 keV band is 8.9 +- 1.8 x 10-8 erg/cm2. The 1-sec peak photon flux measured from T+0.87 sec in the 15-150 keV band is 0.6 +- 0.1 ph/cm2/sec. All the quoted errors are at the 90% confidence level. With a duration longer than 2 s and a soft spectrum, this burst is likely to be in the "long" category. The results of the batgrbproduct analysis are available at http://gcn.gsfc.nasa.gov/notices_s/338338/BA/