TITLE: GCN GRB OBSERVATION REPORT NUMBER: 5363 SUBJECT: GRB 060728: Refined analysis of the Swift-BAT burst DATE: 06/07/29 16:09:28 GMT FROM: Scott Barthelmy at NASA/GSFC D. Palmer (LANL), L. Barbier (GSFC), S. D. Barthelmy (GSFC), J. Cummings (GSFC/ORAU), E. Fenimore (LANL), N. Gehrels (GSFC), D. Hullinger (BYU-Idaho), H. Krimm (GSFC/USRA), M. Koss (GSFC/UMD), C. Markwardt (GSFC/UMD), C. Pagani (PSU), A. Parsons (GSFC), T. Sakamoto (GSFC/ORAU), G. Sato (GSFC/ISAS), M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC) on behalf of the Swift-BAT team: Using the data set from T-239 to T+963 sec from recent telemetry downlinks, we report further analysis of BAT GRB 060728 (trigger #221627) (Pagani, et al., GCN Circ. 5360). The BAT ground-calculated position is RA,Dec = 16.646,-41.390 deg {1h 6m 35.1s, -41d 23' 23.1"} (J2000) +- 3.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%. The mask-weighted lightcurve shows a smooth (at 10-sec binning) single peak starting at T_zero and ending at ~T+70 sec. T90 (15-350 keV) is 60 +- 10 sec (estimated error including systematics). The time-averaged spectrum from T+0.0 to T+64.0 is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.40 +- 0.45. The fluence in the 15-150 keV band is 2.4 +- 0.7 x 10^-07 erg/cm2. All the quoted errors are at the 90% confidence level. Given (a) the weakness of this event and (b) the lack of an x-ray counterpart (Pagani et al., GCN Circ 5361) and optical counterpart (Pagani et al., 5360), we can not rule out the possibility that this is a hard x-ray transient, but is most likely a burst. We note that Swift has not had a long GRB with no XRT detection when a prompt slew occurred.