D. C. Morris (GWU/GSFC), A. P. Beardmore (U Leicester), V. D'Elia (ASDC), P. A. Evans (U Leicester), J. M. Gelbord (PSU), C. Guidorzi (U Ferrara), S. T. Holland (CRESST/USRA/GSFC), H. A. Krimm (CRESST/GSFC/USRA), O. M. Littlejohns (U Leicester), C. B. Markwardt (CRESST/GSFC/UMD), J. P. Osborne (U Leicester), C. Pagani (U Leicester), K. L. Page (U Leicester), A. Rowlinson (U Leicester), B. Sbarufatti (INAF-OAB/IASFPA), M. H. Siegel (PSU), M. Stamatikos (OSU/NASA/GSFC) and R. L. C. Starling (U Leicester) report on behalf of the Swift Team:

At 10:31:55 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 100728B (trigger=430172). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 44.053, +0.280 which is RA(J2000) = 02h 56m 13s Dec(J2000) = +00d 16' 46" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single-peaked structure with a duration of about 10 sec plus some possible precursor activity. The peak count rate was ~2000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 10:33:32.9 UT, 97.0 seconds after the BAT trigger. No source was detected in the first 2.5 seconds image. The XRT observed the field for approximately 80 seconds in WT mode. The prompt spectrum from the WT data shows evidence of a source. Due to an observing constraint no additional PC data were taken to allow XRT to centroid on the afterglow and derive a position. We are waiting for the full dataset to detect and localise the XRT counterpart. UVOT took a finding chart exposure of 150 seconds with the White filter starting 100 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 02:56:13.46 = 44.05607 DEC(J2000) = +00:16:51.9 = 0.28109 with a 90%-confidence error radius of about 0.75 arc sec. This position is 11.7 arc sec. from the center of the BAT error circle. The estimated magnitude is 17.30 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.07. Burst Advocate for this burst is D. C. Morris (david.c.morris AT nasa.gov). Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/too.html.)

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