TITLE: GCN CIRCULAR NUMBER: 13525 SUBJECT: GRB 120728A: Swift detection of a burst DATE: 12/07/28 22:37:55 GMT FROM: Scott Barthelmy at NASA/GSFC J. R. Cummings (NASA/UMBC), S. D. Barthelmy (GSFC), M. M. Chester (PSU), H. A. Krimm (CRESST/GSFC/USRA), N. P. M. Kuin (UCL-MSSL), C. B. Markwardt (NASA/GSFC), A. Maselli (INAF-IASFPA), D. M. Palmer (LANL), M. H. Siegel (PSU) and T. N. Ukwatta (MSU) report on behalf of the Swift Team: At 22:25:11 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 120728A (trigger=529021). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 137.100, -54.437, which is RA(J2000) = 09h 08m 24s Dec(J2000) = -54d 26' 13" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a peak with structure with a total duration of about 20 sec. The peak count rate was ~1500 counts/sec (15-350 keV), at ~3 sec after the trigger. The XRT began observing the field at 22:27:38.8 UT, 147.8 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source located at RA, Dec 137.09412, -54.43783 which is equivalent to: RA(J2000) = 09h 08m 22.59s Dec(J2000) = -54d 26' 16.2" with an uncertainty of 4.1 arcseconds (radius, 90% containment). This location is 12 arcseconds from the BAT onboard position, within the BAT error circle. This position may be improved as more data are received; the latest position is available at http://www.swift.ac.uk/sper. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 5.18 x 10^21 cm^-2 (Kalberla et al. 2005). UVOT took a finding chart exposure of 250 seconds with the U filter starting 148 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. Because of the density of catalogued stars, further analysis is required to report an upper limit for any afterglow in the sub-image. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. Because of the density of catalogued stars, further analysis is required to report an upper limit for any afterglow in the region. No correction has been made for the large, but uncertain extinction expected. Burst Advocate for this burst is J. R. Cummings (jayc AT milkyway.gsfc.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.)