TITLE: GCN CIRCULAR NUMBER: 18087 SUBJECT: GRB 150728A: Swift detection of a GRB DATE: 15/07/28 13:14:33 GMT FROM: Scott Barthelmy at NASA/GSFC H. A. Krimm (CRESST/GSFC/USRA), S. D. Barthelmy (GSFC), A. A. Breeveld (UCL-MSSL), D. N. Burrows (PSU), S. B. Cenko (GSFC), P. A. Evans (U Leicester), C. Gronwall (PSU), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), P. Romano (INAF-IASFPA), T. Sakamoto (AGU), B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 12:51:11 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 150728A (trigger=650617). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 292.288, +33.904 which is RA(J2000) = 19h 29m 09s Dec(J2000) = +33d 54' 14" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows two weak peaks with a total duration of about 20 sec. The peak count rate was ~1000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 12:52:21.1 UT, 70.2 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source with an enhanced position: RA, Dec 292.2269, 33.9165 which is equivalent to: RA(J2000) = 19h 28m 54.46s Dec(J2000) = +33d 54' 59.4" with an uncertainty of 3.8 arcseconds (radius, 90% containment). This location is 188 arcseconds from the BAT onboard position, outside 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. We cannot determine whether the source is fading at the present time. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 2.59 x 10^21 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 75 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 none of the XRT error circle. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.20. Burst Advocate for this burst is H. A. Krimm (hans.krimm 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.)