TITLE: GCN CIRCULAR NUMBER: 17888 SUBJECT: GRB 150530A: Swift detection of a burst DATE: 15/05/30 11:59:01 GMT FROM: Rhaana Starling at U of Leicester R. L. C. Starling (U Leicester), N. Gehrels (NASA/GSFC), J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC) and K. L. Page (U Leicester) report on behalf of the Swift Team: At 11:42:18 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 150530A (trigger=642018). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 327.523, +57.490 which is RA(J2000) = 21h 50m 06s Dec(J2000) = +57d 29' 25" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single peak with some structure with a duration of about 10 sec. The peak count rate was ~2500 counts/sec (15-350 keV). The XRT began observing the field at 11:44:00.8 UT, 102.7 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source with an enhanced position: RA, Dec 327.5109, 57.5164 which is equivalent to: RA(J2000) = 21h 50m 02.61s Dec(J2000) = +57d 30' 58.9" with an uncertainty of 3.3 arcseconds (radius, 90% containment). This location is 96 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. 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 1.00 x 10^22 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 106 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. The typical 3-sigma upper limit has been about 19.6 mag. 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 large, but uncertain extinction expected. Burst Advocate for this burst is R. L. C. Starling (rlcs1 AT star.le.ac.uk). 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.)