TITLE: GCN CIRCULAR NUMBER: 17515 SUBJECT: GRB 150301B: Swift detection of a burst with an optical afterglow DATE: 15/03/01 19:50:01 GMT FROM: David Palmer at LANL A. Y. Lien (GSFC/UMBC), D. N. Burrows (PSU), M. M. Chester (PSU), J. R. Cummings (NASA/UMBC), M. De Pasquale (INAF-IASFPA), A. Maselli (INAF-IASFPA), K. L. Page (U Leicester), D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 19:38:04 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 150301B (trigger=633180). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 89.157, -57.977 which is RA(J2000) = 05h 56m 38s Dec(J2000) = -57d 58' 37" 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 20 sec. The peak count rate was ~4000 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 19:39:26.4 UT, 82.4 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 89.16636, -57.97051 which is equivalent to: RA(J2000) = 05h 56m 39.93s Dec(J2000) = -57d 58' 13.8" with an uncertainty of 3.6 arcseconds (radius, 90% containment). This location is 29 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 6.16 x 10^20 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 85 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 05:56:39.86 = 89.16608 DEC(J2000) = -57:58:10.4 = -57.96956 with a 90%-confidence error radius of about 0.61 arc sec. This position is 31.9 arc sec. from the center of the BAT error circle. The estimated magnitude is 16.22 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.08. Burst Advocate for this burst is A. Y. Lien (amy.y.lien 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.)