TITLE: GCN CIRCULAR NUMBER: 17314 SUBJECT: GRB 150120B: Swift detection of a burst DATE: 15/01/20 07:51:54 GMT FROM: David Palmer at LANL V. D'Elia (ASDC), L. Izzo (URoma/ICRA), N. P. M. Kuin (UCL-MSSL), A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester), D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 07:21:55 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 150120B (trigger=627170). Swift slewed to this burst after a short delay due the Earth limb constraint. The BAT on-board calculated location is RA, Dec 39.294, +8.079 which is RA(J2000) = 02h 37m 11s Dec(J2000) = +08d 04' 43" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with a duration of about 30 sec. The peak count rate was ~1000 counts/sec (15-350 keV), at ~2 sec after the trigger. The XRT began observing the field at 07:29:01.2 UT, 425.7 seconds after the BAT trigger. The position determined from promptly downlinked data differs significantly from the on-board position, suggesting that the XRT may have centroided on a cosmic ray; the initial XRT position notice should be treated with caution. Using promptly downlinked data we find an uncatalogued X-ray source with an enhanced position: RA, Dec 39.2927, 8.0781 which is equivalent to: RA(J2000) = 02h 37m 10.25s Dec(J2000) = +08d 04' 41.2" with an uncertainty of 2.7 arcseconds (radius, 90% containment). This location is 5.7 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 1.16 x 10^21 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 432 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 V. D'Elia (delia AT asdc.asi.it). 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.)