TITLE: GCN CIRCULAR NUMBER: 17464 SUBJECT: GRB 150213B: Swift detection of a burst with an optical afterglow DATE: 15/02/13 22:47:24 GMT FROM: Scott Barthelmy at NASA/GSFC C. Pagani (U Leicester), S. D. Barthelmy (GSFC), N. Gehrels (NASA/GSFC), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), F. E. Marshall (NASA/GSFC), C. J. Mountford (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL) and B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team: At 22:31:30 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 150213B (trigger=631051). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 253.425, +34.179 which is RA(J2000) = 16h 53m 42s Dec(J2000) = +34d 10' 43" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is typical for image triggers (64 sec), the realtime TDRSS light curve does not show anything significant. The XRT began observing the field at 22:33:54.1 UT, 143.3 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 253.4532, 34.1886 which is equivalent to: RA(J2000) = +16h 53m 48.77s Dec(J2000) = +34d 11' 19.0" with an uncertainty of 5.0 arcseconds (radius, 90% containment). This location is 90 arcseconds from the BAT onboard position, within the BAT error circle. No event data are yet available to determine the column density using X-ray spectroscopy. The initial flux in the 2.5 s image was 2.84e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 152 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 16:53:48.65 = 253.45271 DEC(J2000) = +34:11:19.4 = 34.18872 with a 90%-confidence error radius of about 0.63 arc sec. This position is 1.5 arc sec. from the center of the XRT error circle. The estimated magnitude is 18.46 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.02. Burst Advocate for this burst is C. Pagani (cp232 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.)