TITLE: GCN CIRCULAR NUMBER: 17665 SUBJECT: GRB 150403A: Swift detection of a burst with an optical counterpart DATE: 15/04/03 22:17:05 GMT FROM: Scott Barthelmy at NASA/GSFC A. Y. Lien (GSFC/UMBC), S. D. Barthelmy (GSFC), V. D'Elia (ASDC), N. Gehrels (NASA/GSFC), J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester) and B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team: At 21:54:16 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 150403A (trigger=637044). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 311.505, -62.702, which is RA(J2000) = 20h 46m 01s Dec(J2000) = -62d 42' 04" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows two overlapping peaks with a total duration of about 60 sec. The peak count rate was ~19,000 counts/sec (15-350 keV), at ~7 sec after the trigger. The XRT began observing the field at 21:55:31.5 UT, 74.7 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 311.5019, -62.7118 which is equivalent to: RA(J2000) = +20h 46m 0.46s Dec(J2000) = -62d 42' 42.5" with an uncertainty of 4.8 arcseconds (radius, 90% containment). This location is 35 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 6.34e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 84 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 20:46:01.14 = 311.50476 DEC(J2000) = -62:42:41.0 = -62.71138 with a 90%-confidence error radius of about 0.61 arc sec. This position is 5.0 arc sec. from the center of the XRT error circle. The estimated magnitude is 14.85 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.05. 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.)