TITLE: GCN CIRCULAR NUMBER: 19343 SUBJECT: GRB 160425A: Swift detection of an unusual burst with an optical afterglow DATE: 16/04/26 00:05:22 GMT FROM: Hans Krimm at NASA-GSFC H. A. Krimm (CRESST/GSFC/USRA), A. D'Ai (INAF-IASFPA), N. Gehrels (NASA/GSFC), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), A. Y. Lien (GSFC/UMBC), A. Maselli (INAF-IASFPA), D. M. Palmer (LANL), J. L. Racusin (NASA/GSFC) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 23:26:11 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 160425A (trigger=684098). Swift slewed to the burst after a short delay due to an observing constraint. The BAT on-board calculated location is RA, Dec 280.313, -54.359 which is RA(J2000) = 18h 41m 15s Dec(J2000) = -54d 21' 33" 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 2 sec, followed by a 50 second-long peak at T+250s. There may also be a precursor peak of ~100 s duration at T-380s. The peak count rate was ~6000 counts/sec (15-350 keV), at ~0.2 sec after the trigger. The XRT began observing the field at 23:29:34.7 UT, 203.4 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 280.3274, -54.3601 which is equivalent to: RA(J2000) = 18h 41m 18.56s Dec(J2000) = -54d 21' 36.2" with an uncertainty of 2.0 arcseconds (radius, 90% containment). This location is 30 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 in excess of the Galactic value (7.74 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.5 (+2.72/-2.31) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 208 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 18:41:18.57 = 280.32738 DEC(J2000) = -54:21:36.1 = -54.36004 with a 90%-confidence error radius of about 0.68 arc sec. This position is 2.6 arc sec. from the center of the XRT error circle. The estimated magnitude is 19.91 with a 1-sigma error of about 0.17. No correction has been made for the expected extinction corresponding to E(B-V) of 0.06. It is extremely unusual for a short GRB to have a long pulse a few minutes later with much more fluence. It is more common for a long GRB to have a short precursor pulse. Further characterization of this event will require the full downlinked data set. Burst Advocate for this burst is H. A. Krimm (hans.krimm 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.)