TITLE: GCN CIRCULAR NUMBER: 14377 SUBJECT: GRB 130418A: Swift detection of a burst with an optical counterpart DATE: 13/04/18 19:16:51 GMT FROM: David Palmer at LANL M. De Pasquale (UCL-MSSL), W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester), M. M. Chester (PSU), S. T. Holland (STScI), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), N. P. M. Kuin (UCL-MSSL), C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC), A. Maselli (INAF-IASFPA), P. T. O'Brien (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL), B. Sbarufatti (INAF-OAB/PSU), E. Sonbas (NASA/GSFC/Adiyaman Univ.), M. C. Stroh (PSU), C. A. Swenson (PSU) and E. Troja (NASA/GSFC/UMCP) report on behalf of the Swift Team: At 19:00:53 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 130418A (trigger=553847). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 149.043, +13.649 which is RA(J2000) = 09h 56m 10s Dec(J2000) = +13d 38' 58" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is usual with image triggers, the BAT light curve shows no significant features. The XRT began observing the field at 19:03:03.2 UT, 129.7 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 149.0377, 13.6654 which is equivalent to: RA(J2000) = +09h 56m 9.05s Dec(J2000) = +13d 39' 55.4" with an uncertainty of 5.3 arcseconds (radius, 90% containment). This location is 61 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. UVOT took a finding chart exposure of 150 seconds with the White filter starting 138 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 09:56:08.92 = 149.03718 DEC(J2000) = +13:40:02.8 = 13.66744 with a 90%-confidence error radius of about 0.74 arc sec. This position is 7.6 arc sec. from the center of the XRT error circle. The estimated magnitude is 14.96 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.03. This source was detected on the first image trigger after it came within the BAT's field of view, so its start time may be significantly before the nominal trigger time. There is no significant fading in the XRT over the ~300 second XRT observation time. Thus it is likely that this is a very long duration event. The position of the source is 47 degrees from the Galactic plane, but that doesn't preclude a nearby source. Further determination of the characteristics of the source will await further data analysis. Burst Advocate for this burst is M. De Pasquale (m.depasquale AT ucl.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.)