TITLE: GCN CIRCULAR NUMBER: 7453 SUBJECT: GRB 080319D: Swift detection of a burst with a possible optical afterglow DATE: 08/03/19 17:42:09 GMT FROM: Scott Barthelmy at NASA/GSFC P. J. Brown (PSU), S. D. Barthelmy (GSFC), D. N. Burrows (PSU), M. M. Chester (PSU), N. Gehrels (NASA/GSFC), O. Godet (U Leicester), S. T. Holland (CRESST/USRA/GSFC), S. Immler (CRESST/GSFC/UMD), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), P. T. O'Brien (U Leicester), J. P. Osborne (U Leicester), C. Pagani (PSU), K. L. Page (U Leicester), D. M. Palmer (LANL), M. Perri (ASDC), J. L. Racusin (PSU), M. Stamatikos (NASA/ORAU), R. L. C. Starling (U Leicester), E. Troja (U Leicester/INAF-IASFPA) and T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team: At 17:05:09 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 080319D (trigger=306793). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 99.480, +23.933 which is RA(J2000) = 06h 37m 55s Dec(J2000) = +23d 56' 01" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is usual with an image trigger, the prompt light curve does not not show any clear structure. We will be able to tell more with the full data set. The XRT began observing the field at 17:07:36.9 UT, 147.5 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued, variable X-ray source with an enhanced position: RA, Dec 99.47270, 23.94306 which is equivalent to: RA(J2000) = 06h 37m 53.45s Dec(J2000) = +23d 56' 35.5" with an uncertainty of 3.0 arcseconds (radius, 90% containment). This location is 44 arcseconds from the BAT onboard position, within the BAT error circle. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 2.17e+21 cm^-2 (Kalberla et al. 2005), so we cannot constrain the redshift at this time using the relation from Grupe et al. (2007). A summary of the promptly downlinked data is given at http://www.swift.ac.uk/sper/306793/. UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm) filter starting 152 seconds after the BAT trigger. A possible afterglow candidate has been found in the initial data products, at a magnitude of ~19 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.19. The position of the tentative UVOT source is: RA(J2000) = 06h 37m 53.6s Dec(J2000) = +23d 56' 34.1" Burst Advocate for this burst is P. J. Brown (pbrown AT astro.psu.edu). 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.)