TITLE: GCN CIRCULAR NUMBER: 7647 SUBJECT: GRB 080430: Swift detection of a burst with optical afterglow DATE: 08/04/30 20:16:10 GMT FROM: Scott Barthelmy at NASA/GSFC C. Guidorzi (INAF-OAB), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester), D. N. Burrows (PSU), J. R. Cummings (NASA/UMBC), N. Gehrels (NASA/GSFC), O. Godet (U Leicester), S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), W.L Landsman (GSFC), C. B. Markwardt (CRESST/GSFC/UMD), K. M. McLean (GSFC/UMD), P. T. O'Brien (U Leicester), D. M. Palmer (LANL), D. Perez (U Leicester), P. Romano (INAF-IASFPA), T. Sakamoto (NASA/UMBC), M. Stamatikos (NASA/ORAU), R. L. C. Starling (U Leicester), G. Tagliaferri (INAF-OAB), T. N. Ukwatta (GSFC/GWU), D. E. Vanden Berk (PSU) and H. Ziaeepour (UCL-MSSL) report on behalf of the Swift Team: At 19:53:02 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 080430 (trigger=310613). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 165.322, +51.689 which is RA(J2000) = 11h 01m 17s Dec(J2000) = +51d 41' 22" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single FRED peak structure with a duration of about 20 sec. The peak count rate was ~6000 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 19:53:50.9 UT, 48.9 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 165.31041, 51.68546 which is equivalent to: RA(J2000) = 11h 01m 14.50s Dec(J2000) = +51d 41' 07.7" with an uncertainty of 2.7 arcseconds (radius, 90% containment). This location is 28 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 in excess of the Galactic value (9.59e+19 cm^-2, Kalberla et al. 2005), with an excess column of 3 (+1.33/-1.17) x 10^21 cm^-2 (90% confidence). The relation of Grupe et al. (2007) implies that this burst has a redshift z<3.5, although high redshift fits to the absorbed XRT spectrum are possible if paired with an anomalously large column. A summary of the promptly downlinked data is given at http://www.swift.ac.uk/sper/310613/. The initial flux in the 2.5 s image was 1.26e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 400 seconds with the v filter starting 164 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 11:01:14.71 = 165.3113 DEC(J2000) = +51:41:08.5 = 51.6857 with a 1-sigma error radius of about 0.5 arc sec. This position is 2.2 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.6 with a 1-sigma error of about 0.5 mag. A white filter finding chart exposure was also taken before the v filter, showing that the afterglow is fading. No correction has been made for the expected extinction of about 0.04 magnitudes. Burst Advocate for this burst is C. Guidorzi (cristiano.guidorzi AT brera.inaf.it). 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.)