TITLE: GCN CIRCULAR NUMBER: 7351 SUBJECT: GRB 080303: Swift detection of a burst with optical afterglow DATE: 08/03/03 09:41:42 GMT FROM: David Palmer at LANL T. Sakamoto (NASA/UMBC), W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester), G. Cusumano (INAF-IASFPA), P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC), C. Gronwall (PSU), E. A. Hoversten (PSU), N. P. M. Kuin (MSSL), J. Mao (INAF-OAB), A. Moretti (INAF-OAB), P. T. O'Brien (U Leicester), S. R. Oates (UCL-MSSL), J. P. Osborne (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL), D. Perez (U Leicester), P. Romano (INAF-IASFPA), M. Stamatikos (NASA/ORAU), R. L. C. Starling (U Leicester), G. Stratta (ASDC), G. Tagliaferri (INAF-OAB), E. Troja (U Leicester/INAF-IASFPa) and H. Ziaeepour (UCL-MSSL) report on behalf of the Swift Team: At 09:10:35 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 080303 (trigger=304549). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 112.073, -70.240 which is RA(J2000) = 07h 28m 18s Dec(J2000) = -70d 14' 23" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a FRED-like structure with a duration of about 30 sec. The peak count rate was ~2600 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 09:11:47.7 UT, 72.4 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 112.05892, -70.23448 which is equivalent to: RA(J2000) = 07h 28m 14.14s Dec(J2000) = -70d 14' 04.1" with an uncertainty of 2.6 arcseconds (radius, 90% containment). This location is 26 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 1.26e+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/304549/. The initial flux in the 2.5 s image was 4.53e-10 erg cm^-2 s^-2 (0.2-10 keV). UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm) filter starting 80 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 07:28:14.14 = 112.0589 DEC(J2000) = -70:14:01.7 = -70.2338 with a 1-sigma error radius of about 0.6 arc sec. This position is 2.4 arc sec. from the center of the XRT error circle. The estimated magnitude is 18.4 with a 1-sigma error of about 0.5 mag. A 400s V band image was taken beginning 186 s after the burst. The estimated magnitude is 18.3 with a 1 sigma error of 0.5 magnitudes. At 986 s after the burst the source is undetected in a 108 s V band exposure. The typical limiting magnitude is 18.0 magnitudes in an exposure of this length. No correction has been made for the expected extinction corresponding to E(B-V) of 0.19. Burst Advocate for this burst is T. Sakamoto (Taka.Sakamoto 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.)