TITLE: GCN CIRCULAR NUMBER: 7427 SUBJECT: GRB 080319B: Swift detection of an intense burst with a bright optical counterpart DATE: 08/03/19 06:32:26 GMT FROM: David Palmer at LANL J. L. Racusin (PSU), N. Gehrels (NASA/GSFC), S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU), C. B. Markwardt (CRESST/GSFC/UMD), C. Pagani (PSU), D. M. Palmer (LANL) and M. Stamatikos (NASA/ORAU) report on behalf of the Swift Team: At 06:12:49 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 080319B (trigger=306757). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 217.926, +36.303 which is RA(J2000) = 14h 31m 42s Dec(J2000) = +36d 18' 10" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed one bright but complex peak with a duration of about 50 sec, with an extended tail. The peak count rate was ~70,000 counts/sec (15-350 keV), at ~20 sec after the trigger. The XRT began observing the field at 06:13:49.7 UT, 60.5 seconds after the BAT trigger. XRT found a very bright fading, uncatalogued X-ray source located at RA, Dec 217.9196, +36.3041 which is equivalent to: RA(J2000) = 14h 31m 40.7s Dec(J2000) = +36d 18' 14.7" with an uncertainty of 4.7 arcseconds (radius, 90% containment). This location is 18 arcseconds from the BAT onboard position, within the BAT error circle. No event data are yet available to assess possible redshift constraints using X-ray spectroscopy and the nH-z relation from Grupe et al. (2007). UVOT took a finding chart exposure of 400 seconds with the V filter starting 175 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image. This position is consistent with the XRT error circle, but the source is so bright that a precise UVOT position is not possible due to saturation effects. The estimated magnitude is 11.5 with a 1-sigma error of about 0.5 mag. No correction has been made for the expected extinction of about 0.04 magnitudes. Burst Advocate for this burst is J. L. Racusin (racusin 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.)