TITLE: GCN CIRCULAR NUMBER: 9223 SUBJECT: GRB 090424: Swift detection of a burst with an optical counterpart DATE: 09/04/24 14:23:57 GMT FROM: Scott Barthelmy at NASA/GSFC J. K. Cannizzo (NASA/UMBC), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester), D. N. Burrows (PSU), P.A. Curran (MSSL-UCL), P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC), C. Gronwall (PSU), C. Guidorzi (U Ferrara), S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), J. Mao (INAF-OAB), R. Margutti (Univ Bicocca&OAB), F. E. Marshall (NASA/GSFC), P. T. O'Brien (U Leicester), K. L. Page (U Leicester), P. Romano (INAF-IASFPA), B. A. Rowlinson (U Leicester), M. H. Siegel (PSU), M. C. Stroh (PSU), G. Tagliaferri (INAF-OAB) and L. Vetere (PSU) report on behalf of the Swift Team: At 14:12:09 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 090424 (trigger=350311). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 189.538, +16.829 which is RA(J2000) = 12h 38m 09s Dec(J2000) = +16d 49' 46" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows several bright peaks in the first 5 seconds, and then several smaller peaks at T+8, +15, and +50 sec. Total duration of about 60 sec. The peak count rate was ~50000 counts/sec (15-350 keV), at ~3 sec after the trigger. The XRT began observing the field at 14:13:33.8 UT, 84.5 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 189.5213, +16.8364 which is equivalent to: RA(J2000) = 12h 38m 05.11s Dec(J2000) = +16d 50' 11.0" with an uncertainty of 4.8 arcseconds (radius, 90% containment). This location is 63 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 91 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 12:38:05.11 = 189.52128 DEC(J2000) = +16:50:15.1 = 16.83753 with a 90%-confidence error radius of about 0.74 arc sec. This position is 4.1 arc sec. from the center of the XRT error circle. The estimated magnitude is 15.29 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. Burst Advocate for this burst is J. K. Cannizzo (cannizzo AT milkyway.gsfc.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.)