TITLE: GCN CIRCULAR NUMBER: 9198 SUBJECT: GRB 090423: Swift detection of a burst DATE: 09/04/23 08:14:38 GMT FROM: Hans Krimm at NASA-GSFC H. A. Krimm (CRESST/GSFC/USRA), A. P. Beardmore (U Leicester), P. A. Evans (U Leicester), O. Godet (U Leicester), C. Gronwall (PSU), C. Guidorzi (U Ferrara), P. T. O'Brien (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL), M. Perri (ASDC), B. Sbarufatti (INAF-IASFPA), P. Schady (MSSL-UCL), G. Stratta (ASDC), G. Tagliaferri (INAF-OAB) and T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team: At 07:55:19 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 090423 (trigger=350184). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 148.895, +18.160 which is RA(J2000) = 09h 55m 35s Dec(J2000) = +18d 09' 37" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a double-peaked structure with a duration of about 20 sec. The peak count rate was ~2000 counts/sec (15-350 keV), at ~5 sec after the trigger. The XRT began observing the field at 07:56:31.8 UT, 72.5 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 148.8879, 18.1497 which is equivalent to: RA(J2000) = 09h 55m 33.08s Dec(J2000) = +18d 08' 58.9" with an uncertainty of 2.3 arcseconds (radius, 90% containment). This location is 44 arcseconds from the BAT onboard position, within the BAT error circle. This position may be improved as more data are received; the latest position is available at http://www.swift.ac.uk/sper. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 2.89e+20 cm^-2 (Kalberla et al. 2005). UVOT took a finding chart exposure of 150 seconds with the White filter starting 77 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 25% of the BAT error circle. The typical 3-sigma upper limit has been about 19.6 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the BAT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.03. Burst Advocate for this burst is H. A. Krimm (krimm 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.)