TITLE: GCN CIRCULAR NUMBER: 9066 SUBJECT: GRB 090401B: Swift detection of a burst DATE: 09/04/01 09:02:29 GMT FROM: David Palmer at LANL P. Schady (MSSL-UCL), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester), O. Godet (U Leicester), H. A. Krimm (CRESST/GSFC/USRA), J. Mao (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), A. M. Parsons (GSFC), P. Romano (INAF-IASFPA), B. A. Rowlinson (U Leicester) and H. Ziaeepour (UCL-MSSL) report on behalf of the Swift Team: At 08:35:25 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 090401B (trigger=348152). The BAT on-board calculated location is RA, Dec 95.101d, -8.963d which is RA(J2000) = 06h 20m 24s Dec(J2000) = -8d 57' 47" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows at least four overlapping peaks with a total duration of ~20 seconds. The peak count rate was ~55,000 counts/sec (15-350 keV), at ~8 sec after the trigger. The XRT began observing the field at 08:36:37.9 UT, 73.2 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 95.0889, -8.9734 which is equivalent to: RA(J2000) = 06h 20m 21.33s Dec(J2000) = -08d 58' 24.2" with an uncertainty of 4.7 arcseconds (radius, 90% containment). This location is 57 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 81 seconds after the BAT trigger. No credible afterglow candidate has been found in the automatic analysis. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag, although we note that there are two bright known sources within the XRT error circle that may be confusing source detection. One of these sources appears to be brighter and slightly offset from the DSS position, suggesting the possible detection of the optical afterglow. However, we shall need more data to check for any variability. The coverage of the XRT error circle by the 8'x8' region for the list of sources generated on-board is uncertain because the large number of sources filled the available telemetry. No correction has been made for the expected extinction corresponding to E(B-V) of 0.67. Burst Advocate for this burst is P. Schady (ps AT mssl.ucl.ac.uk). 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.)