TITLE: GCN CIRCULAR NUMBER: 9149 SUBJECT: GRB 090418: Swift detection of a burst with optical afterglow DATE: 09/04/18 11:35:24 GMT FROM: David Palmer at LANL V. Mangano (INAF-IASFPA), S. D. Barthelmy (GSFC), D. N. Burrows (PSU), P.A. Curran (MSSL-UCL), N. Gehrels (NASA/GSFC), S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU), J. Mao (INAF-OAB), P. T. O'Brien (U Leicester), D. M. Palmer (LANL), A. M. Parsons (GSFC), M. Perri (ASDC), B. A. Rowlinson (U Leicester), B. Sbarufatti (INAF-IASFPA), G. Tagliaferri (INAF-OAB), T. N. Ukwatta (GSFC/GWU), L. Vetere (PSU) and H. Ziaeepour (UCL-MSSL) report on behalf of the Swift Team: At 11:07:40 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 090418 (trigger=349510). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 269.334, +33.409 which is RA(J2000) = 17h 57m 20s Dec(J2000) = +33d 24' 31" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows two main peaks with a duration of about 70 sec. The peak count rate was ~2000 counts/sec (15-350 keV), at ~45 sec after the trigger. The XRT began observing the field at 11:09:16.3 UT, 96.1 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 269.3129, 33.4055 which is equivalent to: RA(J2000) = 17h 57m 15.09s Dec(J2000) = +33d 24' 19.9" with an uncertainty of 2.1 arcseconds (radius, 90% containment). This location is 64 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 3.55e+20 cm^-2 (Kalberla et al. 2005). The initial flux in the 2.5 s image was 1.05e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 250 seconds with the u filter starting 859 seconds after the BAT trigger. An afterglow candidate has been found in the initial data products at RA(J2000) = 17:57:15.1 = 269.31458 Dec(J2000) = +33:24:21 = +33.40583 with an estimated uncertainty of 1.0 arcsec (radius, 90% confidence, statistical + systematic). This is 1.1 arcsec from the XRT position. The estimated magnitude is u = 17.3 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.04. Burst Advocate for this burst is V. Mangano (vanessa AT ifc.inaf.it). 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.)