TITLE: GCN CIRCULAR NUMBER: 10612 SUBJECT: GRB 100418A: Swift detection of a burst with a possible optical afterglow DATE: 10/04/18 21:27:58 GMT FROM: David Palmer at LANL F. E. Marshall (NASA/GSFC), A. P. Beardmore (U Leicester), J. M. Gelbord (PSU), S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU), J. A. Kennea (PSU), O. M. Littlejohns (U Leicester), C. B. Markwardt (CRESST/GSFC/UMD), P. T. O'Brien (U Leicester), C. Pagani (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL), A. Rowlinson (U Leicester), M. C. Stroh (PSU), T. N. Ukwatta (GSFC/GWU) and L. Vetere (PSU) report on behalf of the Swift Team: At 21:10:08 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 100418A (trigger=419797). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 256.376, +11.464 which is RA(J2000) = 17h 05m 30s Dec(J2000) = +11d 27' 50" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single-peaked structure with a duration of about 4 sec. The peak count rate was ~700 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 21:11:27.1 UT, 79.1 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 256.3635, 11.4619 which is equivalent to: RA(J2000) = 17h 05m 27.24s Dec(J2000) = +11d 27' 42.7" with an uncertainty of 3.1 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 4.78e+20 cm^-2 (Kalberla et al. 2005). The initial flux in the 2.5 s image was 1.27e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 87 seconds after the BAT trigger. A possible afterglow candidate has been found in the initial data products at the location of the XRT afterglow. The preliminary UVOT position is RA(J2000.0) = 17:05:26.96 = 256.36233 (deg) Dec(J2000.0) = +11:27:41.9 = +11.46164 (deg) with an estimated uncertainty of 1.0 arcsec (radius, 90% confidence, statistical + systematic). The estimated white magnitude is 20.7 +/- 0.3. No correction has been made for the expected extinction corresponding to E(B-V) of 0.07. Burst Advocate for this burst is F. E. Marshall (marshall 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.)