TITLE: GCN CIRCULAR NUMBER: 11087 SUBJECT: GRB 100814A: Swift detection of a burst with an optical afterglow DATE: 10/08/14 04:22:40 GMT FROM: David Palmer at LANL A. P. Beardmore (U Leicester), J. R. Cummings (NASA/UMBC), N. Gehrels (NASA/GSFC), J. M. Gelbord (PSU), N. P. M. Kuin (UCL-MSSL), O. M. Littlejohns (U Leicester), C. B. Markwardt (NASA/GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL), C. J. Saxton (UCL-MSSL), R. L. C. Starling (U Leicester), E. Troja (NASA/GSFC/ORAU) and T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team: At 03:50:11 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 100814A (trigger=431605). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 22.481, -17.986 which is RA(J2000) = 01h 29m 55s Dec(J2000) = -17d 59' 07" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed three separate peaks over a total interval of at least 150 sec. The peak count rate was ~6000 counts/sec (15-350 keV), at ~6 sec after the trigger. The XRT began observing the field at 03:51:38.6 UT, 87.4 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source located at RA, Dec 22.47220, -17.99643 which is equivalent to: RA(J2000) = 01h 29m 53.33s Dec(J2000) = -17d 59' 47.1" with an uncertainty of 3.8 arcseconds (radius, 90% containment). This location is 48 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 1.75e+20 cm^-2 (Kalberla et al. 2005). The initial flux in the 2.5 s image was 6.18e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 250 seconds with the U filter starting 153 seconds after the BAT trigger. A bright optical afterglow candidate was found at RA=22.4749, dec=-17.9932 which is RA = 01h 29m 53.98s Dec=-17d 59m 35.5s (J2000). The estimated magnitude is U=16.8 . Burst Advocate for this burst is C. J. Saxton (cjs2 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.)