TITLE: GCN CIRCULAR NUMBER: 5823 SUBJECT: GRB 061121: Swift detection of a bright burst with an optical counterpart DATE: 06/11/21 15:39:45 GMT FROM: Scott Barthelmy at NASA/GSFC K. L. Page (U Leicester), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester), P. J. Brown (PSU), D. N. Burrows (PSU), P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC), C. Guidorzi (Univ Bicocca&INAF-OAB), J. A. Kennea (PSU), H. A. Krimm (GSFC/USRA), W. L. Landsman (NASA/GSFC), V. Mangano (INAF-IASFPA), F. E. Marshall (NASA/GSFC), J. P. Osborne (U Leicester), C. Pagani (PSU), D. M. Palmer (LANL), M. Perri (ASDC), P. Romano (Univ. Bicocca & INAF-OAB), T. Sakamoto (NASA/ORAU), M. Stamatikos (NASA/ORAU), R. L. C. Starling (U Leicester), E. Troja (INAF-IASFPA), D. E. Vanden Berk (PSU) and H. Ziaeepour (UCL-MSSL) report on behalf of the Swift Team: At 15:22:29 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 061121 (trigger=239899). Swift slewed immediately to the burst. The BAT on-board calculated location is RA,Dec 147.252, -13.183 {09h 49m 00s, -13d 10' 57"} (J2000) with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows an initial pulse of ~3000 cnts/sec lasting ~10 sec. Then a second much brighter peak starts at ~T+50 sec, peaking at T+70 sec (at 32,000 cnts/sec), and ending at ~T+80 sec. The XRT began observing the field at 15:23:24 UT, 55 seconds after the BAT trigger. XRT found a very bright, fading, uncatalogued X-ray source, located at RA(J2000) = 09h 48m 54.5s, Dec(J2000) = -13d 11' 46.4" with an estimated uncertainty of 4.7 arcseconds (90% confidence radius). This is a ground calculated position based on prompt downlinked data. This location is 102 arcseconds from the BAT on-board position, within the BAT error circle. The initial flux in the 2.5s image was 1.8e-08 erg/cm2/s (0.2-10 keV). There is a large flare peaking at about 70 s after the BAT trigger. UVOT took a finding chart exposure of 526 seconds with the V filter starting 167 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 09:48:54.58 = 147.2274 DEC(J2000) = -13:11:42.7 = -13.1952 with a 1-sigma error radius of about 0.5 arc sec. This position is 5.0 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.2 with a 1-sigma error of about 0.5 mag. No correction has been made for the expected extinction of about 0.1 magnitudes. We note that the BAT triggered on a precursor allowing the XRT and UVOT to observe during the main emission of the burst.