TITLE: GCN CIRCULAR NUMBER: 29444 SUBJECT: GRB 210210A: Swift detection of a burst with an optical counterpart DATE: 21/02/10 02:27:51 GMT FROM: David Palmer at LANL S. Laha (GSFC/UMBC/CRESST), J.D. Gropp (PSU), H. A. Krimm (NSF), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL), B. Sbarufatti (PSU) and A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift Observatory Team: At 02:00:27 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210210A (trigger=1031728). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 262.811, +14.648 which is RA(J2000) = 17h 31m 15s Dec(J2000) = +14d 38' 53" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). Due to a telemetry gap, only the first 8 seconds of the BAT light curve after the trigger are available, showing that the emission has at least two peaks and continues beyond that time. The peak count rate was ~6000 counts/sec (15-350 keV), at ~4 sec after the trigger. The XRT began observing the field at 02:01:50.0 UT, 82.1 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 262.77059, 14.66385 which is equivalent to: RA(J2000) = 17h 31m 04.94s Dec(J2000) = +14d 39' 49.9" with an uncertainty of 3.5 arcseconds (radius, 90% containment). This location is 151 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 https://www.swift.ac.uk/sper. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (8.82 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 2.2 (+1.27/-1.18) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 85 seconds after the BAT trigger. There is a candidate afterglow in the list of sources generated on-board at RA(J2000) = 17:31:05.00 = 262.77085 DEC(J2000) = +14:39:45.8 = 14.66272 with a 90%-confidence error radius of about 1.10 arc sec. This position is 4.6 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.06. No correction has been made for the expected extinction corresponding to E(B-V) of 0.097. Burst Advocate for this burst is S. Laha (sib.laha AT gmail.com). 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/)