TITLE: GCN CIRCULAR NUMBER: 28730 SUBJECT: GRB 201021C: Swift detection of a burst with an optical counterpart DATE: 20/10/21 20:40:16 GMT FROM: Amy Lien at GSFC A. Y. Lien (GSFC/UMBC), M. G. Bernardini (INAF-OAB), D. N. Burrows (PSU), P. D'Avanzo (INAF-OAB), J.D. Gropp (PSU), J. A. Kennea (PSU), N. J. Klingler (PSU), H. A. Krimm (NSF), S. Laha (GSFC/UMBC/CRESST), F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB), M. J. Moss (GWU), T. Sakamoto (AGU), T. Sbarrato (INAF-OAB) and B. Sbarufatti (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 20:27:18 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 201021C (trigger=1001130). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 12.550, -55.839 which is RA(J2000) = 00h 50m 12s Dec(J2000) = -55d 50' 20" 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 25 sec. The peak count rate was ~1200 counts/sec (15-350 keV), at ~2 sec after the trigger. The XRT began observing the field at 20:29:00.6 UT, 102.0 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source located at RA, Dec 12.53057, -55.86729 which is equivalent to: RA(J2000) = 00h 50m 07.34s Dec(J2000) = -55d 52' 02.2" with an uncertainty of 3.7 arcseconds (radius, 90% containment). This location is 109 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 consistent with the Galactic value of 2.05 x 10^20 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 105 seconds after the BAT trigger. There is a candidate afterglow in the list of sources generated on-board at RA(J2000) = 00:50:06.85 = 12.52855 DEC(J2000) = -55:51:56.7 = -55.86576 with a 90%-confidence error radius of about 1.10 arc sec. This position is 2.8 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.19. No correction has been made for the expected extinction corresponding to E(B-V) of 0.02. Burst Advocate for this burst is A. Y. Lien (amy.y.lien AT 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/)