TITLE: GCN CIRCULAR NUMBER: 27509 SUBJECT: GRB 200409A: Swift detection of a burst with an optical counterpart DATE: 20/04/09 03:30:47 GMT FROM: David Palmer at LANL J.D. Gropp (PSU), S. D. Barthelmy (GSFC), J. A. Kennea (PSU), N. J. Klingler (PSU), S. Laha (GSFC/UMBC/CRESST), A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), M. J. Moss (GWU), D. M. Palmer (LANL), M. H. Siegel (PSU) and A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift Observatory Team: At 03:19:59 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200409A (trigger=965484). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 60.553, -50.247, which is RA(J2000) = 04h 02m 13s Dec(J2000) = -50d 14' 49" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows multiple peaks with a total duration of about 20 sec. The peak count rate was ~2000 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 03:21:13.1 UT, 73.4 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 60.53820, -50.22509 which is equivalent to: RA(J2000) = 04h 02m 09.17s Dec(J2000) = -50d 13' 30.3" with an uncertainty of 3.7 arcseconds (radius, 90% containment). This location is 85 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. We cannot determine whether the source is fading at the present time. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (1.27 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 5 (+4.06/-3.42) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 77 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 04:02:08.75 = 60.53646 DEC(J2000) = -50:13:28.4 = -50.22455 with a 90%-confidence error radius of about 0.77 arc sec. This position is 4.6 arc sec. from the center of the XRT error circle. The estimated magnitude is 19.14 with a 1-sigma error of about 0.16. No correction has been made for the expected extinction corresponding to E(B-V) of 0.01. Burst Advocate for this burst is J.D. Gropp (jdg44 AT psu.edu). 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/)