TITLE: GCN CIRCULAR NUMBER: 27701 SUBJECT: GRB 200509A: Swift detection of a burst with a possible optical counterpart DATE: 20/05/09 07:08:31 GMT FROM: Kim Page at U.of Leicester E. Troja (NASA/GSFC/UMCP), V. D'Elia (SSDC), J.D. Gropp (PSU), N. J. Klingler (PSU), K. L. Page (U Leicester), B. Sbarufatti (PSU), M. H. Siegel (PSU) and A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift Observatory Team: At 06:47:17 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200509A (trigger=970886). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 116.450, -4.641 which is RA(J2000) = 07h 45m 48s Dec(J2000) = -04d 38' 28" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a multi-peaked structure with a duration of about 20 sec. There is also possible further activity for several hundred seconds The peak count rate was ~1400 counts/sec (15-350 keV), at ~4 sec after the trigger. The XRT began observing the field at 06:49:16.9 UT, 119.6 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 116.42333, -4.63131 which is equivalent to: RA(J2000) = 07h 45m 41.60s Dec(J2000) = -04d 37' 52.7" with an uncertainty of 3.5 arcseconds (radius, 90% containment). This location is 101 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 (1.58 x 10^21 cm^-2, Willingale et al. 2013), with an excess column of 5.2 (+4.12/-3.39) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of nominal 150.000 seconds with the White filter starting 123 seconds after the BAT trigger. There is a candidate afterglow in the list of sources generated on-board at RA(J2000) = 07:45:41.20 = 116.42166 DEC(J2000) = -04:37:49.4 = -4.63040 with a 90%-confidence error radius of about 1.10 arc sec. This position is 6.4 arc sec. from the center of the XRT error circle. The estimated magnitude is 19.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.13. The 2.7'x2.7' sub-image does not cover the region of this potential counterpart. Burst Advocate for this burst is E. Troja (eleonora.troja 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/)