TITLE: GCN CIRCULAR NUMBER: 42322 SUBJECT: GRB 251017A: Swift detection of a burst with an optical counterpart DATE: 25/10/17 08:54:42 GMT FROM: K.L. Page at U Leicester klp5@leicester.ac.uk
R. Gupta (NASA GSFC), R. Brivio (INAF-OAB), S. Dichiara (PSU), R. A. J. Eyles-Ferris (U Leicester), A. Mei (INAF-OAB), K. L. Page (U Leicester), C. Salvaggio (INAF-OAB) and T. Sbarrato (INAF-OAB) report on behalf of the Neil Gehrels Swift Observatory Team:
At 08:40:44 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 251017A (trigger=1404904). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 335.386, +7.051 which is RA(J2000) = 22h 21m 33s Dec(J2000) = +07d 03' 02" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with a duration of about 25 sec. The peak count rate was ~2136 counts/sec (15-350 keV), at ~6 sec after the trigger.
The XRT began observing the field at 08:42:34.1 UT, 109.5 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 335.40109, 7.04043 which is equivalent to: RA(J2000) = 22h 21m 36.26s Dec(J2000) = +07d 02' 25.5" with an uncertainty of 4.0 arcseconds (radius, 90% containment). This location is 65 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 (8.90 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 6.7 (+3.92/-3.27) x 10^21 cm^-2 (90% confidence).
The initial flux in the 2.5 s image was 2.03e-09 erg cm^-2 s^-1 (0.2-10 keV).
UVOT took a finding chart exposure of 150 seconds with the White filter starting 120 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 22:21:36.24 = 335.40098 DEC(J2000) = +07:02:23.6 = 7.03990 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 20.42 with a 1-sigma error of about 0.21. No correction has been made for the expected extinction corresponding to E(B-V) of 0.108.
Burst Advocate for this burst is R. Gupta (rahulbhu.c157 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/)
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