TITLE: GCN CIRCULAR NUMBER: 40467 SUBJECT: GRB 250516A: Swift detection of a burst DATE: 25/05/16 14:57:08 GMT FROM: K.L. Page at U Leicester klp5@leicester.ac.uk
A. P. Beardmore (U Leicester), P. A. Evans (U Leicester), R. Gupta (NASA GSFC), K. L. Page (U Leicester), S. P. R. Shilling (Lancaster U.) and M. A. Williams (PSU) report on behalf of the Neil Gehrels Swift Observatory Team:
At 14:35:29 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250516A (trigger=1314210). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 334.561, -9.130 which is RA(J2000) = 22h 18m 15s Dec(J2000) = -09d 07' 48" 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 30 sec. The peak count rate was ~5000 counts/sec (15-350 keV), at ~5 sec after the trigger.
The XRT began observing the field at 14:37:13.2 UT, 104.3 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 334.60979, -9.11941 which is equivalent to: RA(J2000) = 22h 18m 26.35s Dec(J2000) = -09d 07' 09.9" with an uncertainty of 1.9 arcseconds (radius, 90% containment). This location is 177 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 (5.84 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 1.8 (+1.47/-1.34) x 10^21 cm^-2 (90% confidence).
UVOT took a finding chart exposure of 150 seconds with the White filter starting 108 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers none of the XRT error circle. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.062.
Burst Advocate for this burst is A. P. Beardmore (apb AT star.le.ac.uk). 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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