TITLE: GCN CIRCULAR NUMBER: 39182 SUBJECT: GRB 250207A: Swift detection of a burst with optical counterpart DATE: 25/02/07 01:34:53 GMT FROM: Jamie Kennea at Penn State jak51@psu.edu

M. Ferro (INAF-OAB), R. Gupta (NASA GSFC), J. A. Kennea (PSU), K. L. Page (U Leicester) and B. Sbarufatti (INAF-OAB) report on behalf of the Neil Gehrels Swift Observatory Team:

At 01:16:07 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250207A (trigger=1287821). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 16.115, -12.159 which is RA(J2000) = 01h 04m 28s Dec(J2000) = -12d 09' 30" 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 ~3847 counts/sec (15-350 keV), at ~6 sec after the trigger.

The XRT began observing the field at 01:17:53.2 UT, 105.9 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 16.0892, -12.1633 which is equivalent to: RA(J2000) = 01h 04m 21.41s Dec(J2000) = -12d 09' 47.9" with an uncertainty of 5.9 arcseconds (radius, 90% containment). This location is 92 arcseconds from the BAT onboard position, within the BAT error circle. Despite the onboard localisation, no X-ray source was detected in 85 s of promptly-downlinked data, suggesting that the initial centroid may equally have been a cosmic ray. This position should therefore be treated with caution.

The initial flux in the 2.5 s image was 7.54e-10 erg cm^-2 s^-1 (0.2-10 keV).

UVOT took a finding chart exposure of 150 seconds with the White filter starting 113 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 01:04:21.41 = 16.08919 DEC(J2000) = -12:09:52.5 = -12.16458 with a 90%-confidence error radius of about 0.74 arc sec. This position is 4.6 arc sec. from the center of the XRT error circle. The estimated magnitude is 15.04 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.027.

Burst Advocate for this burst is M. Ferro (matteo.ferro AT inaf.it). 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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