TITLE: GCN CIRCULAR NUMBER: 40760 SUBJECT: GRB 250617B: Swift detection of a burst with an optical counterpart DATE: 25/06/17 21:24:27 GMT FROM: David Palmer at LANL palmer@lanl.gov

K. L. Page (U Leicester), R. Gupta (NASA GSFC), A. Y. Lien (U Tampa), M. J. Moss (GSFC), D. M. Palmer (LANL), T. Sakamoto (AGU), M. H. Siegel (PSU) and M. A. Williams (PSU) report on behalf of the Neil Gehrels Swift Observatory Team:

At 21:01:50.11 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250617B (trigger=1325580). Swift did not slew immediately to the burst. The BAT on-board calculated location is RA, Dec 332.902, +32.727 which is RA(J2000) = 22h 11m 37s Dec(J2000) = +32d 43' 38" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The currently available BAT light curve showed a complex structure with a duration of at least 10 sec. However, the light curve data from ~T+8 s to ~T+100 s is unavailable in the immediate data downlink. The peak count rate was ~900 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 21:15:03.4 UT, 793.4 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source with an enhanced position: RA, Dec 332.89735, 32.73270 which is equivalent to: RA(J2000) = 22h 11m 35.36s Dec(J2000) = +32d 43' 57.7" with an uncertainty of 1.9 arcseconds (radius, 90% containment). This location is 24 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 (9.66 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 5.7 (+4.15/-3.46) x 10^21 cm^-2 (90% confidence).

UVOT took a finding chart exposure of 150 seconds with the White filter starting 796 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 22:11:35.25 = 332.89686 DEC(J2000) = +32:43:56.9 = 32.73247 with a 90%-confidence error radius of about 0.62 arc sec. This position is 1.9 arc sec. from the center of the XRT error circle. The estimated magnitude is 18.17 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.087.

Burst Advocate for this burst is K. L. Page (klp5 AT leicester.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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