TITLE: GCN CIRCULAR NUMBER: 42343 SUBJECT: GRB 251018A: Swift detection of a burst with an optical counterpart DATE: 25/10/18 03:25:45 GMT FROM: David Palmer at LANL palmer@lanl.gov

R. Gupta (NASA GSFC), R. Brivio (INAF-OAB), S. Dichiara (PSU), M. Ferro (INAF-OAB), D. M. Palmer (LANL), T. Sbarrato (INAF-OAB) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team:

At 03:12:02 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 251018A (trigger=1405083). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 282.071, -37.028 which is RA(J2000) = 18h 48m 17s Dec(J2000) = -37d 01' 41" 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 60 sec. The peak count rate was ~2000 counts/sec (15-350 keV), at ~1 sec after the trigger.

The XRT began observing the field at 03:13:16.7 UT, 74.4 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 282.0433, -37.0265 which is equivalent to: RA(J2000) = 18h 48m 10.39s Dec(J2000) = -37d 01' 35.4" with an uncertainty of 6.3 arcseconds (radius, 90% containment). This location is 79 arcseconds from the BAT onboard position, within the BAT error circle. The initial flux in the 2.5 s image was 5.19e-10 erg cm^-2 s^-1 (0.2-10 keV).

UVOT took a finding chart exposure of 150 seconds with the White filter starting 83 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 18:48:10.04 = 282.04183 DEC(J2000) = -37:01:42.2 = -37.02840 with a 90%-confidence error radius of about 0.62 arc sec. This position is 8.0 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.96 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.100.

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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