TITLE: GCN CIRCULAR NUMBER: 41403 SUBJECT: GRB 250818A: Swift detection of a burst DATE: 25/08/18 01:48:57 GMT FROM: K.L. Page at U Leicester klp5@leicester.ac.uk

S. B. Cenko (GSFC), C. Gronwall (PSU), R. Gupta (NASA GSFC), H. A. Krimm (NSF), A. Y. Lien (U Tampa), K. L. Page (U Leicester), D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team:

At 01:31:22 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250818A (trigger=1343270). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 213.833, -58.007 which is RA(J2000) = 14h 15m 20s Dec(J2000) = -58d 00' 26" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). Due to a communications gap, only the BAT light curve after T+8s is available. This later lightcurve shows a complex structure out to at least T+150s. The peak count rate in the available lightcurve was ~5500 counts/sec (15-350 keV), at ~80 sec after the trigger.

The XRT began observing the field at 01:33:00.6 UT, 97.9 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 213.8002, -58.0134 which is equivalent to: RA(J2000) = 14h 15m 12.05s Dec(J2000) = -58d 00' 48.2" with an uncertainty of 4.9 arcseconds (radius, 90% containment). This location is 66 arcseconds from the BAT onboard position, within the BAT error circle. Despite the onboard localisation, no X-ray source was detected in 45 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 0.1 s image was 1.09e-07 erg cm^-2 s^-1 (0.2-10 keV).

UVOT took a finding chart exposure of 250 seconds with the U filter starting 162 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. Because of the density of catalogued stars, further analysis is required to report an upper limit for any afterglow in the region. No correction has been made for the large, but uncertain, extinction expected.

Burst Advocate for this burst is S. B. Cenko (brad.cenko AT nasa.gov). 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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