TITLE: GCN CIRCULAR NUMBER: 41358 SUBJECT: GRB 250814A: Swift detection of a burst coincident with a subthreshold GW trigger DATE: 25/08/14 07:58:28 GMT FROM: K.L. Page at U Leicester klp5@leicester.ac.uk

R. Caputo (GSFC), A. P. Beardmore (U Leicester), P. A. Evans (U Leicester), R. A. J. Eyles-Ferris (U Leicester), C. Gronwall (PSU), A. Y. Lien (U Tampa) and K. L. Page (U Leicester) report on behalf of the Neil Gehrels Swift Observatory Team:

At 07:34:59 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250814A (trigger=1342439). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 306.329, +48.391 which is RA(J2000) = 20h 25m 19s Dec(J2000) = +48d 23' 27" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure structure with a duration of about 20 sec. The peak at ~T+26 s in the 15-25 keV band raw light curve is likely due to detector noise, though further ground data is required to confirm this. The peak count rate was ~500 counts/sec (15-350 keV), at ~10 sec after the trigger.

The XRT began observing the field at 07:37:54.1 UT, 174.3 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 306.30802, 48.39903 which is equivalent to: RA(J2000) = 20h 25m 13.92s Dec(J2000) = +48d 23' 56.5" with an uncertainty of 2.2 arcseconds (radius, 90% containment). This location is 57 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 consistent with the Galactic value of 6.51 x 10^21 cm^-2 (Willingale et al. 2013).

The initial flux in the 2.5 s image was 7.47e-10 erg cm^-2 s^-1 (0.2-10 keV).

UVOT took a finding chart exposure of 250 seconds with the U filter starting 179 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 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.2 mag. 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 18.0 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 1.066.

This GRB is temporally and spatially coincident with the subthreshold GW trigger S250814bg which occurred 6 s after the BAT trigger. The XRT position is towards the edge of the GW localisation. Given the low significance of the GW event (a false alarm rate of ~10/day) this is probably a chance coincidence; however, follow up of this GRB is advised.

Burst Advocate for this burst is R. Caputo (regina.caputo 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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