TITLE: GCN CIRCULAR NUMBER: 41392 SUBJECT: GRB 250814A: detection of the afterglow candidate with Liverpool Telescope DATE: 25/08/15 13:59:44 GMT FROM: Amit Kumar at Royal Holloway - UoL/ U of Warwick, UK amitkundu515@gmail.com

A. Kumar, J. R. Maund (RHUL), N. C. Sun (UCAS), W. X. Li, Y. N. Wang (NAOC), and K. Wiersema (Herts) report:

We observed the field of the Fermi/GBM and Swift/BAT GRB 250814A (Fermi GBM Team, GCN 41357; Caputo et al., GCN 41358), which is spatially coincident with the sub-threshold GW trigger S250814bg (LVK, GCN 41364), using the IO:O imager on the 2.0m Liverpool Telescope. Multi-band (B, g, V, r, i, and z bands) imaging was obtained between 2025-08-14 23:34:48.3 UT and 2025-08-15 01:29:02.2 UT (corresponding to ~15.99 and 17.90 hours post-trigger, respectively).

Based on our preliminary analysis, we detect the counterpart candidate in our stacked z-band image obtained ~16.86 hours post-trigger at RA = 20:25:13.8 and Dec = +48:23:56.5 (J2000), within the Swift-XRT localisation region (Salvaggio et al., GCN 41379); see also Karambelkar et al. (GCN 41377), Castro-Tirado et al. (GCN 41381), Ahumada et al. (GCN 41382) and Source 1 of D'Avanzo et al. (GCN 41391). No source is detected at this position in the B, g, V, r, and i bands. A summary of our observations and preliminary photometric results is provided below.

T_obs (start time) T_obs - T0 Exp Filter Mag -------------------------------------------------------------------------------------------- 2025-08-15T00:26:48.9 16.86 2x600 s z 21.3 ± 0.3

2025-08-14T23:34:48.3 15.99 600 s g >21.9 2025-08-14T23:45:18.2 16.16 2x600 s r >22.1 2025-08-15T00:06:03.3 16.51 2x600 s i >21.8 2025-08-15T00:47:35.3 17.21 2x600 s V >22.2 2025-08-15T01:08:24.4 17.55 3x600 s B >20.5

Photometric calibration was performed using stars from the Pan-STARRS (for g, r, i and z bands), USNO-B1 (for B band), and APASS (for V band) catalogues. The quoted magnitudes are not corrected for Galactic extinction in the direction of the GRB.

This circular may be cited.

View this GCN Circular online at https://gcn.nasa.gov/circulars/41392. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...