TITLE: GCN CIRCULAR NUMBER: 41465 SUBJECT: GRB 250814A: CMO SAI and SAO RAS izJ observations. DATE: 25/08/21 13:09:03 GMT FROM: Alina Volnova at IKI RAS alinusss@gmail.com

A. Volnova (IKI), A. Moskvitin (SAO RAS), A. Pozanenko (IKI), N. Pankov (HSE, IKI), P. Minaev (IKI), S. Kotov (SAO RAS), A.M. Tatartnikov (SAI MSU), V.K. Postnikova (SAI MSU) report on behalf of IKI-GRB-FuN:

We observed the field of GRB 250814A detected by Swift/BAT and Fermi/GBM (Caputo et al. GCN Circ. 41358; Fermi team, GCN Circ. 41357) and possibly associated with the sub-threshold GW trigger S250814bg (LVK, GCN Circ. 41364) with 2.5-meter SAI-25 telescope of the Caucasian Mountain Observatory of Sternberg Astronomical Institute (SAI CMO), equipped with the IR camera ASTRONIRCAM. The observations in the J-filter were carried out on 2025-08-16 starting UT 23:20:51. We also observed this field with the BTA 6-m telescope of SAO RAS, equipped with the Scorpio-2 multi-mode focal reducer (Afanasiev & Moiseev, 2011, BaltA, 20, 363), and obtained series of images in SDSS i,z bands on 2025-08-17 starting 20:47:31 UT.

The two NIR sources reported previously (Karambelkar et al. GCN Circ. 41377; Castro-Tirado et al., GCN Circ. 41381; Ahumada et al. GCN Circ. 41382; Pankov et al. GRB Circ. 41389; D'Avanzo et al. GCB Circ. 41391; Kumar et al. GCN Circ. 41392; D'Avanzo et al. GCN Circ. 41408) are clearly detected. The preliminary photometry of the sources Source 1 and Source 2 according to D'Avanzo et al. (GCB Circ. 41391) is:

date |UT start|t-T0,d |Filter|Exp,s|Source 1 |Source 2 |UL3sigma|FWHM"|Telescope/Site ========================================================================================= 25-08-16|23:20:51|2.68958|J |4224 |21.72(0.07)|21.96(0.08)|22.9 |1.2 |SAI-25/CMO 25-08-17|20:47:31|3.56255|z |1200 |22.39(0.04)|22.46(0.04)|24.2 |1.4 |BTA/SAO 25-08-17|21:25:21|3.58647|i |1200 |23.39(0.05)|23.31(0.05)|24.6 |1.4 |BTA/SAO

All magnitudes are in AB system and not corrected for the Galactic extinction. The photometry is calibrated using Pan-STARRS i,z magnitudes and 2MASS J magnitudes for the following stars:

RA Dec i_PS1 z_PS1 J_2MASS* 20:25:12.01 +48:24:08.4 16.924(0.004) 16.703(0.008) 15.750 20:25:17.82 +48:24:04.6 17.234(0.003) 16.952(0.005) 15.627 * - J magnitudes of the reference stars are in Vega system.

(We are grateful to the management of the observatories for observational time and the staff for excellent observations.)

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