TITLE: GCN CIRCULAR NUMBER: 39565 SUBJECT: EP250302a: Mondy Observations and Optical Light Curve DATE: 25/03/03 09:54:00 GMT FROM: Nicolai Pankov at HSE, IKI RAS colinsergesen@gmail.com

N. Pankov (HSE, IKI), A. Pozanenko (IKI), A. Moskvitin (SAO RAS), E. Klunko (ISTP) report on behalf of IKI-GRB-FuN:

We performed optical observations of the field of a fast X-Ray transient EP250302a (Zhu et. al, GCN 39550; Busmann et. al, GCN 39551; Leonini et. al, GCN 39553; Wu et. al, GCN 39555; Dai et. al, GCN 39556; Page et. al, GCN 39557; Xin et. al, GCN 39558; Lipunov et. al, GCN 39559; Adami et. al, GCN 39560; Yang et al., GCN 39561; Becerra et al., GCN 39562; Izzo et al., GCN 39564) in the R filter with the AZT-33IK 1.5-meter telescope of Mondy observatory. The observations began on 2025-03-02 (UT) 15:50:41.578, i.e. 0.01083 days since trigger. The optical counterpart (firstly reported by Zhu et. al, GCN 39550) is clearly detected in each image of an exposure of 120 sec. The optical light curve displays variable behavior. Initially, the light curve fades until ~1.5e-2 days since trigger, then it is brightening until it reaches peak magnitude of R = 17.26 +/- 0.03 mag at 2025-03-02 (UT) 16:14:42 (0.02751 days since trigger). The further brightness evolution is a decay by the times indicated in Adami et. al, GCN 39560. Our photometry is based on nearby stars from the USNO-B1.0 catalog (see the list below) and has not been corrected for the Galactic extinction.

Ref. stars RA Dec R2 11:18:11.14 +33:32:45.34 15.74 11:17:47.64 +33:30:50.19 17.69

The light curve image is available at: http://grb.rssi.ru/EP250302a/EP250302a_LC.jpg

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