TITLE: GCN CIRCULAR NUMBER: 39002 SUBJECT: EP250108a/AT2025kg: 1.3m DFOT Optical observations DATE: 25/01/20 16:38:19 GMT FROM: Amit Kumar Ror at ARIES mitturor77894@gmail.com

Amit K. Ror, Anshika Gupta, Kiran, Shashi B. Pandey, Kuntal Mishra (ARIES) report:

We observed the field of EP250108a detected by the Wide-field X-ray Telescope on board the Einstein Probe (Einstein Probe team, Li et al. 2025, GCN 38861), with the 1.3m Devasthal Fast Optical Telescope (DFOT), located at the Devasthal Observatory of the Aryabhatta Research Institute of Observational Sciences (ARIES), India. The observations were started on 2025-01-19 at 16:15:38 UT, i.e., ~ 11.16 days after the Einstein Probe trigger. We have taken multiple frames with an exposure time of 300 s in the R filter. We stacked the images after the alignment. We detected an optical emission in our stacked image at the position of the optical counterpart candidate EP250108a / AT 2025kg by Eyles-Ferris et al. (2025, GCN 38878). We obtain the following preliminary magnitude in the stacked image:

Date Start_UT T_start-T0 (days) Filter Exp time (s) Magnitude ================================================== 2025-01-19 16:15:38 ~11.16 R 300s*12 19.89 +/- 0.04

Our detection is consistent with Lipunov et al. 2025, GCN 38865; Eyles-Ferris et al. 2025, GCN 38878; Zhu et al. 2025, GCN 38885; Junjie-Jin et al. 2025, GCN 38891; Malesani et al. 2025, GCN 38902; Kumar et al. 2025, GCN 38907; Zhu et al. 2025, GCN 38908; Levan et al. 2025, GCN 38909; Izzo et al. 2025, GCN 38912; Zou et al. 2025, GCN 38914; Moskvitin et al. 2025, GCN 38925; Zhu et al. 2025, GCN 38937; Carotenuto et al. 2025, GCN 38958; Schroeder et al. 2025, GCN 38970; Song et al. 2025, GCN 38972; Eyles-Ferris et al. 2025, GCN 38983; Xu et al. 2025, GCN 38984; Levan et al. 2025, GCN 38987; and Tao et al. 2025, GCN 38998.

The magnitude is not corrected for the Galactic extinction in the direction of the burst. Photometric calibration is performed using the standard stars from the USNO-B1.0 catalogue. This circular may be cited.

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