TITLE: GCN CIRCULAR NUMBER: 38433 SUBJECT: EP241202b: Kinder optical counterpart candidate DATE: 24/12/03 17:35:31 GMT FROM: Janet Chen at National Central University janetstars@gmail.com

C.-C. Ngeow (NCU), S. Yang (HNAS), A. Aryan, T.-W. Chen, Y. J. Yang, Y.-C. Cheng, M.-H. Lee, W.-J. Hou (all NCU), A. K. H. Kong (NTHU), S. J. Smartt (Oxford/QUB), J. Gillanders (Oxford), A. Sankar.K, Y.-H. Lee, H.-Y. Miao, Y.-C. Pan, C.-H. Lai, H.-C. Lin, H.-Y. Hsiao, C.-S. Lin, J.-K. Guo (all NCU), Z. N. Wang, L. L. Fan, G. H. Sun (all HNAS), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, T. Moore, K. W. Smith, C. Angus, A. Aamer (all QUB), A. Schultz and M. Huber (both IfA, Hawaii) report:

We observed the field of the fast X-ray transient EP241202b (Zhou et al., GCN 38426) using the 1m LOT at the Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2024 arXiv:2406.09270). The first LOT epoch of observations started at 10:40 UTC on 3rd December 2024 (MJD 60647.444), 19.45 hr after the EP-WXT trigger.

We utilized the Kinder pipeline (Yang et al. A&A 646, A22) to stack the images and visually identified a faint, plausible optical transient candidate at RA = 03:01:20.862, Dec = +02:26:27.04, approximately 2.11 arcminutes from the reported coordinates of EP241202b. To confirm this, we subtracted the stacked image from the DESI Legacy Survey (Dey et al. 2019) DR10 template images. The resulting difference image also revealed the trace of a tenuous source characterized by a well-defined Gaussian profile.

The details of the observations and measured aperture magnitude without template subtraction (in the AB system) of the possible counterpart of EP241202b are as follows: -------------------------------------------------------------------------------------------------Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass LOT | r | 60647.444 | 19.45 | 300 * 6 | 22.41 +/- 0.16 | 1".51 | 1.56

Following the detection of the plausible optical counterpart candidate, we began another set of observations about 1.47 hr after the first set of observations. The optical counterpart candidate disappeared in the second set. The details of observations and measured 3-sigma upper limit for the second epoch of observations are as follows: -------------------------------------------------------------------------------------------------Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass LOT | r | 60647.524 | 21.36 | 300 * 6 | > 22.9 | 1".02 | 1.13

The plausible optical counterpart candidate has faded by 0.5 magnitudes over 1.9 hours; however, this variation could be attributed to statistical fluctuations, given the source's faintness.

The presented magnitudes were calibrated using the field stars from the Pan-STARRS1 catalog (Chambers et al., 2016 arXiv:1612.05560) and were not corrected for the expected Galactic foreground extinction corresponding to a reddening of A_r = 0.21 mag in the direction of the transient (Schlafly & Finkbeiner 2011).

Further follow-up observations are encouraged to confirm this candidate.

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