TITLE: GCN CIRCULAR NUMBER: 38418 SUBJECT: EP241201a: Kinder optical counterpart candidate DATE: 24/12/02 15:45:07 GMT FROM: Janet Chen at National Central University janetstars@gmail.com

M.-H. Lee (NCU), S. Yang (HNAS), A. Aryan, T.-W. Chen, Y. J. Yang, W.-J. Hou (all NCU), S. J. Smartt (Oxford/QUB), A. K. H. Kong (NTHU), J. Gillanders (Oxford), C.-C. Ngeow, A. Sankar.K, Y.-H. Li, 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 EP241201a (Chen et al., GCN 38415) 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 10:25 UTC on 2nd December 2024 (MJD 60646.434), 13.40 hr after the EP-WXT trigger.

We utilized the Kinder pipeline (Yang et al. A&A 646, A22) to stack the images and subtract them from the DESI Legacy Survey (Dey et al. 2019) DR10 template images. Although no definitive candidates were detected in the difference images, we conducted a manual inspection of both the target and difference images. During this process, we identified a plausible optical transient candidate at RA=18:50:38.079, Dec=+66:03:11.36, located approximately 2.28 arcminutes from the reported coordinates of EP241201a. Despite the low signal-to-noise ratio (SNR) of this candidate, we believe it to be real due to its well-defined Gaussian profile.

The details of the observations and measured PSF magnitude with template subtraction (in the AB system) of the possible counterpart of EP241201a are as follows: --------------------------------------------------------------------------------------------------Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass LOT | r | 60646.434 | 13.40 | 300 * 6 | 22.69 +/- 0.41 | 1".43 | 2.06

The presented magnitude was calibrated using the field stars from the Pan-STARRS1 catalog (Chambers et al., 2016 arXiv:1612.05560) and was not corrected for the expected Galactic foreground extinction corresponding to a reddening of A_r = 0.13 mag in the direction of the transient (Schlafly & Finkbeiner 2011).

We note an extended source in the Legacy Survey image 5".8 west and 3".8 south from the plausible optical transient candidate, at RA=18:50:37.128, Dec=+66:03:07.56, is possibly its host galaxy, with r-band magnitude as 20.47 mag and photo-z = 0.313 +/- 0.056, the distance module is then 40.91. Assuming the r-band can be roughly K-corrected to the g-band, with the Milky Way extinction correction, the optical counterpart candidate has the rest-frame M_g = -18.35 mag.

Further follow-up observations are encouraged to confirm this candidate, as it is beyond our 3-sigma detection limit of r > 21.6 mag.

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