TITLE:   GCN CIRCULAR
NUMBER:  38061
SUBJECT: EP241101a: Optical upper limits with Kinder observations
DATE:    24/11/03 15:29:37 GMT
FROM:    Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com>

A. Aryan, T.-W. Chen, W.-J. Hou (all NCU), A. K. H. Kong (NTHU), J. Gillanders (Oxford), S. J. Smartt (Oxford/QUB), Y. J. Yang, A. Sankar. K, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, H.-C. Lin, C.-H. Lai, H.-Y. Hsiao, C.-S. Lin, J.-K. Guo (all NCU), S. Yang, 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 EP241101a (Liang et al., GCN 38039; Perez-Garcia et al., GCN 38047; Lipunov et al., GCN 38049; Adami et al., GCN 38060) using the 1m LOT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2024arXiv240609270C). The first LOT epoch of observations started at 16:59 UTC on the 2nd of November 2024 (MJD = 60616.707), 0.71d after the EP WXT trigger. 

We utilized the astroalign (Beroiz et al. 2020, A&C, 32, 100384) and astropy (Astropy Collaboration et al. 2022, ApJ, 935, 167) packages to align and stack the individual frames. We do not find any evidence of a new and uncataloged source in the stacked images within the 2.8 arcminute error circle of EP-WXT localization. 

Moreover, we utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform the PSF photometry on our stacked frames. The details of the observations and measured 3-sigma upper limits  (in the AB system) are as follows:

Telescope | Filter | MJD (start) | t-t0 (d) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass
LOT | r | 60616.707 | 0.71  | 300  * 6 | >22.9| 1".47 | 1.08
LOT | g | 60616.730 | 0.73  | 300 * 6 | >23.0| 1".39 | 1.16

We notice that the probable optical counterpart candidate identified in the MISTRAL images (Adami et al., GCN 38060) is already present in the SDSS DR12 (SDSS J023053.10+224348.0) images, as well as in the DESI Legacy Survey images. 

The presented magnitudes are calibrated using the field stars from the SDSS catalog and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E(B-V) = 0.17 mag in the direction of the transient (Schlafly & Finkbeiner 2011).