TITLE: GCN CIRCULAR NUMBER: 39400 SUBJECT: EP250220a: Optical upper limit with Kinder observations DATE: 25/02/21 08:46:24 GMT FROM: Janet Chen at National Central University janetstars@gmail.com
L. L. Fan (HNAS), A. Aryan (NCU), A. K. H. Kong (NTHU), T.-W. Chen, W.-J. Hou (both NCU), 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, 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 EP250220a (Wang et al., GCN 39387) using the 1m LOT at 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 11:23:50 UTC on the 20th of February 2025 (MJD = 60726.475), about 9.20 hr 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 2.81 arcminute error circle of EP-WXT localization.
Moreover, we employed the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction using the "sfft" (Hu, 2022, ApJ, 936, 157) and "hotpants" (Becker A., 2015, ascl.soft. ascl:1504.004) algorithms using a template from Pan-STARRS1 (Chambers et al. 2016, arXiv:1612.05560). We found no evidence of any prominent candidate optical counterpart in the difference image as well.
We further employed AutoPhOT to perform the PSF photometry. The details of the observations and measured 3-sigma upper limit (in the AB system) are as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass LOT | r | 60726.475 | 9.20 | 300 * 6 | > 22.65| 1".12 | 1.13
The non-detection of any optical counterpart in our observations is consistent with Xinglong observations, as reported by Zhang et al. (GCN 39393).
The presented magnitudes are calibrated using the field stars from the Pan-STARRS1 catalog and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E(B-V) = 0.12 mag in the direction of the transient (Schlafly & Finkbeiner 2011).
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