TITLE: GCN CIRCULAR NUMBER: 38131 SUBJECT: EP241107a: Kinder optical follow-up observations DATE: 24/11/08 16:55:04 GMT FROM: Janet Chen at National Central University janetstars@gmail.com
A. K. H. Kong (NTHU), A. Aryan, T.-W. Chen, C.-S. Lin (all NCU), Z. N. Wang (HNAS), J. Gillanders (Oxford), S. J. Smartt (Oxford/QUB), Y. J. Yang, A. Sankar. K, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, C.-H. Lai, H.-Y. Hsiao, W.-J. Hou, H.-C. Lin, J.-K. Guo (all NCU), S. Yang, 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 EP241107a (Zhou et al., GCN 38112; Odeh et al., GCN 38115; SVOM/C-GFT team, GCN 38116; Lipunov et al., GCN 38117; Mohan et al., GCN 38118; Busmann et al., GCN 38120; Adami et al., GCN 38122; Quirola-Vasquez et al., GCN 38126; Li et al., 38127; Odeh et al., GCN 38128) 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 14:38 UTC on the 8th of November 2024 (MJD = 60622.610), 1.02d 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 detect the optical counterpart reported by Odeh et al. (GCN 38115) in the stacked image.
Further, we utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform the PSF photometry on our stacked frame. The details of the observations and measured PSF photometry (in the AB system) are as follows:
Telescope | Filter | MJD (start) | t-t0 (d) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass LOT | r | 60622.610 | 1.02 | 300 * 6 | 21.53 +/- 0.32 | 1".34 | 1.06
The presented magnitudes are calibrated using the field stars from the PanSTARRS-1 catalog and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E(B-V) = 0.04 AB mag in the direction of the transient (Schlafly & Finkbeiner 2011).
The potential host galaxy has an r-band magnitude of 22.69 in the DESI Legacy Surveys DR10 catalog, and our observed optical counterpart appears brighter than the host galaxy, though this measurement lacks template subtraction. Given the host redshift z=0.456 (Quirola-Vasquez et al., GCN 38126; see also Adami et al., GCN 38122), the current r-band absolute magnitude of about -20.45 (after Galactic foreground extinction correction) for this optical counterpart indicating it may be an afterglow candidate.
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