TITLE: GCN CIRCULAR NUMBER: 39509 SUBJECT: EP250226a/GRB 250226A: Kinder optical follow-up observations DATE: 25/02/27 05:20:39 GMT FROM: Amar Aryan at National Central University, Institute of Astronomy (NCUIA) amararyan941@gmail.com
A. Aryan, T.-W. Chen, H.-Y. Hsiao (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, W.-J. Hou, M.-H. Lee, H.-C. Lin, C.-H. Lai, C.-S. Lin, J.-K. Guo (all NCU), S. Yang, L. L. Fan, 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 EP250226a/GRB 250226a (Jiang et al., GCN 39482; The Fermi team, GCN 39479) using the Lulin One-meter Telescope (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 in the g band started at 19:34 UT on the 26th of February 2025 (MJD = 60732.815), ~12.98 hrs after the EP trigger; while the first LOT epoch of observations in the r band started at 19:59 UT on the 26th of February 2025 (MJD = 60732.826), ~13.25 hrs after the EP 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. In the stacked r-band image, we marginally detected the optical counterpart candidate proposed by An et al. (GCN 39486) and confirmed by several other observations (e.g., Zhu et al., GCN 39487; Magnani et al., GCN 39488; and Li et al., GCN 39489).
We employed the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform PSF photometry on our stacked frames. The details of the observations and measured photometry (in the AB system) are as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass LOT | g | 60732.815 | 12.98 | 300 * 3 | >21.1 | 0".92 | 1.02 LOT | r | 60732.826 | 13.25 | 300 * 3 | 21.73 +/- 0.17 | 0".91 | 1.01
The presented magnitudes were calibrated using the field stars from the Pan-STARRS1 catalog and were not corrected for the expected Galactic foreground extinction corresponding to a reddening of A_g = 0.16 mag and A_r = 0.11 mag, respectively, in the direction of the transient (Schlafly & Finkbeiner 2011).
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