GCN Circular 42803
Subject
EP251122a / GRB 251122A: optical counterpart candidate with Kinder observations
Event
Date
2025-11-22T16:25:24Z (4 days ago)
From
Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com>
Via
Web form
Y.-C. Cheng (NCU), J.Y.B Choo (UKM), A.N. Garcia (PUP), A.L.A.M. Nasir (UniSZA), J.-L. Yang (NCU) on behalf of the 47th International School for Young Astronomers (ISYA) in Taipei, A. Aryan, T.-W. Chen, Y.-H. Lee, C.-S. Lin (all NCU), S. Yang (HNAS), A. K. H. Kong (NTHU), J. Gillanders, S. J. Smartt (both Oxford), Y. J. Yang (NYUAD), A. Sankar.K, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, C.-H. Lai, W.-J. Hou, H.-C. Lin, H.-Y. Hsiao, 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 EP251122a (Wang et al., GCN 42799) using the 1m LOT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). A follow-up observation by SVOM found a temporally and spatially coincident long GRB 251122A (Lin et al. GCN 42800). The first LOT epoch of observations began at 14:13 UTC on the 22nd of November 2025 (MJD 61001.592), 0.31 hr after the EP-WXT trigger.
We used the Kinder pipeline (Yang et al. A&A 646, A22) to stack the images and perform template subtraction with the DESI Legacy Survey (Dey et al. 2019, AJ 157, 168) DR10 image. In the difference image, we find a faint, uncataloged optical counterpart candidate at RA, DEC = 02:32:05.1284, +14:07:43.826; lying within the 20" EP-FXT localization circle. The counterpart candidate is likely associated with the DESI Legacy Survey (DR10) galaxy at RA, Dec = 38.0213, 14.1289, with r = 24.64 mag and a photometric redshift of 1.113 ± 0.368.
We perform PSF photometry on the position of the source with the stacked frames. The details of the observations and measured PSF magnitude (in the AB system) of the proposed counterpart of EP251122a are as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass
LOT | r | 61001.592 | 0.31 | 300 * 6 | 23.1 +/- 0.2 | 0".93 | 1.01
We caution that the optical counterpart candidate is detected at a level close to the detection limit (>23.2 mag). Our observation is consistent with Wu et al. (GCN 42801) and deeper than their limits.
The presented magnitude is calibrated using the field stars from the Pan-STARRS1 DR2 and is not corrected for the expected Galactic foreground extinction of A_r = 0.35 mag in the direction of the transient (Schlafly & Finkbeiner 2011). The methodology, details on the Lulin observatory telescopes, and a compilation of our optical follow-up campaign for FXTs discovered within the first year of operation of the Einstein-Probe mission can be found in Aryan et al. 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69.