GCN Circular 40192

Y.-H. Lee, 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, M.-H. Lee, Y.-C. Pan, C.-C. Ngeow, A. Sankar. K, C.-H. Lai, C.-S. Lin, H.-Y. Hsiao, H.-C. 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 EP-WXT trigger 01709135179 at the time of 2025-04-21T16:16:28.949 using the Lulin One-meter Telescope (LOT) at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2025, ApJ, 983, 86). The first LOT epoch of observations started at 16:43:39 UT on 21st of April 2025 (MJD = 60786.697), 27 minutes after the EP trigger. 

We detected an optical transient in the stacked frame, at RA = 11:29:40.757, Dec = -24:40:31.98 (which is 22.3 arcsec east and 26.8 arcsec north away from the reported coordinate of the EP-WXT trigger 01709135179, and within the sky localisation error of 0.05 deg). 

We used the Kinder pipeline (Yang et al. A&A 646, A22) to stack the images, and utilised the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, 62) to perform PSF photometry on our stacked frames The details of the observations and measured PSF magnitude (in the AB system) of the proposed counterpart of the EP-WXT trigger 01709135179 (possibly EP250421a) are as follows: 

Telescope | Filter | MJD | t-t0 | Exposure | Magnitude | avg. Seeing | med. Airmass
LOT | r | 60786.697| 0.45 hrs | 300 sec * 6 | 21.44 +/- 0.09 | 1".15 | 3.06

The multiband observations using 40cm SLT and 1m LOT are ongoing. We encourage further follow-up observations to identify the nature of this optical counterpart candidate. 

We also note that at this optical counterpart position, a very faint, uncatalogued, extended source is visible in the DESI Legacy Survey image. A visual comparison with nearby objects suggests it is likely fainter than r > 24 mag. This faint, extended source could be the host of the transient.

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 A_r = 0.15 mag in the direction of the transient (Schlafly & Finkbeiner 2011).