TITLE: GCN CIRCULAR NUMBER: 39862 SUBJECT: EP-WXT trigger 01709133021: Optical upper limit through Kinder observations DATE: 25/03/24 14:56:49 GMT FROM: Janet Chen at National Central University janetstars@gmail.com
C.-H. Lai, Y.-H. Lee, A. Aryan, T.-W. Chen, Y. J. Yang (all NCU), S. Yang (HNAS), A. K. H. Kong (NTHU), J. Gillanders (Oxford), S. J. Smartt (Oxford/QUB), W.-J. Hou, H.-Y. Hsiao, C.-C. Ngeow, Y.-C. Pan, A. Sankar. K, M.-H. Lee, C.-S. Lin, H.-C. Lin, J.-K. Guo (all NCU), 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 Einstein Probe (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics) discovered X-ray transient with WXT trigger ID 01709133021 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 in the r band started at 11:29 UT on the 24th of March 2025 (MJD = 60758.4017), ~1.84 hr post 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. In the stacked frames, we do not detect any potential uncataloged optical afterglow counterpart candidate within EP-WXT localization circle with a radius of ~3 arcminutes. Moreover, we used the Kinder pipeline (Yang et al. 2021, A&A, 646, 22) to subtract the stacked images using the templates from Pan-STARRS1 (Chambers et al. 2016 arXiv:1612.05560). We also utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, 62) to perform template subtraction using the "sfft" (Hu, 2022, ApJ, 936, 157) and "hotpants" (Becker A., 2015, ascl.soft. ascl:1504.004) algorithms. We found no evidence of any prominent optical counterpart candidate in the difference images as well.
We further employed AutoPhOT to perform PSF photometry on the combined frames. The details of the observation and 3-sigma upper limits (in the AB system) were as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | tmid-t0 (hr)| Exposure (s) | Magnitude | Average Seeing | Median Airmass LOT | r | 60758.4017 | 1.84 | 2.65 | 300 * 12 | >22.77 | 1".15 | 1.56
The non-detection of any optical afterglow counterpart candidate is consistent with the previous report by Ducoin et al. (GCN 39860).
The presented upper limit was calibrated using the field stars from the SDSS catalog and was not corrected for the expected Galactic foreground extinction corresponding to a reddening of A_r = 0.05 mag, respectively, in the direction of the transient (Schlafly & Finkbeiner 2011).
We are grateful to the EP team for providing useful communications through GCN Notices.
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