TITLE: GCN CIRCULAR NUMBER: 39839 SUBJECT: FRB 20250316A: Kinder optical upper limits of the Einstein Probe candidate X-ray source EP J120944.2+585060 DATE: 25/03/22 18:49:41 GMT FROM: Janet Chen at National Central University janetstars@gmail.com

A. Aryan, T.-W. Chen (both NCU), S. Yang (HNAS), C.-C. Ngeow, Y. J. Yang, Y.-H. Lee, A. Sankar. K, W.-J. Hou, H.-Y. Hsiao (all NCU), A. K. H. Kong (NTHU), T. Hashimoto, V.V. Vignesh, M. Mohanraj, T.-C. Yang (all NCHU), T. Goto (NTHU), C.-H. Niu (CCNU), S. C.-C. Ho (ANU), E. Kilerci-Eser (Sabanci U.), Y.-H. Zhu (NAOC), D. Li (Tsinghua U.), Y. Niino (U. Tokyo), S. Yamasaki (NCHU), Y.-A. Chen (NTHU), J.-S. Zhang (CAS), P. Wang (CAS), J. Gillanders (Oxford), S. J. Smartt (Oxford/QUB), M.-H. Lee, Y.-C. Pan, C.-H. Lai, 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 fast radio burst 20250316A (Ng et al., ATel#17081; Leung et al., ATel#17086) using the 1m LOT and 40cm SLT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al. 2024, arXiv:2406.09270). A candidate associate X-ray source was detected by the EP mission (Sun et al., GCN 39834). The first LOT epoch of observations in the r band started at 11:30 UT on the 22nd of March 2025 (MJD = 60756.479), ~6.12 days after the CHIME detection (3.84 days after the EP X-ray source), while the first SLT epoch of observations in the z band started at 12:57 UT on the 22nd of March 2025 (MJD = 60756.540), ~7.18 days after the CHIME detection (3.90 days after the EP X-ray source).

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 the updated localization provided by Leung et al. (ATel#17086). Moreover, we used the Kinder pipeline (Yang et al. 2021, A&A, 646, A22) 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, A62) to perform template subtraction utilizing 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 afterglow 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-t_CHIME (d) | t-t_EP (d) | Exposure (s) | Magnitude | Average Seeing | MedianAirmass LOT | r | 60756.479 | 6.12 | 3.84 | 300 * 12 | >22.31 | 1".4 | 1.59 LOT | g | 60756.501 | 6.14 | 3.86 | 300 * 6 | >22.32 | 1".3 | 1.66 LOT | i | 60756.529 | 6.17 | 3.89 | 300 * 12 | >22.40 | 1".2 | 1.40 SLT | z | 60756.540 | 7.18 | 3.90 | 300 * 31 | >20.51 | 1".4 | 1.30

The non-detection of optical afterglow counterpart candidate is consistent with previous reports by Becerra et al. (ATel#17082), Niino et al. (ATel#17083), and our previous observations reported in Hashimoto et al. (Atel#17095). t_CHIME is the discovery time of CHIME FRB 20250316A, and t_EP is detection time of the corresponding EP X-ray source EP J120944.2+585060.

The presented upper limits 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 = 0.07 mag, A_r = 0.05, A_i = 0.03 mag, and A_z = 0.02 mag, respectively, in the direction of the burst (Schlafly & Finkbeiner 2011).

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