FRB 20250316A: Optical upper limits from Lulin Observatory

ATel #17095; 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), A. Aryan, T.-W. Chen, Y.-H. Lee, Y. J. Yang, M.-H. Lee, Y.-C. Pan, C.-C. Ngeow, A. Sankar. K, C.-H. Lai, W.-J. Hou, H.-Y. Hsiao (all NCU), A. K. H. Kong (NTHU), S. Yang, L. L. Fan, Z. N. Wang, G. H. Sun (all HNAS), J. Gillanders (Oxford), S. J. Smartt (Oxford/QUB), C.-S. Lin, H.-C. Lin, J.-K. Guo (all NCU), 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)
on 20 Mar 2025; 21:44 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Amar Aryan (amararyan941@gmail.com)

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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) and LOT 2025A semester proposal, R12. The first LOT epoch of observations in the r band started at 16:08 UT on the 20th of March 2025 (MJD = 60754.6725), ~4.3158 days after the CHIME detection, while the first SLT epoch of observations in the g band started at 16:23 UT on the 20th of March 2025 (MJD = 660754.6827), ~4.3258 days after the CHIME detection.

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 frame, we do not detect any potential uncataloged optical afterglow counterpart candidate within the updated localization provided by Leung et al. (ATel#17086). Moreover, we employed 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, using templates from Pan-STARRS1 (Chambers et al. 2016 arXiv:1612.05560). 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-t0 (d) | Exposure (s) | Magnitude | Average Seeing | MedianAirmass

LOT | r | 60754.6725 | 4.3158 | 300 * 6 | >22.354 | 1".53 | 1.22

SLT | g | 60754.6827 | 4.3258 | 300 * 12 | >20.961 | 1".83 | 1.24

LOT | i | 60754.7257 | 4.3687 | 300 * 12 | >22.235 | 1".18 | 1.31

LOT | z | 60754.7800 | 4.4233 | 300 * 6 | >20.939 | 0".91 | 1.45

The non-detection of any optical afterglow counterpart candidate is consistent with previous reports by Becerra et al., (ATel#17082) and Niino et al., (ATel#17083).

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, and A_i = 0.03 mag, respectively, in the direction of the burst (Schlafly & Finkbeiner 2011).