TITLE: GCN CIRCULAR NUMBER: 39683 SUBJECT: GRB 250311A/EP250311a: Optical upper limits with Kinder observations DATE: 25/03/12 13:01:59 GMT FROM: Janet Chen at National Central University janetstars@gmail.com

A. Sankar. K, M.-H. Lee, A. Aryan, Y.-H. Lee, C.-H. Lai, T.-W. Chen, H.-Y. Hsiao, W.-J. Hou (all NCU), A. K. H. Kong (NTHU), J. Gillanders (Oxford), S. J. Smartt (Oxford/QUB), Y. J. Yang, C.-S. Lin, Y.-C. Pan, C.-C. Ngeow, 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 fast X-ray transient GRB 250311A/EP250311a (Tatano et al., GCN 39660; Mao et al., GCN 39664) using the 40cm SLT and 1 meter LOT telescopes at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al. 2024, arXiv:2406.09270). The first SLT epoch of observations in the r band started at 15:28 UT on the 11th of March 2025 (MJD = 60745.6442), ~13.52 hrs after the EP trigger, while the first LOT epoch of observations in the r band started at 17:01 UT on the 11th of March 2025 (MJD = 60745.7090), ~ 15.08 hrs after the EP trigger.

We utilised 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. We did not detect any uncataloged optical counterpart candidate within the EP-WXT localization error circle of radius 2.8 arcminutes in the stacked image.

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) algorithm using a template from SkyMapper DR4 (Onken et., 2024, PASA, 41, e061). We found no evidence of any prominent candidate optical counterpart in the difference image as well.

We further employed the Python-based package AutoPhOT to perform PSF photometry on our stacked frames. The details of the observations and measured 3-sigma upper limits (in the AB system) were as follows:

Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass SLT | r | 60745.6442 | 13.52 | 300 * 9 | >19.6 | 1".5 | 2.04 LOT | r | 60745.7090 | 15.08 | 300 * 6 | >21.2 | 1".3 | 1.36

The non-detection of any prominent optical counterpart is consistent with Li et al. (GCN 39667).

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_r = 0.27 mag, respectively, in the direction of the transient (Schlafly & Finkbeiner 2011).

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