TITLE: GCN CIRCULAR NUMBER: 41375 SUBJECT: GRB 250814A: Optical observations with MDM Hiltner 2.4m telescope DATE: 25/08/14 18:24:24 GMT FROM: Janet Chen at National Central University janetstars@gmail.com

L. E. Salazar Manzano (Umich), H.-W. Lin (UMich), A. Aryan, T.-W. Chen (both NCU), and S. Yang (HNAS) report:

We observed the field of the GRB 250814A (Fermi GBM team, GCN41357) using the 2.4m Hiltner telescope (HT) at the Michigan-Dartmouth-MIT (MDM) Observatory in Arizona, United States. The Swift-XRT detection of GRB 250814A was proposed to be temporally and spatially coincident with the subthreshold GW trigger S250814bg, which occurred 6 s after the BAT trigger (Caputo et al., GCN41358). The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration, with the Fermi GBM team and Swift team, reported the identification of a GW candidate possibly associated with this GRB (GCN41364).

The first epoch of observations started at 10:22 UTC on the 14th of August 2025 (MJD = 60901.432), 2.79 hr after the Swift trigger.

We do not find any credible evidence of a new and uncataloged source in the difference images (subtraction using the Pan-STARRS1 template images) within the 2.2 arcseconds uncertainty circle of Swift-XRT localization. We utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction using the 'hotpants' (Becker A., 2015, ascl.soft. ascl:1504.004) algorithm and perform the PSF photometry. The details of the observations and the measured 3-sigma upper limits (in the AB system) are as follows:

Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing 2.4m HT | g | 60901.432 | 2.79 | 300 * 5 | > 22.03 | 1".32 2.4m HT | r | 60901.436 | 2.88 | 300 * 7 | > 22.98 | 0".99 2.4m HT | i | 60901.462 | 3.51 | 300 * 4 | > 21.96 | 1".21

Our (detection) limits are consistent with those reported by Becerra et al. (GCN41362), Postigo et al. (GCN41367), and Karambelkar et al. (GCN41372).

The presented magnitudes are calibrated using the field stars from the PanStarrs1 catalog and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of A_g = 3.5 mag, A_r = 2.4 mag, and A_i = 1.8 mag in the direction of the transient (Schlafly & Finkbeiner 2011).

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