TITLE: GCN CIRCULAR NUMBER: 39903 SUBJECT: EP250315a: Pan-STARRS r and i-band imaging and photometry DATE: 25/03/28 10:04:31 GMT FROM: James Gillanders at University of Oxford jhgillanders.astro@gmail.com

J. H. Gillanders (Oxford), M. Huber, K. C. Chambers (IfA, Univ. Hawaii), S. J. Smartt, K. W. Smith, S. Srivastav (Oxford), M. Nicholl, D. Young, M. Fulton (QUB), T.-W. Chen (NCU, Taiwan) A. S. B. Schultz, T. de Boer, J. Fairlamb, G. Paek, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii).

We observed the optical counterpart of EP250315a (Peng et al., GCN 39731; Quirola-Vasquez et al., GCN 39733), using the Pan-STARRS telescope system (Chambers et al., 2016, arXiv e-prints, 1612.05560) on MJD 60755.50 (2025-03-21 12:00 UTC), approximately 6.25 days after the EP-WXT detection (Peng et al., GCN 39731). The Pan-STARRS system consists of two 1.8m telescope units located at the summit of Haleakala on the Hawaiian island of Maui, employing an SDSS-like filter system denoted as grizy, and a broad w-filter, which is a composite of the gri-filters.

Our observation consisted of 6x150s exposures in both the r and i filters with Pan-STARRS1. The images were processed with the Pan-STARRS pipeline. After astrometric and photometric calibration, reference images were subtracted from the target stacked images (Magnier et al., 2020a, ApJS, 251, 3; Magnier et al., 2020b, ApJS, 251, 6; Waters et al., 2020, ApJS, 251, 4).

From these difference images, we do not detect the optical counterpart discovered by Quirola-Vasquez et al., GCN 39733. At the reported optical counterpart position, we measure 3.5-sigma limiting AB magnitudes of r~21.7 and i~21.7.

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