TITLE: GCN CIRCULAR NUMBER: 40605 SUBJECT: GRB 250521C / AT2025mgj: Pan-STARRS multi-band and multi-epoch imaging and photometry DATE: 25/06/02 15:01:40 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, (Oxford/QUB), 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 fast-fading transient AT2025mgj (Andreoni et al., GCN 40566), tentatively associated with the short-GRB 250521C (Fermi GBM Team, GCN 40515) using the Pan-STARRS telescope system (Chambers et al., 2016, arXiv e-prints, 1612.05560) on MJDs 60825.26 and 60827.26 (approximately 8.7 and 10.7 days post-GRB, respectively). 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 observations consisted of 120s (MJD 60825.26) and 200s exposures (MJD 60827.26) in grizy-bands. All images were processed with the Pan-STARRS pipeline. After astrometric and photometric calibration, reference images were subtracted from the target 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 recover the following photometry measurements:

MJD Filter AB mag

60825.26 g 18.97 +/- 0.06 60825.26 r 19.18 +/- 0.08 60825.26 i 19.29 +/- 0.08 60825.26 z 19.5 +/- 0.2

60827.26 g 19.29 +/- 0.05 60827.26 r 19.39 +/- 0.06 60827.26 i 19.62 +/- 0.06 60827.26 z 19.57 +/- 0.08

From our multi-band Pan-STARRS photometry, we estimate a fade rate of ~0.1 mag/day in r-band. Our rate of fading across our photometric bands match those reported previously by Martin-Carrillo et al. (GCN 40584), Becerra et al. (GCN 40590), and Busmann et al. (GCN 40597).

The rate of fading and persistent blue colour, with some indication of cooling, are all reminiscent of typical CV behaviour. While the connection to GRB 250521C cannot be definitively ruled out, the data match the lightcurve behaviour of CVs. Late-time, deep imaging is required to unambiguously uncover the nature of AT2025mgj. A point source would confirm AT2025mgj is indeed a CV, whereas an extended source in its vicinity would indicate that it was the afterglow of GRB 250521C.

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