TITLE: GCN CIRCULAR NUMBER: 40309 SUBJECT: EP250428b: Pan-STARRS follow-up, upper limit on the optical counterpart, and possible host galaxy. DATE: 25/05/01 20:14:32 GMT FROM: Stephen Smartt at University of Oxford s.smartt@qub.ac.uk

J. H. Gillanders (Oxford), M. Huber, K. C. Chambers (IfA, Univ. Hawaii), S. J. Smartt, K. W. Smith, S. Srivastav, F. Stoppa (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 EP250428b (Liu et al., GCN 40277), using the Pan-STARRS telescope system (Chambers et al., 2016, arXiv e-prints, 1612.05560) on MJD 60795.48 (2025-04-30 11:31 UTC), 2.2 days after the EP-WXT detection (Liu et al., GCN 40277). 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. We used PS1 with 6x150s exposures in both r and i. The PS reference frames were subtracted from the stacked 900s images, and no source was detected to r > 22.6 and i > 22.5.

There is a red source in the Legacy Surveys (Dey et al. 2019) DR10 at i=22.07 (r-i=0.76) morphologically classified as a galaxy (source type E), at 0.4" separation from the nominal centre of the EP250428b error circle (220.4575, 2.0989) from Wu et al. GCN 40285. The source is also visible in the Pan-STARRS 3Pi survey data and our night stack i-band. The only other source visible within 10 arcsec radius is a fainter object at 8.8 arcsec offset i=23.9 in the Lgeacy Survey. It is classified with a PSF-like morphology, although the significance of the detection would preclude a definitive stellar classification. We suggest the brigher, i=22.07, source may be the host galaxy of EP250428b.

There is also a spatially coincident source in the unWISE catalogue (at 220.4576984 +2.0988141) in the 3.4mu band at 128 +/- 7 nMgy (Vega nanomaggies), or AB mag = 19.9 +/- 0.1. The source is visible as excess flux in the legacysurvey.org browser. This could suggest a high redshift host galaxy, but we caution that there is no recorded flux in the 4.6mu band in the unWISE catalogue

The non-detection is consistent with the deeper limits reported in Busman et al. (GCN 40290) and Xin et al. GCN 40283) and also other GCN optical searches (Lipunov et al., GCN 40278; Konno et al., GCN 40281; Moskvitin et al., GCN 40282; Salgundi et al., GCN 40284).

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