EP260617a: optical observations with LCO

ATel #17850; Gregory Corcoran (UCD), Andrew J. Levan (Radboud and Warwick), Daniele B. Malesani (DAWN/NBI and Radboud), Rob A. J. Eyles-Ferris (Leicester), Jonathan Quirola-Vasquez (Radboud), Antonio Martin-Carrillo (UCD), Peter G. Jonker (Radboud), Jennifer Chacon (PUC)
on 19 Jun 2026; 14:37 UT
Credential Certification: Daniele Malesani (daniele.malesani@nbi.ku.dk)

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We observed the location of the X-ray transient EP260617a (Jiang et al., ATel #17848; ATel #17849) using an LCO 1m telescope located at the Teide Observatory (Spain) equipped with the SINISTRO instrument. We obtained 6x300 s exposures in the SDSS-r filter starting on 2026-06-18 at 21:05:41 UT (mid time of the exposure: 43.35 hr after the first detection by EP/WXT).

Image subtraction was carried out against the Legacy Survey images of this field. No new sources are visible within the EP/WXT error localization. The 3-sigma limiting magnitude of our LCO image is r > 22.7 (AB), calibrated against the Pan-STARRS catalog and not corrected for Galactic extinction.

We note the presence of an historically strongly variable star inside the WXT error circle, at coordinates:

RA = 13:56:02.39
Dec = -01:36:26.4

In our observation, the star has a magnitude r = 15.55 +- 0.01, which is consistent with recent measurements from ZTF (e.g. https://alerce.online/object/ZTF18abcvnmf). The star distance, while not measured, is > 1 kpc given the lack of parallax measurement in Gaia. This previous monitoring is suggestive of large amplitude variability (spanning one magnitude peak to trough, although with some variation in amplitude) with a period of 210 days. The relationship of this star with the EP event remains unclear, though we note that such high-amplitude variables are rare in the sky, especially outside the Galactic plane.