GCN Circular 45972
EP261015a: Tentative fading of the candidate optical counterpart from the NOT
From: Gregory Corcoran at University College Dublin
Issued: 2026-10-17 16:03 UTC
Type: X-ray Transient, Optical Transient

G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), P. G. Jonker (Radboud) A. L. Bouquin (NOT), A. Caballero Almagro (NOT), E. Georgiou (NOT and Turku Univ.) report on behalf of a larger collaboration

We observed the field of the Einstein Probe fast X-ray transient EP261015a (Li et al., GCN 45956) using the ALFOSC camera on the Nordic Optical Telescope (NOT). Observations consisted of a sequence of 5 x 300 s exposures in SDSS-r band and started on 2026-10-17 at 02:12:26 UT (1.54 days after event start). Due to the telescope pointing and dither pattern, there are some artifacts caused by a bright star to the north of the target that affect part of our images.

After dropping two images affected by the bright star and performing image subtraction against a Dark Energy Survey stacked image, we clearly detect the candidate discussed by Corcoran et al. (GCN 45955), Mendez et al (GCN 45960), and Ma et al. (GCN 45963). We measure a difference magnitude of r = 23.18 +/- 0.10 (AB) calibrated against nearby stars in the Pan-STARRS catalog and not corrected for Galactic extinction at a midtime of 2026-10-17 02:20:22 UT (1.54 days after event start).

We also refined our photometry on the previous epoch presented in GCN 45955 to r = 22.64 +/- 0.12. This shows fading between epochs with a decay of t^{-0.51+/-0.15}. We note that the uncertainties on our photometry neglect the difficult to estimate systematic uncertainties inherent to image subtraction between instruments, therefore the statistical significance of the fading cannot be reliably determined.

The nature of this object is still not fully clear. An X-ray source has been reported by EP/FXT within the EP/WXT error circle (Li et al., GCN 45969), positionally inconsistent with the above candidate. This source is however the likely counterpart of a spectroscopically confirmed QSO (DESI spectral viewer (legacysurvey.org)), thus likely unrelated to the EP/WXT event (no significant variability is detected in the QSO flux between our two epochs). The optical fading would then favor an association of the candidate with EP261015a, though an unrelated transient (e.g. a SN) cannot be fully discarded.

Further monitoring is planned and encouraged to clarify the nature of this object.

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