GCN Circular 45955

G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), P. G. Jonker (Radboud), J. Chacon (PUC), and F. E. Bauer (SSI and UTA), O. Stalder (NOT) report on behalf of a larger collaboration:

We observed the field of the Einstein Probe fast X-ray transient EP261015a (WXT trigger 11900947714) using the ALFOSC camera on the Nordic Optical Telescope (NOT). Observations consisted of a sequence of 3 x 300 s exposures in the SDSS r-band, and started on 2026-10-16 at 02:58:58 UT (13.72 hr after the EP/WXT trigger).

After performing image subtraction against Dark Energy Survey stacked images, we identify a potential counterpart at coordinates (J2000):

RA: 02:57:19.83
Dec: -13:21:17.3

We measure a difference magnitude of r = 22.74 +/- 0.21 (AB) at a midtime of 2026-10-16 03:06:55 UT (13.86 hr after the EP/WXT trigger), calibrated using nearby stars in the Pan-STARRS catalog and not corrected for Galactic extinction.

At a similar mid-time (2026-10-16 03:02:53 UT), we performed additional observations using an LCO 1m telescope located at the Cerro Tololo Interamerican Observatory (Chile) equipped with the SINISTRO instrument. Here observations consisted of 9 x 300 s exposures in SDSS r-band. We see the above candidate source in our difference image as well, at r = 22.83 +/- 0.16 (AB).

We caution it is possible that this source is an artefact of the image subtraction due to presence of an underlying galaxy, however the source’s detection from both telescopes and clean residuals in the photometry lend credence to it being real. Given our two observations are at similar times, we cannot make a useful estimate of its temporal evolution.