TITLE:   GCN CIRCULAR
NUMBER:  42839
SUBJECT: EP251124b: NOT r- and z-band optical upper limits
DATE:    25/11/26 00:24:55 GMT
FROM:    Agnes van Hoof at Radboud University <agnes.vanhoof@ru.nl>

A. P. C. van Hoof (Radboud), G. Corcoran (UCD), J. N. D. van Dalen, A. Martin-Carrillo (UCD), D. Xu (NAOC), S.Q. Jiang (NAO, CAS), T. Pursimo (NOT, Aarhus University), J.P.U. Fynbo (DAWN/NBI), P. G. Jonker (Radboud) report on behalf of a larger collaboration:

We observed the field of the fast X-ray transient EP251124b (Feng et al., GCN 42834) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations were obtained in the r (3x300 s) and z (5x200 s) bands, starting on 2025-11-25 at 22:19:58.38 UTC (1.2 days after the WXT trigger). 

No new source is detected within the EP/WXT uncertainty region (Feng et al., GCN 42834). From the stacked images we obtain the following 3-sigma upper limits:

r > 24.4
z > 23.2

These upper limits are in AB magnitudes and were calibrated using nearby stars from the Pan-STARRS catalog and are not corrected for Galactic extinction.

We also performed image subtraction in r band using a reference image from the Legacy Survey. No new source is revealed compared to the reference. However, we note that there are artefacts across the field.