GCN 43581: EP260131a: NOT early detection of the counterpart in "rapid response mode"

D. B. Malesani (DAWN/NBI and Radboud), J. N. D. van Dalen (Radboud), J. A. Quirola-Vasquez (Radboud), P. G. Jonker (Radboud), K. Valeckas (NOT) report on behalf of a larger collaboration:

We observed the field of the fast X-ray transient EP260131a (Ding et al., GCN 43574) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations were triggered using the recently implemented automatic “Rapid Response Mode” and obtained in the SDSS r and z bands, consisting of 5x180 s and 9x150 s exposures, respectively, starting on 2026-01-31 at 04:56:50 UTC (~2.5 min after the GCN Notice, or 52 min after the WXT trigger).

We detect the counterpart reported by Li et al. (GCN 43577) and Sánchez Álvarez et al. (GCN 43578). In our first frame (mid time 53.5 min after trigger), we measure r' = 21.18 +- 0.10 (AB) and from a stacked z-band image we measure z' = 20.75 +- 0.06 at time 75.2 min after the WXT trigger.

The above magnitudes are calibrated against nearby stars from the Pan-STARRS catalog and are not corrected for Galactic extinction.