TITLE:   GCN CIRCULAR
NUMBER:  45659
SUBJECT: EP260920a: NOT optical upper limits
DATE:    26/09/20 12:04:25 GMT
FROM:    Zipei Zhu at NAOC <zpzhu@nao.cas.cn>

Z.-P. Zhu, X. Liu, D. Xu, J. An, L.-B. He, S.-Q. Jiang (NAOC), S.-Y. Fu, L. Lei (HUST), J.P.U. Fynbo (DAWN/NBI), T. Pursimo, A. Krzyzaniak (NOT) report on behalf of a large group:

We observed the field of EP260920a detected by the Einstein Probe (EP) (Ni et al., GCN 45649 ), using the ALFOSC camera mounted on the Nordic Optical Telescope (NOT). We obtained 2x300 s frames in each of SDSS g-, r-, and i- filters as well as 4x180s in the z filter, starting on 2026-09-20 at 02:41:57 UTC, i.e., 2.6 hrs after the EP trigger.

No new source is detected in our stacked images within or at the border of the refined EP-FXT error circle (Ni et al., GCN 45655), down to the following 3-sigma limits:

g > 23.8
r > 23.2
i > 22.9
z > 22.1

calibrated using nearby stars from the Pan-STARRS catalog and are not corrected for Galactic extinction.

Our results are consistent with previous reports by LCO (Liu et al., GCN 45653), COLIBRI (Dornic et al., GCN 45654), and SVOM/VT (Li et al., GCN 45657).

Given the deep optical upper limits shortly after the EP trigger, EP260920a would be a good candidate of high redshift, and thus near-infrared follow ups are strongly encouraged.