TITLE:   GCN CIRCULAR
NUMBER:  37840
SUBJECT: EP241021a: NOT likely optical counterpart
DATE:    24/10/23 04:38:25 GMT
FROM:    Dong Xu at NAOC/CAS <dxu@nao.cas.cn>

S.Y. Fu, Z.P. Zhu, J. An, S.Q. Jiang, X. Liu, D. Xu (NAOC), J.P.U. Fynbo (DAWN/NBI), M. Turkki (NOT) report on behalf of a large collaboration:

We observed the field of EP241021a detected by Einstein Probe (EP, Hu et al., GCN 37834), using the ALFOSC instrument mounted on the Nordic Optical Telescope (NOT), with 5 x 200 s and 5 x 300 s photometry in the SDSS-z and SDSS-r bands, respectively. Observations started at 23:31:48.4 UT on 2024-10-22, i.e. 1.77 days after the EP trigger.

An uncatalogued source is detected in the individual as well as the stacked frames within the EP/WXT error circle (Hu et al., GCN 37834) and localized at coordinates

R.A. (J2000) =  01:55:23.41
Dec. (J2000) = +05:56:18.01

with an uncertainty of ~ 0.5 arcsec. The source has z = 21.60 +/- 0.11 at 1.77 days post-trigger and r = 21.95 +/- 0.06 at 1.91 days post-trigger, calibrated with nearby PanSTARRS stars and not corrected for Galactic extinction. The source is not present in the Legacy Survey, which is well deeper than the r-/z-band detection. We thus conclude that the source is very likely the optical counterpart of EP241021a.