TITLE:   GCN CIRCULAR
NUMBER:  36202
SUBJECT: EP240420a: NOT optical counterpart candidate and potential host
DATE:    24/04/21 05:16:55 GMT
FROM:    Dong Xu at NAOC/CAS <dxu@nao.cas.cn>

J. An, Z.P. Zhu, X. Liu, S.Q. Jiang, S.Y. Fu, D. Xu (NAOC), D. B. Malesani (DAWN/NBI and Radboud), P. Jonker (Radboud), M. A. Diaz Teodori (NOT) report on behalf of a large collaboration:

We observed the field of the X-ray transient, EP 240420a, detected by EP/WXT (Zhang et al., GCN 36194) using the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera, and obtained 5 x 200 s frames in the Sloan z-band with a median time of 2024-04-21T01:01:05.287, i.e., 0.54 day after the EP trigger.

The stacked z-band image has a 5-sigma limiting magnitude of z ~ 22.3. Within the 3 arcmin error circle of the EP/WXT, we identified an optical transient (OT) localized at

R.A. =  15:14:55.08 (J2000)
Dec. = +14:48:17.8 (J2000)

with an uncertainty of radius ~ 0.3" and z ~ 21.4 mag, calibrated with the PanSTARRS field.

The OT is ~ 1” south-west to a known source in the Legacy Survey (LS) which has r ~ 23.3 , z ~ 22.9,  and a photometric redshift of z_phot ~ 0.82. This LS source could be the host of the OT.

Further observations are encouraged.