TITLE:   GCN CIRCULAR
NUMBER:  36736
SUBJECT: EP240618a: NOT optical counterpart candidate
DATE:    24/06/22 18:53:13 GMT
FROM:    Dong Xu at NAOC/CAS <dxu@nao.cas.cn>

X. Liu, S.Y. Fu, J. An, Z.P. Zhu, S.Q. Jiang, D. Xu (NAOC), J. Terwel (NOT) report on behalf of a large collaboration:

We observed the field of EP240618a detected by EP/WXT (Sun et al., GCN 36690) using the the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations started at 00:07:28 UT on 2024-06-20, i.e., ~ 1.77 day after the EP trigger, and 12x120 s Sloan z-filter images were obtained.

An uncatalogued optical transient (OT), within the 30 arcsec EP/FXT error circle (Sun et al., GCN 36690), is detected in the stacked z-band image at coordinates:

R.A. (J2000)= 18:46:35.78
Dec. (J2000)= +23:50:13.17

with an uncertainty of ~ 0.2 arcsec. The OT is 1.3 arcsec away from a nearby 18.4 magnitude star, and its preliminary photometry is z ~ 21 mag at 1.78 day since the EP trigger, calibrated against nearby Pan-STARRS stars and not corrected for Galactic extinction.

The OT is not detected by early TRT observation (Tinyanont et al., GCN 36710) down to R ~ 20.3 mag, and not by LCO observation (Li et al., GCN 36731) down to r ~ 21.5 mag.

If EP240618a is a GRB, the z-r color might indicate high redshift.

A finding chart of the candidate is available at

https://drive.google.com/file/d/1pPAKrFyVvk3D7QnpgCfBQHim1ieGQ8zq/view?usp=sharing

A second epoch in z-band at the NOT has been scheduled for the night of June 22.