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