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GCN Circular 36741

Subject
EP240618a: NOT further optical observations
Date
2024-06-23T02:56:10Z (a month ago)
From
Dong Xu at NAOC/CAS <dxu@nao.cas.cn>
Via
Web form
X. Liu, S.Y. Fu, J. An, Z.P. Zhu, S.Q. Jiang, D. Xu (NAOC), M. Turkki (NOT) report on behalf of a large collaboration:

Following the first epoch of the observations of EP240618a (Sun et al., GCN 36690) at the the 2.56-m Nordic Optical Telescope (NOT, Liu et al., GCN 36736), we have carried out the second epoch at the NOT.

Observations started at 2024-06-23T00:13:37, i.e., ~ 4.8 day after the EP trigger, and again 12x120 s Sloan z-filter images were obtained.

The candidate in the 1st epoch is clearly detected in the 2nd epoch with comparable brightness in z-band. Specifically, it has z = 21.03 +/- 0.05 in the 1st epoch and z = 21.05 +/- 0.06 mag in the 2nd epoch, more or less affected by the nearby bright star ~1.3 arcsec away and the photometric method. 

As the candidate is probably mixed with the bright star in the Pan-STARRS image, we also measure the candidate+star in the NOT images. Both NOT epochs yield the same magnitude of z ~ 18.4 mag, being consistent with the value by Pan-STARRS.

Hence, we conclude that the candidate is likely not the counterpart of EP240618a.

Meanwhile, we searched for other possible candidates by image subtraction of the two NOT epochs within the 30-arcsec EP/FXT error circle (Sun et al., GCN 36690). Neither uncatalogued optical transient nor apparent brightening of catalogued source is found.
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