TITLE: GCN CIRCULAR NUMBER: 38885 SUBJECT: EP250108a: NOT optical observations DATE: 25/01/10 04:55:01 GMT FROM: Dong Xu at NAOC/CAS dxu@nao.cas.cn
Z.P. Zhu, X. Liu, S.Y. Fu, S.Q. Jiang, J. An, D. Xu (NAOC) report on behalf of large collaboration:
We observed the field of EP250108a (Li et al., GCN 38861) using the Nordic Optical Telescope (NOT) equipped with ALFOSC. The observations were carried out in the g-/r-/i-/z- bands, starting at 22:16:55 UT on 2025-01-09, i.e., 1.4 days after the EP/WXT trigger.
The previously reported likely optical counterpart by LT (Eyles-Ferris et al., GCN 38878) is clearly detected in NOT individual images, and it has brightened from g = 20.79 +/- 0.05 at ~1.31 days post-trigger to g = 20.05 +/- 0.03 at ~ 1.44 days post-trigger.
The SED from g-/r-/i-/z- bands is opposite to the typical GRB optical afterglow, i.e., non-thermal power-law decay. Instead, it appears as quasi-thermal.
The quick brightening and the quasi-thermal SED make this source unlikely a conventional GRB afterglow. It would be more consistent with early behaviors of Fast Blue Optical Transients (FBOT) or novel Fast X-ray Transients by EP, if it is indeed associated with EP250108a.
In addition, we performed image subtraction on the stacked NOT g-/r- band images using Legacy Survey images as templates, and only the LT source is credible in the differencing images, down to limiting magnitudes of g > 22.7 and r > 23.2.
We acknowledge the excellent support from the NOT staff, in particular Samuel Grund Sorensen.
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