TITLE: GCN CIRCULAR NUMBER: 38984 SUBJECT: EP250108a/AT 2025kg: NOT spectroscopy in agreement with SN Ic-BL DATE: 25/01/19 15:49:39 GMT FROM: Dong Xu at NAOC/CAS dxu@nao.cas.cn

D. Xu, Z.P. Zhu, X. Liu (NAOC), J.P.U. Fynbo (DAWN/NBI), X.Z. Zou, B. Kumar, X.W. Liu (SWIFAR, YNU), F.F. Song, R.Z. Li, J.R. Mao (YNAO), Y.Q. Liu, T. An (SHAO), W.X. Li (NAOC), X.F. Wang (THU), J.J. Geng, X.F. Wu (PMO), H. Sun, W.M. Yuan (NAOC), B. Zhang (UNLV), report on behalf of a large collaboration:

We have been doing multi-band photometry of the optical counterpart of EP250108a detected by EP (Li et al., GCN 38861) at different telescopes, including NOT, Mephisto, and GMG (e.g., Zhu et al., GCN 38885; Malesani et al., GCN 38902; Zou et al., GCN 38914; Song et al., GCN 38972).

The g-/r-/i- band light curves show slow decay followed by slow re-brightening over the past days, in agreement with the report by Eyles-Ferris et al. (GCN 38983).

We carried out spectroscopy of the counterpart in the night of Jan. 18, ~ 10 days post-trigger, using the the ALFOSC instrument mounted on the NOT. Observations consisted of 4 exposures of 1200 s each, and the spectra cover the wavelength range 3600 - 9000 AA.

The target, including the counterpart and its host galaxy, has r ~ 20.2 mag (AB), while the host galaxy has r ~ 23.2 mag (AB) from Legacy Survey. Therefore the spectrum is certainly dominated by the counterpart itself.

The spectral continuum doesn't have a high S/N, but the spectral shape is clearly present, due to its significant fluctuation. Using Superfit, we found the spectrum is in good agreement with the broad-lined Ic (Ic-BL) SN 1998bw associated with GRB 980425, the prototype of GRB/XRF-SN association events, about 6 days before its maximum, i.e., ~ 15 days post-burst.

Further observations are planned.

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