TITLE: GCN CIRCULAR NUMBER: 38987 SUBJECT: EP250108a / SN2025kg: Gemini GMOS-S confirmation of an associated broad-lined SN Ic DATE: 25/01/19 19:44:15 GMT FROM: Andrew Levan at Radboud University a.levan@astro.ru.nl

Andrew J. Levan (Radboud), Jillian C. Rastinejad (Northwestern), Daniele B. Malesani (DAWN/ NBI and Radboud), Wen-fai Fong (Northwestern), Nial R. Tanvir (Leicester), P.G. Jonker (Radboud), Rob A. J. Eyles-Ferris (Leicester) report on behalf of a larger collaboration:

We obtained spectroscopic observations of the optical counterpart AT2025kg (“the kangaroo”; Eyles-Ferris, GCN 38878; Zhu et al., GCN 38885; Malesani et al., GCN 38902; Kumar et al., GCN 38907; Zhu et al, GCN 38908; Levan et al., GCN 38909; Izzo, GCN 38912; Zou et al., GCN 38914; Moskvitin & Spiridonova, GCN 38925; Song et al., GCN 38972; Eyles-Ferris et al., GCN 38983; Xu et al., GCN 38984) to EP250108a (Li et al., GCN 38861) with the GMOS spectrograph mounted on the Gemini-South telescope (PI: Rastinejad; program GS-2024B-Q-105). We obtained 4 x 400 s spectroscopy beginning at 2025-01-19 01:29 UT, approximately 10.5 days after the EP/WXT trigger. Spectroscopy was taken with the B480 grism, covering the spectral range from 4250-8500 AA.

The optical counterpart is well detected across the spectral range. In contrast to earlier observations which showed a hot, blue continuum (Zhu et al., GCN 38908), it is now characterised by a redder shape. In addition, broad features are now visible in the spectrum with high signal to noise. Both the spectral shape and broad features resemble those found in broad-lined type Ic supernovae. Our findings find further support for the conclusions based on NOT spectroscopic observations (Xu et al., GCN 38394, and are also consistent with the optical re-brightening reported by Eyles-Ferris et al. (GCN 38983).

On this basis we therefore conclude that the FXT and FBOT EP250108a is related to the core-collapse of a massive star. The earlier bright blue emission may be consistent with a rapidly cooling black body, similar to that seen in GRB/XRF 060218 (Campana et al. 2006, doi:10.1038/nature04892; Pian et al. 2006, doi:10.1038/nature05082), although it appears to last for a much longer period (days instead of hours) in the case of EP250108a.

We thank the staff of Gemini, especially Murilo Marinello and Joanna Thomas-Osip for their excellent support in securing these observations.

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