TITLE: GCN CIRCULAR NUMBER: 38983 SUBJECT: EP250108a / AT 2025kg: the kangaroo is hopping, evidence for optical rebrightening DATE: 25/01/19 13:30:18 GMT FROM: Rob Eyles-Ferris at U of Leicester raje1@leicester.ac.uk

R. A. J. Eyles-Ferris (Leicester), D. B. Malesani (DAWN/NBI and Radboud), P. T. O’Brien (Leicester), P. G. Jonker (Radboud), A. J. Levan (Radboud and Warwick), J. N. D. van Dalen (Radboud), A. Kumar (RHUL/Warwick), B. Gompertz (Birmingham), N. R. Tanvir (Leicester), report on behalf of several collaborations:

Following the discovery of the optical counterpart (“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) of the fast X-ray transient EP250108a (Li et al., GCN 38861), we started a monitoring campaign using the Liverpool Telescope and the Nordic Optical Telescope, both located on the island of La Palma, Spain. Observations were conducted in the griz filters.

The transient has been evolving rather slowly - certainly much slower than typical GRB or FXT afterglows (host contamination is not an issue, as it is significantly fainter at r ~ 23.2). The spectral shape has shown an overall trend of transitioning from blue to red. Starting on about January 17, the r and i bands are seen to brighten, by about 0.3-0.4 mag in both the LT and NOT observations, also consistent with the report by Song et al. (GCN 38972). The most recent r-band magnitude is 20.30 +/- 0.07 (AB), measured in LT observations starting at 2025-01-18 20:17:45 UT (approximately 10.3 days post trigger).

This rebrightening might be the indication of an emerging SN.

We acknowledge excellent support from the observing staff at the NOT.

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