AstroNote 2025-17

We observed the optical counterpart AT 2025kg (Eyles-Ferris, GCN 38878; Zhu et al., GCN 38885; Malesani et al., GCN 38902; Kumar et al., GCN 38907) of the transient EP250108a (Li et al., GCN 38861) using the ESO VLT UT3 (Melipal) equipped with the X-shooter instrument. Our spectra cover the wavelength range 3000-21000 AA, and consist of 4 exposures by 600 s each. The observation mid time was 2025 Jan 11.08 UT (2.56 days after the EP trigger).

Bright, blue continuum is detected over the UVB and VIS arms. Emission lines from Halpha, [O II] 3727/29, and faint [O III] 5007 are visible at a common redshift z = 0.176. At the same redshift, we also detect (narrow) absorption from Ca II H and K. This is thus the likely redshift of EP250108a.

In the UV, there is a broad absorption feature, at an observed wavelength of ~3600 Å and with a FWHM of ~15,000 km s^-1. This feature is not identified at the moment.

The spectral shape (mostly featureless) and luminosity are overall similar to the early AT 2018cow (e.g., Prentice et al., doi:10.3847/2041-8213/aadd90), both being hot blackbodies. The luminosity (absolute g-band magnitude ~ -20) and blue color robustly put this object in the region of luminous FBOTs (e.g. Ho et al., doi:10.3847/1538-4357/acc533). We tentatively suggest that AT 2025kg belongs to this class of objects, and humorously dub it "the kangaroo".

At this redshift, the X-ray peak flux (Li et al., GCN 38861) corresponds to a luminosity of ~1.3*10⁴⁶ erg s^-1.

We thank excellent support from the observing staff in Paranal, in particular Francesca Lucertini, Rodrigo Palominos, and Linda Schmidtobreick. This communication was originally published as a GCN circular (Zhu et al., GCN 38908), and is reposted here as an astronote to alert a broader community.