GCN Circular 38908
Subject
EP250108a / AT 2025kg (the "kangaroo”): VLT/X-shooter spectroscopic redshift z = 0.176 and LFBOT classification
Event
Date
2025-01-11T12:14:29Z (a year ago)
Edited On
2025-01-14T16:05:55Z (a year ago)
From
Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>
Via
Web form
Z.P. Zhu (NAOC), G. Corcoran (UCD), A. J. Levan (Radboud), L. Izzo (INAF/OACn), R. Eyles-Ferris (Leicester), J. P. U. Fynbo (DAWN/NBI), G. Leloudas (DTU Space), D. B. Malesani (DAWN/NBI and Radboud), A. Saccardi (CEA/Irfu), A. L. Thakur (INAF/IAPS), D. Xu (NAOC), B. P. Gompertz (U. Birmingham), P. G. Jonker (Radboud), report on behalf of the Stargate collaboration.
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.
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*10^46 erg s^-1.
We thank excellent support from the observing staff in Paranal, in particular Francesca Lucertini, Rodrigo Palominos, and Linda Schmidtobreick.
Update: the original version of this circular reported a broad feature at the blue end of our spectrum. As explained in our refined GCN 38937, this turned out to be spurious and should be disregarded.