TITLE: GCN CIRCULAR NUMBER: 38909 SUBJECT: EP250108a / AT 2025kg: Swift UVOT detections DATE: 25/01/11 13:01:52 GMT FROM: Daniele B. Malesani at IMAPP / Radboud University d.malesani@astro.ru.nl
A.J. Levan (Radboud), L. Cotter (UCD), D. B. Malesani (DAWN/NBI & Radboud), A. Martin-Carrillo (UCD), P.G. Jonker (Radboud) report on behalf of a larger collaboration.
The location of EP250108a (Lin et al., GCN 38861; Eyles-Ferris, GCN 38878) was observed with the Neil Gehrels Swift Observatory on 10 Jan 2025, starting at 15:17 UT, that is 2.12 days since the onset of the outburst.
At the location of the optical counterpart AT 2025kg (the “kangaroo”; Eyles-Ferris, GCN 38878; Zhu et al., GCN 38885; Malesani et al., GCN 38902; Kumar et al., GCN 38907) we clearly identify the source in both the UVW1 and UVW2 filters, with preliminary AB magnitudes of UVW1 = 20.6 +/- 0.2 and UVW2 = 21.4 +/- 0.2. These detections confirm the blue shape of the spectral energy distribution, as also indicated by Zhu et al. (GCNs 38885, 38908) and Malesani et al. (GCN 38902), but also indicate that it does not continue to rise strongly through the UV region.
At a redshift of z = 0.176 (Zhu et al., GCN 38908) our measurement corresponds to an absolute AB magnitude of M_UVW1 = -18.9. At 3 days post discovery (likely somewhat later post-onset) AT 2018cow had M_UVW1 = -20.6 AB (Perley et al. 2019, doi:10.1093/mnras/sty3420), suggesting that, in the UV, AT 2025kg is somewhat fainter than AT 2018cow.
At the position of AT 2025kg, no significant X-ray emission is detected by XRT, down to a 3-sigma upper limit of 3.8*10^-3 c s^-1, computed using the online tool at the University of Leicester (https://www.swift.ac.uk/user_objects/).
We thank the Swift team for the very rapid scheduling of these observations.
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