TITLE: GCN CIRCULAR NUMBER: 38970 SUBJECT: EP250108a / AT2025kg: VLA radio upper limit DATE: 25/01/18 03:08:32 GMT FROM: Genevieve Schroeder at Cornell University genevieveschroeder@u.northwestern.edu

We observed the position of AT2025kg (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), the optical counterpart to Fast X-ray Transient EP250108a (Li et al. GCN 38861) with the Karl G. Jansky Very Large Array (VLA) under program 25A-374 (PI: Perley) beginning on 2025 January 15 at 00:21 UT (6.5 days post discovery) at a mean frequency of 10 GHz. Based on preliminary analysis, we do not detect any radio emission at or near the position of AT2025kg to a 3-sigma limit of 16.5 microJy.

At a redshift of z=0.176 (Zhu et al., GCN 38908), the VLA observation corresponds to a 10 GHz upper limit of < 2E28 erg/s/Hz. This is consistent with previous observations of luminous fast blue optical transients: for example, at 10 days post discovery, AT2020xnd was 4E28 erg/s/Hz (e.g., Ho et al. 2022, ApJ, 932, 116), but AT2018cow was over an order of magnitude less luminous (Margutti et al. 2019, ApJ, 872, 18). This luminosity limit is two orders of magnitude lower than a typical GRB radio afterglow, but is consistent with the luminosities of sub-energetic GRBs at a similar epoch (e.g., Margutti et al. 2014, ApJ, 797, 107; Laskar et al. 2023, ApJ, 946, 23).

Additional followup is planned.

We thank the VLA staff for quickly approving and executing these observations.

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