TITLE: GCN CIRCULAR NUMBER: 40488 SUBJECT: EP250427a/GRB250427A: VLA Detection of Radio Counterpart DATE: 25/05/19 03:14:19 GMT FROM: Tao An at SHAO, CAS antao@shao.ac.cn
Tao An and Yuanqi Liu (Shanghai Astronomical Observatory) report on behalf of a large collaboration.
We report the detection of a radio counterpart to the fast X-ray transient EP250427a, discovered by the Einstein Probe on 2025 April 27 (Wang et al., GCN 40257). EP250427a, also designated GRB250427A (Ravasio et al. GCN 40262), is a transient X-ray source notable for its rapid rise and decay in X-ray emission, peaking within minutes of detection. This behavior, combined with its potential association with a gamma-ray burst (GRB), suggests it may belong to a rare class of high-energy astrophysical phenomena. Initial follow-up observations in X-rays and optical wavelengths reported a faint, uncatalogued host galaxy at its position (Liu et al., GCN 40260), with subsequent spectroscopic observations confirming the host galaxy redshift of z=1.52 (Saccardi et al., GCN 40266).
On 2025 May 5, at approximately UT 11:25 (mid-time of the observation), we conducted observations using the Karl G. Jansky Very Large Array (VLA) under DDT program VLA/25A-467. The observations were carried out at central frequencies of 6 GHz (C-band) and 10 GHz (X-band). We detected a radio source coincident with the position of EP250427a, with measured flux densities of 26 +/- 7 μJy at 6 GHz and 77 +/- 7 μJy at 10 GHz. These values yield an inverted radio spectrum with a spectral index \alpha ≈ 2.1 (where S \propto ν^\alpha), hinting at physical processes such as synchrotron self-absorption.
The inverted spectrum is consistent with an early-time radio afterglow where the emission region is still compact. Continued VLA monitoring is planned to track the flux density and spectral evolution of the radio counterpart, which will help refine models of its emission mechanism and constrain the energetics of the event.
We express our gratitude to the VLA TAC for approving the DDT proposal and to the VLA staff for their swift scheduling and support during these observations.
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