Radio Follow-Up of Einstein Probe Fast X-Ray Transients

Fast X-ray transients (FXTs) are brief, luminous bursts of soft X-ray emission whose physical origins remain uncertain. The Einstein Probe (EP) mission has recently enabled prompt discovery of these events, providing opportunities for rapid multi-wavelength follow-up. We present a coordinated radio observing campaign targeting 20 FXTs detected by the EP in 2024. The core consists of 59 epochs with the Allen Telescope Array (ATA), sampling post-burst timescales from about 1 to about 65 days and reaching typical 3 sigma sensitivities of 0.6--1.5,mJy across 1--8,GHz. Two FXTs---EP240315a and EP241021a---have radio counterparts. For EP241021a, AMI-LA monitoring at 15.5,GHz reveals a light curve peaking at about 30 days with F nu about1.0,mJy; equipartition analysis implies a Newtonian equipartition energy of about3 times10 49,erg and a mildly relativistic on-axis solution with on about1.3. Multi-frequency modelling of EP240315a, combining new VLA measurements with published MeerKAT, ATCA, and eMERLIN data, indicates highly relativistic early emission (3) that decelerates with time, consistent with jetted outflow. The detections and ATA non-detections span luminosities overlapping the brightest GRB and relativistic tidal disruption event (TDE) afterglows, suggesting FXTs comprise a heterogeneous population including both relativistic and non-relativistic explosions. Under a GRB-like redshift prior, our 3 sigma ATA limits correspond to observer-frame specific-luminosity thresholds of about10 32 -- 10 33, erg s-1 Hz-1; for nearby events (z 0.2), L nu 10 30--31, erg s-1 Hz-1. Planned ATA upgrades and next-generation arrays (SKA, DSA, ngVLA) will enable sensitive, population-level radio studies of the FXT radio sky.