Multiwavelength analysis of the <i>Einstein Probe</i> X-ray transient EP240305a

Abstract

ABSTRACT We report multiwavelength observations of EP240305a, an uncatalogued X-ray transient detected by the Einstein Probe on 2024 March 5. The source exhibits distinct characteristics across the X-ray, optical, near-infrared, and radio bands. The soft X-ray observations show two significant flares lasting $\sim 120{-}250$ s, accompanied by rapid flux decay in a few days, and the optical and near-infrared data reveal a faint, candidate counterpart. In contrast, the radio observations reveal a long-term spectral evolution from a self-absorbed to an optically thin state within two months, implying discrete jet ejection. We compare EP240305a with known classes of X-ray transients and find that it is unlikely to be associated with long-time-scale transients such as jetted tidal disruption events or X-ray binaries. Its properties also disfavor a short-time-scale stellar flare origin. Although the absence of optical spectroscopy prevents a redshift determination, the source exhibits properties similar to those of gamma-ray-dark gamma-ray burst-like transients, which may be associated with relativistic jets viewed off-axis or with choked jets. The discovery of EP240305a, along with other uncatalogued transients detected by the Einstein Probe, underscores the scientific potential of highly sensitive X-ray survey telescopes and rapid-response multiwavelength follow-up observations in exploring the nature of atypical astronomical transients.