ASKAP J005512.2−255834: A Luminous, Long-lived Radio Transient at <i>z</i> = 0.1—an Orphan Afterglow or an Off-nuclear TDE from an IMBH?

Abstract We report the discovery of a slowly evolving, extragalactic radio transient, ASKAP J005512.2–255834 (hereafter ASKAP J0055–2558), identified using the Australian SKA Pathfinder in a search for orphan afterglows associated with archival gravitational-wave events. Although discovered in this context, there is no evidence that the transient is associated with any known gravitational-wave event. Nonetheless, this source exhibits a 20-fold increase in flux density over &lt;250 days, and it remains in a declining yet detectable state more than 1000 days after the initial detection. Follow-up observations from 0.3 to 9 GHz reveal an evolving spectrum consistent with synchrotron emission. ASKAP J0055–2558 is spatially coincident with a low-mass, star-forming galaxy at redshift z = 0.116 ( d L = 543 Mpc), placing its peak radio luminosity at νL ν ∼ 10 39 erg s −1 . Analysis of its radio light curve, inferred blast-wave velocity, energetics, host-galaxy properties, and the absence of counterparts at other wavelengths suggests that ASKAP J0055–2558 is most consistent with either the late-time phase of an orphan long gamma-ray burst afterglow or a tidal disruption event involving an intermediate-mass black hole spatially offset from the galaxy nucleus. The radio discovery of either of these phenomena is extremely rare, with only a few or no confirmed examples to date.