2026ApJ..1000..118G

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 <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 ∼ 10³⁹ 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.