EP241021a as an Off-nuclear Month-duration Transient in a Dwarf Starburst Galaxy

EP241021a is a peculiar fast X-ray transient (FXT) exhibiting a long X-ray plateau and an optical rebrightening, both challenging the traditional jet afterglow model. Whether its nature is a stellar explosion or a tidal disruption event (TDE) remains debated. Here we present late-time Hubble Space Telescope (HST)/WFC3 and Gemini-N/GMOS observations of its host galaxy at z = 0.748. After ∼300 days posttrigger, the transient emission had faded below detection, revealing a dwarf disk galaxy with an effective radius R e = 1.7 kpc, a stellar mass M ⋆ ∼ 3 × 10 8 M ⊙ , and a high star formation rate of ∼5 M ⊙ yr −1 . Combined with previous very long baseline interferometry measurements, the transient can be located in the galactic disk with a projected physical offset of 0.95 ± 0.13 kpc from the galaxy center. Host-subtracted optical light curves show that the rebrightening ceased at ∼47 days, followed by a sudden drop with a power-law index steeper than −3.3, coincident with a similar drop in the X-ray light curve. This synchronous drop in both bands is consistent with jet shutoff, and the required month-long jet activity suggests a persistently accreting compact object fed by a TDE. Among the TDE scenarios, the off-nuclear location and the starburst host galaxy slightly favor the postsupernova micro-TDE scenario in a binary system, in which a natal kick drives the newborn compact object into a tidal disruption of its companion. Nevertheless, an off-nuclear intermediate-mass black hole (IMBH) TDE or a stellar-mass black hole TDE induced by dynamical scattering in a dense stellar cluster cannot yet be excluded.