JWST and Keck observations of the off-nuclear tidal disruption event TDE 2025abcr: An evolving reprocessing layer

Abstract

Off-nuclear tidal disruption events (TDEs) provide a rare probe of massive black holes (MBHs) outside galactic nuclei. Only a handful are known, including five X-ray-selected candidates and two optically selected events. We present observations of TDE 2025abcr, the second optically selected off-nuclear TDE, discovered at a projected offset of $9.08\pm 0.02$ kpc from the nucleus of its host galaxy. We analyze X-ray, UV, optical, and infrared (IR) data from Swift, Keck, ZTF, and JWST. Broad H and He emission lines in the optical and IR confirm a TDE-H-He classification. From luminosity scaling relations and MOSFiT modeling, we infer a BH mass of $10^6$-$10^7{M}_{\odot }$, substantially smaller than the $10^{8.35\pm 0.41}{M}_{\odot }$ BH inferred for the host-galaxy nucleus. We observe velocity evolution in the N III + He II emission complex, shifting from $-500$ km s${}^{-1}$ at day $-7$ to $+1000$ km s${}^{-1}$ by day $+29$, which we interpret as radiative transfer effects in an evolving reprocessing layer. The IR SED deviates from a thermal blackbody, with $\nu L_{\nu }\propto {\lambda }^{-2.13\pm 0.04}$, significantly shallower than the Rayleigh-Jeans slope of ${\lambda }^{-3}$. We rule out dust as the source of this IR excess. Two possibilities remain: free-free emission from reprocessing gas, or an unresolved stellar cluster at the TDE location. Reprocessed emission provides a natural explanation for the IR excess but an underlying stellar cluster of mass $\log (M_{\ast }/M_{\odot })=7.57\pm 0.02$ and age $<$2 Gyr is also consistent with the data. If interpreted as a stellar cluster, the inferred mass suggests a stripped remnant of a satellite galaxy. The wandering MBH most likely originated in a minor merger with a smaller galaxy, although dynamical ejection from the host nucleus cannot yet be ruled out.

23 pages, 16 figures; comments welcome