TDE 2025abcr: A Tidal Disruption Event in the Outskirts of a Massive Galaxy

Abstract

Tidal disruption events (TDEs) have traditionally been discovered in optical sky surveys through targeted searches of nuclear transients. However, it is expected that some TDEs will occur outside the galaxy nucleus, arising from wandering black holes originating in galaxy mergers. Here we present observations of TDE 2025abcr, the first optical TDE discovered in the outskirts of a host galaxy. The TDE was identified by a custom 'off-nuclear' implementation of the ML classifier $\texttt{tdescore}$, which classifies new ZTF transients based on their lightcurves. Follow-up observations confirm that TDE 2025abcr is a TDE-H+He, occurring 9.5$"$ (10.3 kpc projected distance) from the nucleus of a massive galaxy ($\mathrm{M}_{\star}$ = $10^{11.18 \pm 0.03}\mathrm{M}_{\odot}$) with a central black hole mass of $10^{8.82 \pm 0.65}\mathrm{M}_{\odot}$. TDE 2025abcr itself was likely disrupted by a much lighter black hole ($10^{6.09\pm0.53}\mathrm{M}_{\odot}$, as estimated with peak luminosity scaling relations). The black hole was either dynamically ejected from the nucleus or lies at the center of a very faint tidally-stripped dwarf galaxy undergoing a minor merger. Late-time observations of TDE 2025abcr could confirm the origin of this apparent 'orphan' black hole. The rate of highly offset ($\gtrsim$3 kpc) TDEs can be constrained to $<$10% of the nuclear TDE rate, but our discovery implies that many dozens of similar sources will be detected by the Vera C. Rubin each year with resolvable offsets.

Figures (11)

• Figure 1: Top: Archival composite g/r/i image from Legacy Survey DR10, with the position of the transient (blue circle) and host galaxy (marked with the black + symbol). No source is visible at the transient position to a depth of 23.78 mag (see Section \ref{['sec:scarlet']} for more details), ruling out the possibility of a bright dwarf galaxy origin for TDE 2025abcr. Bottom: Composite u/g/i image of TDE 2025abcr, taken with LDT on 2025 -12-14. The transient (circled in blue) is significantly offset (9.5$"$ / 10.3 kpc) from the center of the host galaxy WISEA J014656.04-152214.7.
• Figure 2: Top: Extinction-corrected UV/optical/NIR lightcurve of TDE 2025abcr, alongside a blackbody fit to the data described in Section \ref{['sec:lc']}. The lightcurve is well described by a single black body with a linearly-evolving temperature. Lower Left: 1$\sigma$ uncertainty contour for the best-fit peak temperature and cooling rate. The temperature appears to change very little over time. Lower Right: Inferred blackbody radius as a function of time. The shaded region corresponds to the envelope of inferred radii when sampling the 1$\sigma$ peak temperature/cooling ellipse.
• Figure 3: Spectral sequence of TDE 2025abcr, with times given in rest frame days relative to the peak date. The source exhibits a blue continuum with little apparent cooling, broad H lines and a blended feature of He II ($\lambda4686$) and N III ($\lambda4640$). A comparison spectrum of TDE 2021mhg 2021mhg_tns, a TDE belonging to the same TDE-H+He spectral subclass at a similar phase, is shown in black at the bottom of the figure. The spectral features of TDE 2025abcr resemble this and other known TDEs.
• Figure 4: Bolometric luminosity (orange) from the UV/optical blackbody fit, alongside the X-ray detections (blue circles) and upper limits (blue triangles).
• Figure 5: Comparison of the optical lightcurve properties of TDE 2025abcr (in orange) and the sample of TDEs reported in final_season (in blue). The median of each distribution is illustrated with a dashed line. The best-fit values for TDE 2025abcr differ slightly from those in Figure \ref{['fig:lc']}, because no UV data was included in these fits. Top Left: Best-fit peak temperature versus cooling rate for TDEs, using the thermal model outlined in Section \ref{['sec:lc']} and fitting only ZTF photometric data. Top Right: Best-fit rise time and fade time for the same sample and fitting procedure, with the time defined as days between peak magnitude and 0.5 magnitude below peak. Lower Left: Redshift versus peak absolute $g-$band magnitude. Lower Right: Bolometric luminosity versus blackbody radius. TDE 2025abcr has a typical peak temperature and cooling rate, as well as a typical rise and fade time. In these lightcurve properties, the source is therefore an unremarkable optical TDE. However, the source has a low peak absolute $g-$band magnitude compared to other ZTF TDEs. As a relatively hot blackbody, TDE 2025abcr has a much lower inferred blackbody radius than the overall ZTF population. This supports the interpretation that TDE 2025abcr is a normal optical TDE but arises from a $\sim$$10^{6}$$\mathrm{M}_{\odot}$ BH, as argued in more detail in Section \ref{['sec:lc']}.