AstroNote 2025-317

We report the classification of optical transient ZTF25abxvhmk/AT 2025abcr as a tidal disruption event (TDE). The source was selected by an ‘offnuclear’ implementation of the tdescore ML classifier (Stein+2024, doi: 10.3847/2041-8213/ad3337), which searches for TDEs using only transient lightcurve properties without any information about proximity to possible host galaxies. It was selected as a candidate TDE due to its slow evolution and persistent blue colour. It was not selected by the classical tdescore implementation owing to the large offset from any nearby galaxy nucleus. 

ZTF25abxvhmk was first detected by ZTF on 2025-10-13 06:42:19 as an alert, and reported as a transient to TNS on 2025-10-21 10:05:54. The transient was rising at the time of discovery and appears to have peaked on 2025-11-06 with a blue (g-r = -0.3 AB mag) color measured just afterwards. The source was first identified as a likely TDE by tdescore on 2025-11-04 and assigned for spectroscopic follow-up. 

A spectrum taken by the Spectral Energy Distribution Machine on 2025-11-05 showed a blue continuum, but with insufficient SNR for a confident classification. A second spectrum of ZTF25abxvhmk was taken with the Goodman High Throughput Spectrograph on the 4m SOAR telescope on 2025-11-08 (PI: Carney), which exhibited a blue continuum. Broad features H 4861 and H 6563 are seen at a redshift of z=0.05. A broad feature consistent with blended He II 4686+N III 4640 is also present at this redshift. The H-beta line has a velocity width of 2100 km s-1 (FWHM). These features are narrower than typical TDE velocities at this early phase. 

The source is offset 9.5 arcseconds/10.3 kpc (using Pan-STARRS astrometry) from a nearby bright galaxy (WISEA J014656.04-152214.7). The galaxy has a spectroscopic redshift of 0.0498 on NED, identical to the observed redshift of ZTF25abxvhmk and confirming that the transient/galaxy alignment is not a coincidence. This large offset makes ZTF25abxvhmk a good candidate for a TDE by an intermediate mass black hole (IMBH), with a spectrum that bears remarkable similarity to that of the off-nuclear IMBH TDE EP240222a (Jin+ 2025, arXiv:2501.09580). 

The luminosity, spectrum, and persistent blue color are all indicative of a TDE origin for this flare. We therefore classify ZTF25abxvhmk/AT 2025abcr as a likely off-nuclear TDE-H+He.

Further multiwavelength observations are in progress.

Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Award #2407588 and a partnership including Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; University of Wisconsin at Milwaukee, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan; Operations are conducted by Caltech's Optical Observatory (COO), Caltech/IPAC, and the University of Washington at Seattle, USA.