AstroNote 2025-251

We report the classification of optical transient ZTF25abcemrc/AT2025spf as a tidal disruption event (TDE). The source was selected by the tdescore ML classifier (Stein+2024, doi: 10.3847/2041-8213/ad3337) as a candidate TDE, primarily due to its nuclear location, persistent blue colour and the lack of AGN-like host signatures.

ZTF25abcemrc/AT2025spf was first detected by ZTF on 2025-07-20 as an alert, and reported as a transient to TNS on 2025-07-29 (Stein et al., TNS Discovery Report No. 2025-2978). The transient was rising at the time of discovery, and has remained persistently blue (g-r=-0.3) after accounting for the galactic extinction. Forced photometry reveals additional early detections of this source, beginning 2025-07-04, with similar blue colour. The candidate was first identified as a likely TDE by tdescore on 2025-07-27, and assigned for spectroscopic follow-up.

A spectrum of ZTF25abcemrc was taken with the Low Resolution Imaging Spectrometer (LRIS) at Keck-I on 2025-07-28 (PI: Stein), revealing a blue continuum without clear emission lines. From CaII absorption features, we derive a redshift of z=0.189.

ToO observations were requested with the Neil Gehrels Swift Observatory on 2025-08-02. These observations reveal a bright UV source at the location of ZTF25abcemrc, which is much brighter than would be expected from the (red) host galaxy. After correcting for extinction, these observations suggest that ZTF25abcemrc is UV-bright, as expected for a TDE.

ZTF25abcemrc reached a peak magnitude of mg=20.1 on 2025-07-29, but there are no recent detections in ZTF to constrain further evolution. With the apparent redshift (z=0.189), the peak magnitude of ZTF25abcemrc would be Mg=-19.9. 

The luminosity, spectrum, lightcurve evolution, nuclear location and UV brightness are all indicative of a TDE origin for this flare. We therefore classify ZTF25abcemrc/AT2025spf as a likely TDE, though given the SNR of the spectrum we cannot confidently constrain the spectral subclass. There is no evidence of broad H-alpha, but both TDE-featurless and TDE-He classifications would be plausible. 

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.