We report the discovery of the optical transient ZTF25aafwfzz/AT2025cyh as a possible 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 their persistent blue colour and the lack of AGN-like WISE colors or variability.
ZTF25aafwfzz/AT2025cyh was first detected by ZTF on 2025-02-15 as an alert, and reported as a transient to TNS on 2025-02-28. The transient was rising at the time of discovery, and has remained persistently blue (g-r=-0.32) since discovery with no clear color evolution. Forced photometry reveals additional early detections of this source with similar blue color. The candidate was first identified as a likely TDE by tdescore on 2025-02-20, and assigned for spectroscopic follow-up. ZTF25aafwfzz reached a peak magnitude of mg=19.13 on 2025-02-23, and has since begun to fade. The post-peak lightcurve of ZTF25aafwfzz has continued to stay extremely blue, with no evidence of any cooling. The source remains optically bright, at a g-band magnitude of 19.3 as of ~15 days post-peak.
A spectrum of ZTF25aafwfzz was taken with the Low Resolution Imaging Spectrometer (LRIS) Keck-I Telescope on 2025-03-05 (PI: Chornock), revealing a blue continuum with narrow lines of hydrogen, sulfur, nitrogen, neon, and oxygen but also broad NIII 4640, 4101 and possible He II 4686. The source redshift was determined to be z=0.105. The classification of this source remains ambiguous given the narrow emission line features consistent with the AGN-like line ratios but lack of strong AGN-like WISE colors and variability. We present this source as a possible TDE candidate, but it may also belong to other classes of nuclear transients, including the Bowen Fluorescence Flares (BFFs) put forth by Trakhtenbrot et al. 2019.
The luminosity, spectrum, lightcurve evolution, nuclear location are all indicative of a possible TDE origin for this flare. Swift UV and X-ray observations have been requested to further characterize the nature of this transient.
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.
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