AstroNote 2025-160

We report the classification of optical transient ZTF25aaptkov/AT 2025kkj and ZTF25aaplxiz/AT 2025lxx as tidal disruption events (TDEs). The sources were selected by the tdescore ML classifier (Stein+2024, doi: 10.3847/2041-8213/ad3337) as candidate TDEs, primarily due to their persistent blue colour and the lack of AGN-like host signatures.

ZTF25aaptkov/AT 2025kkj was first detected by ZTF on 2025-05-09 as an alert, and reported as a transient to TNS on the same day. The transient was rising at the time of discovery, and was blue (g-r=-0.2). There have been no subsequent g-band detections with ZTF, but the r-band lightcurve suggests the transient has risen to peak. The candidate was first identified as a likely TDE by tdescore on 2025-05-21, and assigned for photometric and spectroscopic follow-up. Photometry from the Rainbow Camera of the Palomar 60-inch telescope suggests the transient has remained blue. 

A spectrum of ZTF25aaptkov was taken with the  Low Resolution Imaging Spectrometer (LRIS) at Keck-I on 2025-05-23 (PI: Stein). This spectrum shows a blue continuum, with a strong absorption feature which we identify as Na I at a redshift of z=0.292. At this redshift, Ca II absorption is also visible, but no other obvious features.

ZTF25aaptkov appears to still be rising, with a current magnitude of mr=19.1 as of 2025-05-26. With the measured redshift, the absolute magnitude of ZTF25aaptkov would be Mr=-21.9. 

The luminosity, spectrum, lightcurve evolution, and nuclear location are all indicative of a TDE origin for this flare. We therefore classify ZTF25aaptkov/AT 2025kkj as a likely featureless TDE.

ZTF25aaplxiz/AT 2025lxx was first detected by ZTF on 2025-04-21 as an alert, and reported as a transient to TNS on 2025-05-25. The transient was rising at the time of discovery, though the lightcurve is noisy due to the low SNR of the ZTF detections. The source has remained persistently blue (g-r=-0.3) since discovery with no clear colour evolution. Forced photometry reveals additional early detections of this source, beginning 2025-04-06, with similar blue colour. The candidate was first identified as a likely TDE by tdescore on 2025-05-22, and assigned for spectroscopic follow-up.

A spectrum of ZTF25aaplxiz was taken with the  Low Resolution Imaging Spectrometer (LRIS) at Keck-I on 2025-05-23 (PI: Stein). The spectrum shows a blue continuum, with broad He-II and some H-alpha at a consistent redshift of z=0.145.

ZTF25aaplxiz reached a peak magnitude of mg=20.3 on 2025-05-09, and has since slowly faded. With the apparent redshift, the peak magnitude of ZTF25aaplxiz would be Mg=-19.0. The post-peak lightcurve of ZTF25aaplxiz has continued to stay blue, with no evidence of any cooling. 

The luminosity, spectrum, lightcurve evolution, nuclear location are all indicative of a TDE origin for this flare. We therefore classify ZTF25aaplxiz/AT 2025lxx as a likely TDE-He.

ToO observations have been requested with the Neil Gehrels Swift Observatory for both sources. 

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.