AstroNote 2025-212

We report the classification of optical transients ZTF25aaxdtdm/AT2025own  and ZTF25aaooewz/AT2025iuh as tidal disruption events (TDEs). 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 persistent blue colour and the lack of AGN-like host signatures.

ZTF25aaxdtdm/AT2025own was first detected by ZTF on 2025-06-24 as an alert, and independently reported to TNS as a supernova candidate on 2025-06-25 by the XOSS group (KATS25M018, Zhang et al., TNS Discovery Report No. 2025-2398). The transient was fading at the time of ZTF discovery, and was blue (g-r=-0.25). A request for forced photometry reveals no recent ZTF observations of this source before discovery, so we cannot constrain its prior evolution. The candidate was first identified as a likely TDE by tdescore on 2025-06-27, and assigned for spectroscopic follow-up.

A spectrum of ZTF25aaxdtdm was taken with the Low Resolution Imaging Spectrometer (LRIS) at Keck-I on 2025-06-28 (PI: Kasliwal). This shows a blue continuum with narrow emission lines (H, [O III]) at a consistent redshift of z=0.1535. 

ToO observations were requested with the Neil Gehrels Swift Observatory, and were conducted on 2025-07-09. These observations reveal a bright UV source at the location of ZTF25aaxdtdm, which is much brighter than would be expected from the (red) host galaxy. After correcting for galactic extinction, we find that ZTF25aaxdtdm is bright in all UV bands (e.g m_W2=19.4 AB). These observations suggest that ZTF25aaxdtdm is UV-bright, as expected for a TDE.

ZTF25aaxdtdm was first detected at a magnitude of m_g=19.4 on 2025-06-27, and has since slowly faded. We cannot constrain the true peak magnitude, but with the apparent redshift, the first detection suggests that it must be at least M_g=-19.9. The post-peak lightcurve of ZTF25aaxdtdm has not demonstrated any evidence of any cooling. 

There was no Swift-XRT detection of ZTF25aaxdtdm, yielding an upper limit of 8.2 x 10^-3 cts/s at 0.3 - 10 keV. Given the redshift of z = 0.1535, this gives a luminosity upper limit of approximately 2.4 x 10⁴³ erg/s.

The luminosity, spectrum, lightcurve evolution, nuclear location and UV brightness are all indicative of a TDE origin for this flare. We therefore classify ZTF25aaxdtdm/AT2025own as a likely featureless TDE.

ZTF25aaooewz/AT2025iuh was first detected by ZTF on 2025-04-23. It had previously been discovered and reported to TNS by GOTO (GOTO25ces; O’Neill et al., TNS Discovery Report No. 2025-1594) with a detection on 2025-04-18. The transient was blue (g-r =-0.2). The candidate was first identified as a likely TDE by tdescore on 2025-05-01, and assigned for spectroscopic follow-up.

We obtained several epochs of spectroscopy of ZTF25aaooewz, starting 2025-05-02, all of which were dominated by a blue continuum. An LRIS spectrum taken on 2025 June 25 (PI: Chornock) additionally exhibits broad emission lines at H-alpha and near He II 4686. 

ToO observations were requested with the Neil Gehrels Swift Observatory, and were conducted on 2025-05-09 and 2025-06-11. These observations reveal a bright UV source at the location of ZTF25aaooewz, which is much brighter than would be expected from the (red) host galaxy. After correcting for galactic extinction, we find that ZTF25aaooewz is bright in all UV bands (e.g m_M2=19.1 AB), with little apparent cooling between the epochs. These observations suggest that ZTF25aaooewz is UV-bright, as expected for a TDE.

The lightcurve of ZTF25aaooewz had relatively few observations in ZTF data, but appeared to peak around the first Swift epoch. With the measured redshift, ZTF25aaooewz had a peak magnitude of at least M_M2 = -20.7 in Swift data, and M_g = -20.5 in ZTF data. 

There was no Swift-XRT detection of ZTF25aaooewz, yielding an upper limit of 3.5 x 10^-3 cts/s at 0.3 - 10 keV. Given the redshift of z = 0.182, this gives a luminosity upper limit of approximately 1.4 x 10⁴³ erg/s.

The WISE host colours are W1-W2 = 0.45 and there is some evidence of past WISE variability, indicating a possible AGN nature for the host. However, the luminosity, spectrum, lightcurve evolution, nuclear location, lack of past optical variability in ZTF imaging, and UV brightness of ZTF25aaooewz are more compatible with a TDE origin for this flare. We therefore classify ZTF25aaooewz/AT2025iuh as a likely TDE-H+He. 

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.