AstroNote 2025-50

We report the classification of optical transients ZTF24abwsfdk/AT 2024admt and ZTF25aabebve/AT 2024agqx as tidal disruption events (TDEs). Both 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. 

ZTF24abwsfdk/AT 2024admt was first detected by ZTF on 2024-12-02 as an alert, and reported as a transient to TNS on 2024-12-06 by ALeRCE. The transient was rising at the time of discovery, and 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 2024-11-08, with similar blue colour. The candidate was first identified as a likely TDE by tdescore on 2024-12-13, and assigned for spectroscopic follow-up.

The first spectrum of ZTF24abwsfdk was taken with the Lowell Discovery Telescope on 2024-12-29 (PI: Hammerstein), revealing a blue continuum but with low SNR. Two additional higher-SNR spectra was taken with the Goodman High Throughput Spectrograph on the 4m SOAR telescope on 2025-01-03 and 2025-01-23 (PI: Andreoni), from which a redshift of z=0.111 was derived using host galaxy star light absorption features. No narrow emission lines are detected. A broad emission line consistent with He II 4686 appears to be present.

ToO observations were requested with the Neil Gehrels Swift Observatory, beginning on 2025-01-22. These observations reveal a bright UV source at the location of ZTF24abwsfdk, which is much brighter than would be expected from the (red) host galaxy. Archival SDSS imaging of the host galaxy indicates a u-band magnitude of 22.4, while our Swift U-band observations reveal a point source with m=19.6. These observations suggest that ZTF24abwsfdk is UV-bright, as expected for a TDE.

ZTF24abwsfdk reached a peak magnitude of m_g=19.3 on 2024-12-24, and has since slowly faded. With the apparent redshift, the peak magnitude of ZTF24abwsfdk would be M_g=-19.3. The post-peak lightcurve of ZTF24abwsfdk has continued to stay blue, with no evidence of any cooling. This is seen both for the optical ZTF lightcurve and the UV detections with Swift. 

The luminosity, spectrum, lightcurve evolution, nuclear location and UV brightness are all indicative of a TDE origin for this flare. We therefore classify ZTF24abwsfdk/AT 2024admt as a likely TDE-He event.

ZTF25aabebve/AT 2024agqx was first detected by ZTF on 2024-12-30 as an alert, and reported as a transient to TNS on 2025-01-19. The transient was rising at the time of discovery, and has remained persistently blue (g-r=-0.3) since discovery with no clear colour evolution. The candidate was first identified as a likely TDE by tdescore on 2025-01-22, and assigned for spectroscopic follow-up.

A spectrum of ZTF25aabebve was taken with the Goodman High Throughput Spectrograph on the 4m SOAR telescope on 2025-01-27 (PI: Andreoni), revealing a blue continuum. Ca II absorption lines appear to be present, from which a redshift of z=0.194 was derived. At this redshift, there is some evidence for broad He-II emission. However, in either case, the blue continuum is consistent with a TDE nature for this transient.

ZTF25aabebve reached a peak magnitude of m_g=19.6 on 2025-01-24, and has since slowly faded. With the apparent redshift, the peak magnitude of ZTF25aabebve would be M_g=-20.3. The post-peak lightcurve of ZTF25aabebve has continued to stay blue, with no evidence of any cooling. 

The luminosity, spectrum, lightcurve evolution, and nuclear location are all indicative of a TDE origin for this flare. We therefore classify ZTF25aabebve/AT 2024agqx as a TDE, likely belonging to the TDE-He subclass. 

ToO observations of ZTF25aabebve have been requested with the Neil Gehrels Swift Observatory.

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 Grant No. AST-2034437 and a collaboration including Caltech, IPAC, the Oskar Klein Center at Stockholm University, the University of Maryland, University of California, Berkeley , the University of Wisconsin at Milwaukee, University of Warwick, Ruhr University Bochum, Cornell University, Northwestern University and Drexel University. Operations are conducted by COO, IPAC, and UW.