AstroNote 2024-369

We report the classification of optical transient ZTF24abmaujk/AT 2024ymu 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 their persistent blue colour and the lack of AGN-like host signatures.

ZTF24abmaujk/AT 2024ymu was first detected by ZTF on 2024-10-01 as an alert, and reported as a transient to TNS on 2024-10-20. The transient was rising at the time of discovery, and has remained persistently blue (g-r=-0.5) since discovery with no clear colour evolution. Forced photometry reveals additional early detections of this source, beginning 2024-09-22, with similar blue colour. The candidate was first identified as a likely TDE by tdescore on 2024-11-08, and assigned for spectroscopic follow-up.

The first spectrum of ZTF24abmaujk was taken with the Double Spectrograph at the Palomar 200-in Hale Telescope on 2024-11-10 (PI: Kasliwal), revealing a blue continuum without clear features. A second higher-SNR spectrum was taken with the Goodman High Throughput Spectrograph on the 4m SOAR telescope on 2024-11-18 (PI: Andreoni), from which a tentative redshift of z=0.12 was derived. For this redshift, Na I absorption lines appear to be visible. A third spectrum was taken with the  Low Resolution Imaging Spectrometer (LRIS) at Keck-I on 2024-11-25 (PI: Stein). Despite poor observing conditions, this third spectrum shows a similar blue continuum with little apparent cooling and indications of Na I absorption at the same redshift.

ToO observations were requested with the Neil Gehrels Swift Observatory, beginning on 2024-11-24, and continuing at weekly cadence. These observations reveal a bright UV source at the location of ZTF24abmaujk, which is much brighter than would be expected from the (red) host galaxy. These observations suggest that ZTF24abmaujk is UV-bright, as expected for a TDE.

ZTF24abmaujk reached a peak magnitude of mg=18.1 on 2024-10-28, and has since slowly faded. With the apparent redshift, the peak magnitude of ZTF24abmaujk would be/Mg=-20.7. The post-peak lightcurve of ZTF24abmaujk has continued to stay extremely blue, with no evidence of any cooling. This is seen both for the optical ZTF lightcurve and the UV detections with Swift. The source remains optically bright, at a g-band magnitude of 18.9 as of 40 days post-peak.

The luminosity, spectrum, lightcurve evolution, nuclear location and UV brightness are all indicative of a TDE origin for this flare. We therefore classify ZTF24abmaujk/AT 2024ymu as a likely featureless TDE.

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, University of North Carolina at Chapel Hill, Northwestern University and Drexel University. Operations are conducted by COO, IPAC, and UW.