AstroNote 2025-325

We report the identification of optical transient ZTF25absjioz/AT 2018mqa 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 a lack of cooling and the lack of AGN-like host signatures.

ZTF25absjioz/AT 2018mqa was first detected by ZTF on 2025-09-20 as an alert, and reported as a transient to TNS on 2025-10-18. The transient was rising at the time of discovery, and has maintained a constant colour (g-r=~0.0) since discovery with no significant cooling. Alert forced photometry reveals additional early detections of this source, beginning 2025-09-01, with similar colour. The transient is located in a nearby galaxy at 63 Mpc, and was selected by the CATS150 program (Das. et al, Astronote 2025-64) on 2025-10-07, and was assigned for spectroscopic follow-up. The candidate was independently identified as a candidate TDE by tdescore on 2025-10-16.

We note that this transient was also detected by ZTF under the name ZTF18acmqevd. The automated TNS submissions include ZTF detections in 2018/19 taken under poor conditions that were likely spurious. There are many low-SNR detections in archival ZTF forced photometry, but this is not uncommon for a bright galaxy nucleus. We find no evidence of any coherent flare in past ZTF data which would be comparable to the current transient.

ZTF25absjioz reached a peak magnitude of mg=18.2 on 2025-10-17, and has since slowly faded. With the apparent redshift, the peak magnitude of ZTF25absjioz would be Mg=-15.8. The post-peak lightcurve of ZTF25absjioz has slowly faded with no evidence of cooling.

Multiple spectra of the source were taken with the low-resolution P60/SEDm, and with the red arm of the P200/NGPS, but these were insufficient for a classification. A higher resolution spectrum of ZTF25absjioz was taken with NOT/ALFOSC on 2025-11-17 (PI: Das). The spectrum was mostly dominated by galaxy light. After subtraction with an archival SDSS spectrum, we find that the continuum is bluer than before, and exhibits a broad feature consistent with He 4686. This would be compatible with a faint TDE-He. By contrast, there are no spectroscopic signatures associated with either a supernova or an AGN.

ToO observations were requested with the Neil Gehrels Swift Observatory with a single 2ks snapshot on 2025-11-07 spanning all UV filters. These observations reveal a UV source at the location of ZTF25absjioz, but the flux is consistent with archival measurements of the host galaxy (e.g. SDSS u = 16.2 mag, GALEX FUV = 19.3 mag). We therefore rule out any luminous UV emission. However, given the faint nature of the transient relative to the host galaxy, we cannot easily recover low-luminosity UV emission from this source. No X-ray emission was detected in the concurrent XRT observations.

Based on the spectrum, lightcurve, and nuclear location, we tentatively classify ZTF25absjioz as a possible TDE-He. To partially explain the lack of UV emission and lower than average luminosity, we further suggest that ZTF25absjioz may be dust-obscured.

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.