AstroNote 2025-327

We report spectroscopic and photometric observations of the event AT2025zmb/ZTF25abuovxv, conducted using the Kast spectrograph on the Shane 3m telescope at Lick Observatory on the night of 2025-11-21 and Liverpool Telescope IO:O on the night of 2025-11-25. The event's photometric colours and spectral appearance motivate us to consider this event to be a candidate featureless Tidal Disruption Event (TDE). The observations were performed as part of our classification programme for transients discovered by the Zwicky Transient Facility (ZTF).

The red side of KAST was set to use the 300/7500 grating, with a dispersion of 2.55 Å/pixel, and the blue side used the 600/4310 grism, with a dispersion of 1.02 Å/pixel. The observations were taken with 2.0'' slit; the seeing on site was > 2.0'' for most of the night. The spectra were reduced using the UCSC Spectral Pipeline (Siebert et al. 2019), and the obtained spectral range is approx. 3515-10220 Å.

We have also observed the object with the Liverpool Telescope optical camera IO:O on the night of 2025-11-25; the observations were taken in the SDSS ugriz filters and started at MJD 61005.1378. The image subtraction, especially in redder bands, is affected by a nearby red star. The obtained apparent magnitudes (AB) are u = 18.61 ± 0.05, g = 18.72 ± 0.12, r = 18.80 ± 0.14, i = 18.90 ± 0.19, and z = 19.09 ± 0.27.

AT 2025zmb/ZTF25abuovxv was first reported to TNS by the ZTF alert stream (first reported to TNS at UT 2025-10-02), it was later identified as a candidate SLSN or TDE by NEEDLE (Sheng et al. 2025; AstroNote 2025-307) with similar scores for each classification, and as a candidate SLSN by ZTF (Sankar et al. 2025; AstroNote 2025-310). AT 2025mzb displays a long rise (+60 days), shows a potential pre-rise activity for around ~15 days, and appears to be nuclear. The probable host's WISE colors are w1 - w2 = 0.177 and w2 - w3 = 2.731 (Cutri et al. 2013), suggesting it is a star-forming galaxy with no evidence of an AGN (Wright et al. 2010, Stern et al. 2012). The host has a photometric SDSS redshift of z = 0.339 ± 0.021. At this redshift, photometry measured in the observed SDSS u filter corresponds to the rest-frame near-UV, and the remaining filters are also shifted substantially to bluer rest frames, for example the SDSS i and z bands bracket the rest-frame r-band. The peak absolute magnitude of AT 2025zmb, corrected for Galactic extinction of E(B-V) = 0.38, is calculated to be between approximately M_NUV,rest= -23.85 and M_NUV,rest = -24.23 (AB). After correcting for Galactic extinction and redshift effects, we estimate (NUV - r)_rest = -1.44 ± 0.32. This value, compared to the (NUV - r) vs M_NUV distribution from Gezari 2021, would place the object firmly in the TDE regime. Additionally, the peak absolute magnitude of this event would be consistent with events presented in Yao et al. 2025sample of optically overluminous TDEs.

The spectrum (attached) shows no clear features; the absorption line at 5896 Å is galactic Na D and is consistent with the high galactic extinction at this location; it is also the only absorption feature we identify. There is a potential broad emission feature around 5400 Å, but we cannot match it to known lines; it is also close to the edge of the blue CCD and could be due to noise/edge effects. We therefore consider AT 2025mzb to be a strong candidate to be an optically overluminous featureless TDE.

We encourage additional multiwavelength follow-up of this object.

We would like to thank the staff at Lick Observatory for enabling these observations, in particular Norm Jackson.

Based on observations made with the Liverpool Telescope operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias with financial support from the UK Science and Technology Facilities Council.

Based on observations obtained with the Samuel Oschin Telescope 48-inch 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, and OKC, University of Stockholm, Sweden. Operations are conducted by Caltech's Optical Observatory (COO), Caltech/IPAC, and the University of Washington at Seattle, USA.