AstroNote 2025-66

On 17 Feb 2025 (MJD 60723.32) ZTF detected a new optical transient (ZTF25aaftbug/AT2025cez) with a magnitude of r = 18.98 +/- 0.24. The next night, forced photometry on ZTF images showed that the transient had faded by 0.2 mag in g-band. The closest source in archival Legacy Survey data is a galaxy 3” away, with a photometric redshift of z=0.38.

Due to the rapid fade rate and possible galaxy association, on 18 Feb we obtained a spectrum with Gemini North under ToO Program GN-2025A-Q-129 (PI Ho). The spectrum showed a featureless blue continuum, without the Balmer features that might be expected for a foreground dwarf nova. A spectrum obtained on 22 Feb with SPRAT on the Liverpool Telescope similarly showed no clear features.

On 19 Feb a Swift ToO observation resulted in a non-detection with a 3-sigma upper limit in the 0.3-10 keV band of 1.4E-13 erg/cm2/s. 

Continued ZTF photometry showed steady 0.2 mag/day fading through last night. 

We encourage additional follow-up observations of this transient. 

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.