AstroNote 2026-229

The source was first identified in the ZTF alert stream on 2026-06-17 12:46:41 UT (MJD 61208.53) with a detection at i = 17.41 ± 0.02 AB mag earlier that night (MJD 61208.45; r = 17.38 ± 0.02 at MJD 61208.39). ZTF forced photometry reveals an initial phase of activity beginning around MJD 61181 (2026-05-21, r ~ 19.9), during which the source was first reported to TNS (2026TNSTR2576....1R). It subsequently faded before this new episode of activity. On MJD 61200.45 it was at g = 20.1 and r = 20.0 AB mag, after which it rose rapidly, brightening to g ~ 17 and r ~ 17.2 within ~9 days (~0.4 mag/d in g and ~0.3 mag/d in r). The light curve has since settled onto a plateau at g ~16.8 but continues to rise in r and i, showing undulations of ~0.1-0.2 mag across gri. While the g band has remained roughly constant on that plateau, the source has become progressively redder, with g-r color evolving from g-r = -0.15 at the end of the rise (~61209 MJD) to g-r = +0.25 mag at present (~61237 MJD).

We obtained optical spectra of the transient with the Next Generation Palomar Spectrograph (NGPS; #2024–340) on the Palomar 200-inch telescope on 2026-06-18, 2026-07-10, and 2026-07-16, as part of the Z+LIONS (#2026-43) survey. All three spectra show a featureless continuum, with the only detected features being narrow host-galaxy emission lines - Ha, [N II] 6548, 6583 A, and [S II] 6717 A (rest)- at the NED host redshift of z = 0.029297.

The transient location is ~2” away from the nucleus of its host galaxy in LS, which appears to be an elliptical in archival imaging. The host shows some evidence of prior variability in forced photometry from ZTF (spanning 2018-present), however, the current evolution significantly deviates from the previous activity. At z=0.029297 (D_L ~ 133 Mpc, DM~ 35.6 mag for H0 = 67.4 km/s/Mpc), the current plateau corresponds to an absolute magnitude of M_r ~ -19, and the transient continues to redden while remaining roughly constant in g. The combination of a bright, featureless, blue-to-red continuum, nuclear location, and persistent plateau with undulations does not point to an obvious classification, and we encourage further photometric and spectroscopic follow-up of this source.

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.