AstroNote 2026-139

The nuclear transient AT 2025alls was originally discovered in ZTF photometry on December 11, 2025, by the Fritz platform (Sollerman et al., AT Discovery Report 287482; also designated ZTF25achvfus). The ZTF light-curve (e.g., here) indicates that by early February 2026, the source brightened from an apparent g-band AB magnitude of g~19.8 (measured as late as end of November 2025) to g~19. This constitudes a >0.5 mag increase in total apparent g-band magnitude within ~2 months. The ZTF light-curve then remained rather flat, at least through March 2026 (the currently available data reaches April 9, 2026).

Following this transient discovery, we obtained two new optical spectra, on February 8 and 24, 2026, using the BOSS spectrograph on the Sloan Foundation 2.5m Telescope, located at the Apache Point Observatory. These two spectra were obtained as part of the new rapid target-of-opprotunity (ToO) program within the on-going 5th generation of the Sloan Digital Sky Survey (SDSS-V; Kollmeier et al. 2026, AJ, 71, 52). The SDSS-V/ToO program aims to obtain optical spectroscopy of recently identified transients, whenever the main SDSS-V survey operations and fiber assignment allow for this, with either one of the relevant SDSS-V facilities (at the Apache Point and Las Campanas observatories).

Another SDSS-V spectrum was obtained nearly 5 years prior to the transient detection, on January 13, 2021, as part of the effort to survey X-ray emitting sources in the eROSITA Final Equatorial-Depth Survey (eFEDS; Aydar et al. 2025, A&A 698, 132). This pre-flare SDSS-V spectrum is publicly available through SDSS/DR19 (SDSS Collaboration 2025, arXiv:2507.07093), and yields a redshift of z=0.269719, refined through visual inspection. Yet another, earlier, archival spectrum was obtained on February 28, 2009 as part of the GAMA survey (Liske et al. 2015, MNRAS 452, 2087; available through VizieR). 

All the spectra in hand consistently yield a systemic redshift of z=0.2697, based on prominent narrow emission lines (SDSS-V errors on redshift are <1e-4).

The pre-flare spectra of this source show clear broad Halpha line emission, and hints of broad Hbeta line emission. Moreover, the X-ray emission detected by eROSITA (corresponding to L(0.25-2.9 keV)~7e42 erg/s in eFEDS, November 2019, and 1.3e43 erg/s in eRASS1, May 2020), further supports the classification of this source (prior to the flare) as an active galactic nucleus (AGN). The high-quality pre-flare SDSS-V spectrum also shows  continuum emission dominated by the host galaxy of the AGN, including stellar absorption features (CaII H+K). 

The post-flare SDSS-V/ToO spectra show much stronger broad (Balmer) line emission, including discernible Hgamma, Hdelta and He I λ5876, which were not seen in the pre-flare spectrum. The continuum emission is also markedly stronger and bluer, with no sign of the stellar features seen in the pre-flare spectrum. Another broad emission feature, centered at roughly 4660A, could be associated with the He II λ4686 and/or N III λ4640 transitions, possibly related to Bowen fluorescence--as seen in some flaring AGN and TDEs. There is some evidence for some further variability of these broad emission features between the two post-flare SDSS-V/ToO spectra, suggesting rapid spectral evolution on timescales of <1 month in the rest-frame of the AGN. The spectra are being analyzed by members of the SDSS-V collaboration.

We conclude that AT 2025alls is a flaring event in an already-active galactic nucleus (i.e., an already-accreting SMBH), with spectroscopic changes that resemble those seen in changing-look AGN, and suggesting that the event is most likely driven by an extreme change to the accretion flow that feeds the central SMBH.

Given the design of the SDSS-V survey, we cannot guarantee another spectrum will be obtained in the (near) future. In order to monitor the evolving spectral behavior, we encourage follow-up observations, including multi-wavelength ones. Follow-up observations in the X-ray and UV were carried out in April 2026, by members of the SDSS-V collaboration.

Funding for the Sloan Digital Sky Survey V has been provided by the Alfred P. Sloan Foundation, the Heising-Simons Foundation, the National Science Foundation, and the Participating Institutions. SDSS acknowledges support and resources from the Center for High-Performance Computing at the University of Utah. SDSS telescopes are located at Apache Point Observatory, funded by the Astrophysical Research Consortium and operated by New Mexico State University, and at Las Campanas Observatory, operated by the Carnegie Institution for Science. The SDSS web site is www.sdss.org .