AstroNote 2025-286

The nuclear transient AT 2022aeys/ZTF22aaqccka began rising in ZTF forced photometry by 2024 December 31 (MJD=60675), when it was detected at g=19.4 +/- 0.1 AB mag (the 2022 name likely refers to a spurious detection), although there are previous marginal detections in forced photometry that are near the detection threshold. The transient location is consistent with the nucleus of the host galaxy (SDSS J002440.72-003019.9), which was previously observed spectroscopically by BOSS (MJD = 51782) and DESI (Target ID = 39627772749945001; MJD=59582). Both archival spectra are dominated by star light from an older, quiescent stellar population at redshift z=0.043. Weak narrow [N II] 6583 is the only emission line detected in those spectra. 

The transient has been slowly rising for ~300 days and appears to be nearing a peak now at g~17.9 AB mag. We triggered multiple observations with Swift/UVOT that revealed very blue colors. For example, on 2025 Aug 20.6 (UT) the transient had a color W2-g ~ -0.35 mag. The source is undetected with Swift/XRT, with a stacked upper limit in 4.3 ks total exposure of f_x(0.3-10 keV) < 1.8e-13 erg/cm2/s, assuming a Gamma=2 power-law spectrum, corresponding to a luminosity limit of L_X < 8.2e41 erg/s.

We have obtained a series of optical spectra starting 2025 Aug 02 through 2025 Sep 15 with the SEDM, the Kast spectrograph on the Lick 3 m telescope, the Goodman spectrograph on SOAR, and Keck-I/LRIS. All spectra exhibit the emergence of broad Balmer lines (FWHM~2700 km/s) with complex line profiles. After subtraction of the host contribution using the BOSS spectrum as a template, broad He II 4686 is also apparent (FWHM ~ 4000 km/s). In addition to the broad lines, resolved (FWHM ~ 600 km/s) emission lines including [O III], [Ne III], and [Ne V] are now apparent that were not previously detected in the archival BOSS or DESI spectra.

The very long rise and extremely blue UV-optical colors for hundreds of days would be unusual for a supernova. The long rise and development of the narrow line emission are unlike tidal disruption events. The host galaxy was detected by WISE with W1-W2=0.01 mag, which is consistent with star light and inconsistent with the presence of any luminous obscured AGN. WISE observations over the decade before the current outburst show no sign of variability.

We therefore classify AT 2022aeys/ZTF22aaqccka as an AGN that is turning on (i.e., experiencing an increase in the accretion rate). There may be some similarity with the event in SDSS1115+0544 (Yan et al., 2019, ApJ, 874, 44). 

We encourage further multiwavelength observations to better characterize the ongoing outburst.