Spectral Evolution and Transient Broad-Line Features in the Isolated AGN UNAM-KIAS 613

Abstract We present multi-epoch optical spectroscopy of the isolated elliptical galaxy UNAMKIAS 613, hosting a low-luminosity Type 1 AGN. Analysis of archival Sloan Digital Sky Survey (SDSS) data from 2006 reveals a distinctive double-peaked broad Hα profile, tentatively modeled by a relativistic accretion disk. Follow-up observations in 2018 and 2023 show the disappearance of the red and blue wings, leaving only a single-peaked, central broad component. No significant continuum variability is detected in ASASSN and Catalina light curves over 2012–2025, and multi-wavelength data (radio, midIR, Xray) confirm a subEddington, radioquiet AGN (Eddington ratio ≈ 0.03-0.04, black hole mass ≈107.2 M⊙). We propose that the double-peak structure is in reality transient, and arose from a onetime bipolar outflow event rather than a stable disk or from a Tidal Disruption Event. The midIR SED and radio luminosity place UK 613 on the boundary between AGN and star formation dominance, suggesting residual star formation, while we have found that the isolated environment seems to be prone to the rejuvenation of ellipticals by recent (≲ 1 Gyr) cold gas. We also examined its location within the cosmic web with the aim of identifying possible distinctive effects imprinted on its spectroscopic properties. Ultimately, our results are consistent that UNAM-KIAS 613 might have undergone a “turn-off” of the accretion disk emitting region or a transition between a radiatively-inefficient and radiatively-efficient accretion mode, and highlight the complex interplay of disk, outflow, host processes and environment in lowaccretion, low-black hole mass AGNs, an AGN population still largely unexplored to-date.