Changing-look Active Galactic Nuclei. I. Tracking the Transition on the Main Sequence of Quasars

This paper is the first in a series of papers that prepare and analyze spectral and other properties for a database of already-discovered changing-look active galactic nuclei (CL AGNs). Here we focus on the spectral fitting and analysis of broad emission lines in a sample of 93 CL AGNs collected from the literature with existing Sloan Digital Sky Survey (SDSS)/Baryon Oscillation Spectroscopic Survey (BOSS)/extended-BOSS spectroscopy where the H beta emission-line profile does not completely disappear in any epochs. Additionally, we have gathered older/newer spectral epochs from all the available SDSS data releases to make the database more complete. We use PyQSOFit and perform a homogeneous spectral decomposition of all of our SDSS spectra and tabulate the AGN continuum and emission-line properties per epoch per source, chronologically. This further allows us to categorize the sources in our sample as Turn-On or Turn-Off and subsequently check for repeated occurrences of such phases. We then estimate the black hole mass (M BH) and the Eddington ratio (lambda Edd) per epoch per source where the required parameters are available and well estimated. We demonstrate the movement of the source in the M BH-lambda Edd plane, allowing us to check for systematic changes in the source's fundamental properties. We then track their transition along the optical plane of the Eigenvector 1 schema and categorize sources that either stay within the same population (A or B) or make an interpopulation movement as a function of spectral epoch. We also test the Balmer decrement (H alpha/H beta) of a subset of our sample of CL AGNs as a function of time and AGN luminosity.