Towards an Understanding of Changing-Look Quasars: An Archival Spectroscopic Search in SDSS

TL;DR

This study systematically searches SDSS data to identify changing-look quasars, revealing that their luminosity changes are mainly due to intrinsic accretion rate variations, challenging previous transient explanations.

Contribution

First systematic archival search in SDSS DR12 for changing-look quasars, discovering three such objects and analyzing their spectral evolution to understand the underlying mechanisms.

Findings

Two new changing-look quasars identified.

Luminosity decline driven by decreasing accretion rates.

Intrinsic dimming favored over dust extinction or transient events.

Abstract

The uncertain origin of the recently-discovered `changing-looking' quasar phenomenon -- in which a luminous quasar dims significantly to a quiescent state in repeat spectroscopy over ~10 year timescales -- may present unexpected challenges to our understanding of quasar accretion. To better understand this phenomenon, we take a first step to building a sample of changing-look quasars with a systematic but simple archival search for these objects in the Sloan Digital Sky Survey Data Release 12. By leveraging the >10 year baselines for objects with repeat spectroscopy, we uncover two new changing-look quasars, and a third discovered previously. Decomposition of the multi-epoch spectra and analysis of the broad emission lines suggest that the quasar accretion disk emission dims due to rapidly decreasing accretion rates (by factors of >2.5), while disfavoring changes in intrinsic dust extinction for the two objects where these analyses are possible. Broad emission line energetics also support intrinsic dimming of quasar emission as the origin for this phenomenon rather than transient tidal disruption events or supernovae. Although our search criteria included quasars at all redshifts and transitions from either quasar-like to galaxy-like states or the reverse, all of the clear cases of changing-look quasars discovered were at relatively low-redshift (z ~ 0.2 - 0.3) and only exhibit quasar-like to galaxy-like transitions.