A systematic search for changing-look quasars in SDSS-II using difference spectra.

"Changing-look quasars" (CLQs) are active galactic nuclei (AGN) showing extreme variability that results in a transition from Type 1 to Type 2. The short timescales of these transitions present a challenge to the unified model of AGN and the physical processes causing these transitions remain poorly understood. CLQs also provide interesting samples for the study of AGN host galaxies since the central emission disappears almost entirely. Previous searches for CLQs have utilised photometric variability or SDSS classification changes to systematically identify CLQs, this approach may miss lower luminosity CLQs. In this paper, we aim to use spectroscopic data to asses if analysis difference spectra can be used to detect further changing look quasars missed by photometric searches. We search SDSS-II DR 7 repeat spectra for sources that exhibit either a disappearance or appearance of both broad line emission and accretion disk continuum emission by directly analysing the difference spectrum between two epochs of observation. From a sample of 24,782 objects with difference spectra, our search yielded six CLQs within the redshift range $0.1 \leq z \leq 0.3$, including four newly identified sources. Spectral analysis indicates that changes in accretion rate can explain the changing-look behaviour. While a change in dust extinction fits the changes in spectral shape, the time-scales of the changes observed are too short for obscuration from torus clouds. Using difference spectra was shown to be an effective and sensitive way to detect CLQs. We recover CLQs an order of magnitude lower in luminosities than those found by photometric searches and achieve higher completeness than spectroscopic searches relying on pipeline classification.