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.
