The Discovery of the First "Changing Look" Quasar: New Insights into the Physics & Phenomenology of AGN

TL;DR

This paper reports the discovery of the first known 'changing look' quasar, SDSS J015957.64+003310.5, which transitioned from a Type 1 to Type 1.9 AGN, providing new insights into AGN physics and variability.

Contribution

It presents the first observation of a distant quasar undergoing a dramatic spectral change, challenging simple obscuration models and highlighting intrinsic luminosity variations.

Findings

The quasar's flux decreased by a factor of 6 from 2000 to 2010.

X-ray and optical flux changes are correlated, with no signs of obscuration.

The change is driven by intrinsic dimming of the AGN continuum.

Abstract

SDSS J015957.64+003310.5 is an X-ray selected, $z=0.31$ AGN from the Stripe 82X survey that transitioned from a Type 1 quasar to a Type 1.9 AGN between 2000 and 2010. This is the most distant AGN, and first quasar, yet observed to have undergone such a dramatic change. We re-observed the source with the double spectrograph on the Palomar 5m telescope in July 2014 and found that the spectrum is unchanged since 2010. From fitting the optical spectra, we find that the AGN flux dropped by a factor of 6 between 2000 and 2010 while the broad H$\alpha$ emission faded and broadened. Serendipitous X-ray observations caught the source in both the bright and dim state, showing a similar 2-10 keV flux diminution as the optical while lacking signatures of obscuration. The optical and X-ray changes coincide with $g$-band magnitude variations over multiple epochs of Stripe 82 observations. We demonstrate that variable absorption, as might be expected from the simplest AGN unification paradigm, does not explain the observed photometric or spectral properties. We interpret the changing state of J0159+0033 to be caused by dimming of the AGN continuum, reducing the supply of ionizing photons available to excite gas in the immediate vicinity around the black hole. J0159+0033 provides insight into the intermittency of black hole growth in quasars, as well as an unprecedented opportunity to study quasar physics (in the bright state) and the host galaxy (in the dim state), which has been impossible to do in a single sources until now.