Abstract
ABSTRACTWe present new Gemini/GMOS optical spectroscopy of 16 extreme variability quasars (EVQs) that dimmed by more than 1.5 mag in the g band between the Sloan Digital Sky Survey (SDSS) and the Dark Energy Survey epochs (separated by a few years in the quasar rest frame). These EVQs are selected from quasars in the SDSS Stripe 82 region, covering a redshift range of 0.5 < z < 2.1. Nearly half of these EVQs brightened significantly (by more than 0.5 mag in the g band) in a few years after reaching their previous faintest state, and some EVQs showed rapid (non-blazar) variations of greater than 1–2 mag on time-scales of only months. To increase sample statistics, we use a supplemental sample of 33 EVQs with multi-epoch spectra from SDSS that cover the broad Mg ii λ2798 line. Leveraging on the large dynamic range in continuum variability between the multi-epoch spectra, we explore the associated variations in the broad Mg ii line, whose variability properties have not been well studied before. The broad Mg ii flux varies in the same direction as the continuum flux, albeit with a smaller amplitude, which indicates at least some portion of Mg ii is reverberating to continuum changes. However, the full width at half-maximum (FWHM) of Mg ii does not vary accordingly as continuum changes for most objects in the sample, in contrast to the case of the broad Balmer lines. Using the width of broad Mg ii to estimate the black hole mass with single epoch spectra therefore introduces a luminosity-dependent bias.
Highlights
The canonical unification model of AGN dictates that Type 2 objects are drawn from the same underlying population as Type 1 objects, but the AGN continuum and broad-line emission is obscured by a dust torus (Antonucci 1993; Urry & Padovani 1995)
We present new Gemini/GMOS optical spectroscopy of 16 extreme variability quasars (EVQs) that dimmed by more than 1.5 mag in the g band between the Sloan Digital Sky Survey (SDSS) and the Dark Energy Survey (DES) epochs
We imposed additional criteria: (1) the quasars were restricted to be those that were in the faint state during the DES epochs; (2) the latest DES epoch is brighter than 22 mag in i-band for reasonably good Gemini spectroscopy; (3) the object did not show frequent large-amplitude (> 0.5 mag) and rapid optical variability, which may be due to blazar activity; (4) it is not a broad absorption line quasar according to the SDSS spectrum, excluding objects in which the variability is caused by outflows; (5) we rejected objects with poor or problematic SDSS spectra after visual inspection
Summary
The canonical unification model of AGN dictates that Type 2 objects (with only narrow emission lines) are drawn from the same underlying population as Type 1 objects (with both broad and narrow emission lines), but the AGN continuum and broad-line emission is obscured by a dust torus (Antonucci 1993; Urry & Padovani 1995). The broad Balmer emission lines, including Hα, Hβ, and Hγ, and broad helium lines were observed to have dramatically changed between the dim and bright epochs, even completely disappearing or emerging, following large-amplitude variability in the continuum This population of changing-look quasars (CLQs) challenges the unified model of AGN, and is difficult to understand in the standard accretion disk theory given the observed short timescale of the changes. Because the broad emission lines are presumably powered by the ionizing continuum, they will reverberate to the continuum variability on the BLR light-travel timescales of days to weeks, which is much shorter compared to the BLR dynamical timescale of the order of a few years
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