Spectral Variability of a Sample of Extreme Variability Quasars and Implications for the MgII Broad-line Region

Yang, Qian and Shen, Yue and Desai, Shantanu et al (2019) Spectral Variability of a Sample of Extreme Variability Quasars and Implications for the MgII Broad-line Region. arXiv.

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Abstract

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 (separated by a few years in the quasar rest frame). The quasar sample covers a redshift range of 0:5 < z < 2:1. Nearly half of these EVQs brightened signi_cantly (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 timescales of only months. Leveraging on the large dynamic range in continuum variability between the earlier SDSS and the new GMOS spectra, we explore the associated variations in the broad Mg ii _2798 line, whose variability properties have not been well studied before. The broad Mgii ux varies in the same direction as the continuum ux, albeit with a smaller amplitude, which indicates at least some portion of Mg ii is reverberating to continuum changes. However, the width (FWHM) of Mgii 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 therefore introduces a luminosity-dependent bias. 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 (separated by a fewyears in the quasar rest frame). The quasar sample covers a redshift range of 0:5 <z < 2:1. Nearly half of these EVQs brightened signi_cantly (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 timescales of only months. Leveraging on the large dynamic range in continuum variability between the earlier SDSS and the new GMOS spectra, we explore the associated variations in the broad Mg ii _2798 line, whose variability properties have not been well studied before. The broad Mgii ux varies in the same direction as the continuum ux, albeit with a smaller amplitude, which indicates at least some portion of Mg ii is reverberating to continuum changes. However, the width (FWHM) of Mgii 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 therefore introduces a luminosity-dependent bias.

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