Relation between the Variations in the Mg ii λ2798 Emission Line and 3000 Å Continuum

Abstract

We investigate the relationship between the Mg ii λ2798 emission-line and the 3000Å continuum variations using a sample of 68 intermediate-redshift (z ∼ 0.65−1.50) broad-line quasars spanning a bolometric luminosity range of 44.49 erg s −1 ≤ logL bol ≤ 46.31 erg s −1 (Eddington ratio from ∼ 0.026 to 0.862). This sample is constructed from SDSS-DR7Q and BOSS-DR12Q, each with at least 2 spectroscopic epochs in SDSS-I/II/III surveys. Additionally, we adopt the following signalto-noise ratio (S/N) selection criteria: a) for Mg ii and the 3000Å continuum, S/N ≥ 10; b) for narrow lines, S/N ≥ 5. All our quasar spectra are recalibrated based on the assumption of constant narrow emission-line fluxes. In an analysis of spectrum-to-spectrum variations, we find a fairly close correlation (Spearman ρ = 0.593) between the variations in broad Mg ii and in the continuum. This is consistent with the idea that Mg ii is varying in response to the continuum emission variations. Adopting the modified weighted least squares regression method, we statistically constrain the slopes (i.e., the responsivity α of the broad Mg ii) between the variations in both components for the sources in different luminosity bins after eliminating intrinsic biases introduced by the rescaling process itself. It is shown that the responsivity is quite small (averageᾱ ≈ 0.464) and anti-correlates with the quasar luminosity. Our results indicate that high signal-to-noise flux measurements are required to robustly detect the intrinsic variability and the time lag of Mg ii line.

Quantifying the diffuse continuum contribution of BLR Clouds to AGN Continuum Inter-band Delays

“…5548 that the Mg ii broad-line luminosity does not correlate with continuum variations. For a sample of 68 quasars with multiple SDSS observations, Zhu et al (2017) find that Mg ii responds only weakly to continuum variations (η ∼ 0.46). These findings may preclude the use of this line for reverberation mapping studies, though we note that a handful of significant lag measurements have been reported for this line (e.g., Metzroth et al 2006;Shen et al 2016).…”

The EDGE–CALIFA survey: validating stellar dynamical mass models with CO kinematics

“…Instead of solving the Jeans equations, the Schwarzschild models compute the orbits in a gravitational potential to recover the observed kinematics. A complete description to the methodology of our Schwarzschild models can be found in Zhu et al (2018) and the resulting orbital distribution derived for the CALIFA galaxies and their fitted parameters as adopted here can be found in Zhu et al (2017). Here we give a brief overview of our Schwarzschild models for completeness.…”

“…Schwarzschild 1979;van der Marel et al 1998;Thomas et al 2004;Valluri, Merritt & Emsellem 2004), or by particle-based models such as the Made-to-measure method (e.g. de Lorenzi et al 2007;Long & Mao 2010;Syer & Tremaine 1996;Zhu et al 2014). Having only the line-of-sight information of the velocity field implies that some assumptions must be made.…”

Behaviour of the Mg ii 2798 Å line over the full range of AGN variability

“…This is similar to the method used in e.g. Zhu et al (2017) and Yang et al (2020). Although a study of EW provides important information about the behaviour of broad emission lines (Shen et al 2011;Dong et al 2009;Korista & Goad 2004), considering the line and continuum flux as separate parameters can allow for more insight into the behaviour of each component.…”

“…An investigation into epoch to epoch Mg II variability in a sample of SDSS quasars is presented in Zhu et al (2017), who find that the Mg II line tracks the continuum and that the responsivity of the line decreases with a higher initial continuum state and with a higher Eddington ratio. Both conclusions are supported by the results of Sun et al (2015).…”