The variability of the broad-line Balmer decrement for quasars from the Sloan Digital Sky Survey Reverberation Mapping

TL;DR

This study analyzes the variability of the broad-line Balmer decrement in quasars over 6.5 years, examining its relation to accretion properties and time delays, revealing complex behaviors and size estimates of emission regions.

Contribution

It provides the first detailed analysis of Balmer decrement variability in a large quasar sample, exploring its correlations and lag measurements with continuum flux.

Findings

Mean broad-line Balmer decrement is 0.62 dex with 0.15 dex scatter.

Most quasars show negative correlation between Balmer decrement variance and continuum flux.

Time delays suggest Balmer decrement variations originate from regions between the BLR and the torus.

Abstract

Based on the spectral decomposition through a code of PrepSpec, the light curves (spanning 6.5 years in the observed frame) of the broad-line Balmer decrement, i.e., the flux ratio of the broad \ha to the broad \hb line, are calculated for a sample of 44 Sloan Digital Sky Survey reverberation-mapped quasars ($z<0.53$). It is found that the logarithm of the mean broad-line Balmer decrement is 0.62 with a standard deviation of 0.15 dex. The relations between the mean Balmer decrement and the SMBH accretion properties (the luminosity, black hole mass, Eddington ratio, accretion rate) are investigated and no obvious correlations are found. It is found that there are 27 quasars ($61\%$) showing strong negative correlations between the Balmer decrement variance and the continuum variance, i.e., the Balmer decrement would be smaller with larger continuum flux. Assuming that the dust obscuration leads to the variance in the Balmer decrement and the continuum, an expected slope is $-1/3$, which is not consistent with most of measured slopes. Using the interpolated cross-correlation function, the time delays between the inverse Balmer decrement and the continuum are measured for 14 quasars with the maximum correlation coefficient larger the 0.6. It suggests that the size corresponding to the Balmer decrement lag extends from the BLR size to the torus size.