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

Abstract

ABSTRACT Based on the spectral decomposition through a code of PrepSpec, the light curves (spanning 6.5 yr in the observed frame) of the broad-line Balmer decrement, i.e. the flux ratio of the broad Hα to the broad Hβ 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 supermassive black hole accretion properties (the luminosity, black hole mass, Eddington ratio, and accretion rate) are investigated and no obvious correlations are found. It is found that there are 27 quasars ($61{{\ \rm per\ cent}}$ ) 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 than 0.6. It suggests that the size corresponding to the Balmer decrement lag extends from the broad-line region size to the torus size.