Reverberation responses in light curves of the SBS1520+530 quasar

Abstract

We aim to investigate an accretion disk (AD) temperature profile of a doubly lensed quasar SBS 1520+530. Temperature profiles of accretion disks are known to be diagnostic of many of the physical properties of AGNs and quasars. Our approach involves application of the photometric reverberation mapping to the light curves of SBS 1520+530 obtained in the Johnson-Cousins \(V\) and \(R\) filters. The RM method implies that the time shift between the light curves taken in different spectral ranges determines the light travel time between the AD zones with different physical conditions. In determining the time shifts, we applied a method based on some useful properties of the orthogonal polynomials. The variations in filter \(R\) lag those in filter \(V\), with 1.25±0.63 days for the inter-band time shift averaged between the two image components and over the three seasons. The obtained time lag noticeably exceeds the value following for SBS 1520+530 from the classical model of optically thick geometrically thin AD. We considered two possible ways to resolve the discrepancy between the theory and observations. The first assumes an AD temperature profile flatter than the classical one. The second way is to consider an extended optically thick scattering envelope originated due to matter outflow from an AD interior. Both explanations may be consequences of the super-Eddington accretion in SBS 1520+530. Using bolometric luminosity estimates available for SBS 1520+530 from the literature, we obtained a value of \(\approx 3.4\) for the Eddington ratio, which indeed indicates a moderately super-Eddington accretion regime.