Constraints on the temperature inhomogeneity in quasar accretion discs from the ultraviolet-optical spectral variability

TL;DR

This study tests the inhomogeneous accretion disc model for quasars' UV-optical variability using SDSS data, finding it cannot explain the observed tight inter-band correlations, thus challenging the model's validity.

Contribution

It provides observational evidence against the large temperature fluctuation model for quasar accretion discs based on spectral variability analysis.

Findings

The inhomogeneous disc model cannot reproduce the tight inter-band correlations.

Local temperature fluctuations are unlikely the main driver of long-term UV-optical variability.

The model's assumptions are not supported by multi-epoch SDSS quasar data.

Abstract

The physical mechanisms of the quasar ultraviolet (UV)-optical variability are not well understood despite the long history of observations. Recently, Dexter & Agol presented a model of quasar UV-optical variability, which assumes large local temperature fluctuations in the quasar accretion discs. This inhomogeneous accretion disc model is claimed to describe not only the single-band variability amplitude, but also microlensing size constraints and the quasar composite spectral shape. In this work, we examine the validity of the inhomogeneous accretion disc model in the light of quasar UV-optical spectral variability by using five-band multi-epoch light curves for nearly 9 000 quasars in the Sloan Digital Sky Survey (SDSS) Stripe 82 region. By comparing the values of the intrinsic scatter $\sigma_{\text{int}}$ of the two-band magnitude-magnitude plots for the SDSS quasar light curves and for the simulated light curves, we show that Dexter & Agol's inhomogeneous accretion disc model cannot explain the tight inter-band correlation often observed in the SDSS quasar light curves. This result leads us to conclude that the local temperature fluctuations in the accretion discs are not the main driver of the several years' UV-optical variability of quasars, and consequently, that the assumption that the quasar accretion discs have large localized temperature fluctuations is not preferred from the viewpoint of the UV-optical spectral variability.