An Accretion Disc-Irradiation Hybrid Model for The Optical/UV Variability in Radio-Quiet Quasars

TL;DR

This paper presents a hybrid accretion disc-irradiation model that explains optical/UV variability in radio-quiet quasars, successfully reproducing observed spectral and variability trends and providing insights into the underlying physical mechanisms.

Contribution

The study introduces a hybrid model combining accretion disc dynamics and irradiation effects, improving understanding of quasar variability beyond previous models.

Findings

Reproduces the bluer-when-brighter trend observed in quasars.

Fits the correlation between variability amplitude and wavelength.

Shows a weak positive correlation between variability amplitude and black hole mass.

Abstract

The optical/ultraviolet (UV) variability of quasars has been discovered to be correlated with other quasar properties, such as luminosity, black hole mass and rest-frame wavelength. However,the origin of variability has been a puzzle so far. In this work, we upgrade the accretion disc model (Li & Cao 2008), which assumed the variability is caused by the change of global mass accretion rate, by constraining the disc size to match the viscous timescale of accretion disc to the variability timescale observed and by including the irradiation/X-ray reprocessing to make the emitted spectrum become steeper. We find this hybrid model can reproduce the observed bluer-when-brighter trend quite well, which is used to validate the theoretical model by several works recently. The traditional correlation between the variability amplitude and rest-frame wavelength can also be well fitted by our model. In addition, a weak positive correlation between variability amplitude and black hole mass is present, qualitatively consistent with recent observations.