QSO variability: probing the starburst model

Abstract

The consistency of the Starburst model for AGN is tested using the optical variability observed in large data bases of QSOs. Theoretical predictions for the variabilityluminosity relationship and structure function are presented and compared with observations. If QSOs follow a variability-wavelength relation as that observed in nearby AGN, the model proves successful in reproducing the main characteristics of optical variability. The wavelength dependence (1) flattens the, otherwise, monochromatic Poissonian variability-luminosity relationship; and (2) decreases the asymptotic value of the structure function, which reveals that the elementary pulse driving the variations would have a characteristic time scale of 85-280 days. The upper limit is consistent with the time scale found in nearby Seyfert galaxies. Shorter values of this time scale are expected if the metallicity of high redshift objects is high, as recent observations indicate. If distant QSOs do not follow the variability-wavelength dependence observed in Seyfert nuclei and nearby QSOs, the characteristic pulse of variation needs to be much faster in order to reproduce the variability-luminosity relationship but, then, the single-parametric model explored in this work predicts a more rapidly rising structure function than that inferred from the data.

Variability-selected active galactic nuclei from supernova search in the Chandra deep field south

“…We notice also that, although variability is one of the principal characteristics of active galactic nuclei, its nature is still poorly understood. Independent models have been suggested, including supernova explosions, microlensing, and accretion disk instabilities (Aretxaga et al 1997;Hawkins 1993;Kawaguchi et al 1998;Trevese & Vagnetti 2002). A comparison of these models is discussed in Hawkins (2007).…”

Quasar Spectral Slope Variability in the Optical Band

“…In fact, a variety of mechanisms might in principle be responsible for a hard- ening of the spectrum in the bright phases. For instance, if AGNs are powered by supernovae explosions and variability is caused by a "" Christmas tree ÏÏ e †ect, then there will be an excess of blue emission in the bright phase (Aretxaga et al 1997 ;Cid Fernandes et al 2000). Even gravitational lensing (Hawkins 1996), usually thought of as achromatic, can produce a stronger variability in the blue than in the red band since the ampliÐcation depends on the size of the accretion disk, which is larger in the red than in the blue band (Alexander 1995).…”

Deep Modeling of Quasar Variability

“…The correlation of variability amplitude with black hole mass is still unclear, however, with different studies advocating either positive or negative relationships (e.g., Wold et al 2007;Kelly et al 2009;Zuo et al 2012), depending on the degree to which observational biases have been eliminated. Physical mechanisms underlying the optical/UV variability have been proposed: the superposition of supernovae (Aretxaga et al 1997;Kawaguchi et al 1998), microlensing (Hawkins 1993(Hawkins , 2010, thermal fluctuations from magnetic field turbulence (King et al 2004;Kelly et al 2009Kelly et al , 2011, and instabilities in the accretion disk (Takeuchi et al 1995;Kawaguchi et al 1998).…”