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Quasars are the most luminous objects in the Universe. Their emission is generated by a super-massive black hole in the centre of a distant galaxy, which accretes gas and dust. Friction in the accretion disk heats the gas and dust, such that it emits radition. However, accretion onto the black hole is not uniform, which gives rise to time variations in the quasar emission.
What is the problem?
The variability of quasar emission is an empirically well established fact. Unfortunately, the astrophysical mechanisms behind this phenomenon are still poorly understood (partially because it is very hard to simulate realistic accretion disks with sufficient resolution in time and space).
Given the lack of theoretical understanding, it is not immediately obvious how quasar variability data can be modelled. Numerous different models have been suggested in the literature, based on different astrophysical reasonings. Nevertheless, no real model comparison was ever undertaken, given the substantial amount of observational data that is available. In fact, it is still under debate whether quasar variability is deterministic or stochastic. Figure 1 shows four examples of different stochastic models for quasar variability.
Figure 1. Four different types of stochastic models for quasar lightcurves. The different stochastic models exhibit clearly distinct behaviour.
What am I doing?Given variability data of 6304 quasars from SDSS Stripe 82, I perform a Bayesian model comparison of more than twenty different models.
I find that the variability of the vast majority of quasars is best described by the Ornstein-Uhlenbeck process. Deterministic models generally provide very poor descriptions of the observed quasar variability, i.e., we are likely facing a stochastic phenomenon.