Explaining X-ray Variability in Blazars

Abstract

The X-ray variability of blazars has been observed for many years. The X-ray emission mechanism is now reasonably understood, and, following observations such as the lag of the IR emission (ie the synchrotron component) by the X-rays, is usually explained as synchrotron self-Compton emission from the jet. The X-ray emission is observed to vary very rapidly, usually in a nonlinear way. Sometimes large outbursts can be modelled in terms of a shock propagating down the jet but, even if a shock is the correct physical explanation of the event, what is the source of the underlying variations which drive the shock, and also drive the smaller variations in blazars? We may be able to learn something from the studies of the X-ray variability of non-beamed systems such as Seyfert galaxies and X-ray binary systems. In those systems there are a number of observations, such as the linear relationship between rms variability and flux, and the scaling of characteristic timescales with black hole mass and inversely with accretion rate, which have provided some clues as to the source of the variations. In those systems the variations can be explained quite well as originating as accretion rate variations in the accretion disc which propagate inwards and eventually modulate the X-ray emission region. Thus the key point is that the origin of the variations and the source of the X-rays are physically separated. Here I examine the X-ray observations of blazars such as 3C273 and 3C279 and show that they agree very well with the same model which explains the variations in Seyfert galaxies and X-ray binaries. In particular, the characteristic timescale in 3C273 scales very well with characteristic timescales from Seyfert galaxies. There is no need to adjust the timescale to take account of time dilation, thereby implying that the origin of the variations (note not the source of the X-rays) lies outside the jet. Thus I propose here that X-ray variations which we see from blazars originate in accretion rate fluctuations in the disc which then propagate inwards, modulating any central non-moving X-ray emitting corona, and then carry on and modulate, or possibly produce, the X-ray emission region in the jet.