The uncorrelated long-term γ-ray and X-ray variability of blazars and its implications on disc–jet coupling

Abstract

We examine the long term (~10 years) gamma-ray variability of blazars observed by EGRET and Fermi and find that for six sources the average flux varied by more than an order of magnitude. For two of these sources (PKS 0208-512 and PKS 0528+134), there were extensive observations (at various observing periods) by EGRET. Hence these dramatic variations are not due to a single short time-scale flare, but reflect long term changes in the average flux. Over the last twenty years, these two sources were also the target of several X-ray observatories (e.g. ROSAT, ASCA, RXTE, BeppoSAX, Chandra, Suzaku, XMM-Newton and Swift). While the ratios of the average gamma-ray fluxes between EGRET and Fermi observations are 22.9 +/- 1.9 and 12.6 +/- 1.5, their estimated 2-10 keV X-ray flux do not show such dramatic variations. The X-ray emission from such flat spectrum radio quasars (FSRQs) are believed to be due to synchrotron self Compton, while gamma-rays originate from inverse Comptonization of external soft photons from an accretion disk and/or broad line region. We argue that in this scenario, the only explanation for the uncorrelated variability is that there was an order of magnitude decrease in the external soft photons, while the jet parameters remained more or less constant. This result indicates that perhaps the accretion and jet processes are not tightly coupled in these sources.