1999A&A...347...30V

Multifrequency observations of the BL Lacertae object Mkn 501 show a peculiar behaviour of its spectral energy distribution (SED): the synchrotron spectrum presents a discontinuity between the radio-to-UV part and the X-ray one; moreover, in 1997 BeppoSAX and RXTE detected an exceptional brightening and hardening of the X-ray spectrum, never seen before in any blazar, whereas the lower-frequency emission did not show any substantial change. Classical one-zone and inhomogeneous-jet synchrotron-emission models do not offer any simple and self-consistent explanation for this kind of behaviour. We present a model of inhomogeneous curved-helical jet that allows to explain both phenomena without invoking intrinsic variations of the emission, but assuming that it remains constant in time. The spectacular brightening in the hard X-ray band is interpreted as due to a variation of the jet orientation, probably caused by the orbital motion of the parent black hole in a binary black hole system (BBHS), and hence to a change of the relativistic beaming factor. The predicted flux variations are more noticeable at the highest synchrotron frequencies, which are emitted from the inner and most inhomogeneous part of the jet. A comparison between the model results and the most recent Space VLBI observations reveals a fair agreement and allows to derive an estimate of the BBHS physical parameters. The results of the present work, together with those of previous studies, support the hypothesis of the existence of BBHSs as central engines of active galactic nuclei (AGNs). Their possible role in the ambit of the unified schemes for AGNs is finally discussed.