Proton synchrotron radiation of large-scale jets in active galactic nuclei

The X-radiation of large-scale extragalactic jets poses serious challenge for conventional electron-synchrotron or inverse Compton models suggested to explain the overall nonthermal emission of the resolved knots and hot spots. In this paper I propose an alternative mechanism for X-ray emission synchrotron radiation by extremely high energy protons and discuss implications of this model for the extended jet features resolved by Chandra in several prominent radiogalaxies and AGN like Pictor A, 3C 120, PKS 0637752, and 3C 273. I show that if protons are indeed accelerated to energies Ep ≥ 10 eV, it is possible to construct a realistic model which allows an effective cooling of protons via synchrotron radiation on quite “comfortable” timescales of about 10 − 10 yr, i.e. on timescales which provide effective propagation of protons over the jet structures on kpc scales. This explains quite naturally the diffuse character of the observed X-ray emission, as well as the broad range of spectral X-ray indices observed from different objects. Yet, as long as the proton synchrotron cooling time is comparable with both the particle escape time and the age of the jet, the proton-synchrotron model offers an adequate radiation efficiency. The model requires relatively large magnetic field of about 1 mG, and proton acceleration rates ranging from Lp ∼ 10 to 10 erg/s. These numbers could be reduced significantly if the jet structures are moving relativistically towards the observer. I discuss also possible contributions of synchrotron radiation by secondary electrons produced at interactions of relatively low energy (Ep ≤ 10 eV) protons with the compressed gas in the jet structures. This is an interesting possibility which however requires a very large product of the ambient gas density and total amount of accelerated protons. Therefore it could be treated as a viable working hypothesis only if one can reduce the intrinsic X-ray luminosities assuming that the regions of nonthermal radiation are relativistically moving condensations with Doppler factors δj 1. The kpc scales of knots and hot spots are not sufficient for effective confinement of ≥ 10 eV protons. This suppresses the synchrotron radiation by secondary electrons produced at pγ interactions. At the same time the runaway protons interacting with 2.7 K cosmic microwave background radiation, initiate non-negligible diffuse Xand γ-ray emission in the surrounding cluster environments. I discuss the spectral and angular characteristics of this radiation which essentially depend on the strength of the ambient magnetic field.