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

The X-radiation of large-scale extragalactic jets poses a serious challenge for the conventional electron-synchrotron or inverse Compton models suggested to explain the overall non-thermal emission of the resolved knots and hotspots. 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 radio galaxies and active galactic nuclei (AGN) - Pictor A, 3C 120, PKS 0637-752 and 3C 273. I show that if protons are indeed accelerated to energies E-p greater than or equal to 10(18) eV, it is possible to construct a realistic model that allows an effective cooling of protons via synchrotron radiation on quite 'comfortable' time-scales of about 10(7) -10(8) yr, i.e. on time-scales that 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 L-p similar to 10(43) to 10(46) erg s(-1). 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 (E-p less than or equal to 10(13) 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 non-thermal radiation are relativistically moving condensations with Doppler factors delta(j) >>1. The kpc scales of knots and hotspots are not sufficient for effective confinement of greater than or equal to10(19) eV protons. This suppresses the synchrotron radiation by secondary electrons produced at pgamma interactions. At the same time the runaway protons, interacting with 2.7-K cosmic microwave background radiation, initiate non-negligible diffuse X- and gamma-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.