Introduction

Various hadronic models of active galactic nuclei (AGN) have been proposed [1]. Here, we consider a model in which a power-law distribution of protons is injected in an emission region in an AGN jet, and follow the hadronic and electromagnetic cascades in the radiation and magnetic environment of the emission region. The output from these cascades will be cosmic rays and neutrinos (if the pion photoproduction threshold is exceeded) and gamma-rays which form the high energy part of the AGN spectrum. We apply such a model to the SED of quasars, and use as an example 3C 273.

Figure: Time scale for interaction, energy-loss, and acceleration for an emission region of radius 0.001 pc in the jet at distance 0.01 pc above the accretion disk of 2C273. (a) Protons. (b) Electrons of energy $\gamma'_e m_ec^2$ and photons of energy $\gamma'_\gamma m_ec^2$.

Figure: (a) Jet-frame spectrum of photons at $\cos\theta'=-0.8$ (solid histogram) and $\cos\theta'=+0.8$ (dotted histogram) after hadronic and electromagnetic cascading. (b) Optical depth for model fit to 3C273 SED. Chain line shows optical depth internal to the emission region in an ${E'}^{-1.7}$ spectrum analogous to the cascade spectrum shown as the chain line in part a. Dashed curve gives the optical depth due to accretion disk radiation from the emission region to infinity along the jet axis.

Fig. 3: SED of 3C273 showing assumed accretion disk component (dashed curve) and the best fitting high energy component (solid curve). The data points are taken from [2].

3C 273 is the brightest and nearest (z= 0.158) quasar and has a small scale jet that extends up to 50 kpc from the core [2]. Its broad-band spectrum consists of two spectral components which appear as `humps' in the SED. The low-energy component comprises a 'big blue bump' (BBB) plus a continuum extending from the radio to UV or X-ray frequencies which is generally believed to be synchrotron emission from relativistic electrons in the jet. The origin of the high-energy component, starting at X-ray and extending to energies above 100 MeV [3], is uncertain. We model the BBB with an accretion disk in symbiosis with the jet (an ADJ [4]). A detailed analysis of the UV peak in [5] suggests that the disk makes an inclination angle with the line of sight of up to $\theta \sim 60^\circ$. The uncertainty in $\theta$ allows a large range of possible Doppler factors for 3C 273.