A. Mücke

BL Lac objects in the synchrotron proton blazar model

We calculate the spectral energy distribution (SED) of electromagnetic radiation and the spectrum of high-energy neutrinos from BL Lac objects in the context of the synchrotron proton blazar model. In this model, the high-energy hump of the SED is due to accelerated protons, while most of the low-energy hump is due to synchrotron radiation by co-accelerated electrons. To accelerate protons to sufficiently high energies to produce the high-energy hump, rather high magnetic fields are required. Assuming reasonable emission region volumes and Doppler factors, we then find that in low-frequency peaked BL Lacs (LBLs), which have higher luminosities than high-frequency peaked BL Lacs (HBLs), there is a significant contribution to the high-frequency hump of the SED from pion photoproduction and subsequent cascading, including synchrotron radiation by muons. In contrast, in HBLs we find that the high-frequency hump of the SED is dominated by proton synchrotron radiation. We are able to model the SED of typical LBLs and HBLs, and to model the famous 1997 flare of Markarian 501. We also calculate the expected neutrino output of typical BL Lac objects, and estimate the diffuse neutrino intensity due to all BL Lacs. Because pion photoproduction is inefficient in HBLs, as protons lose energy predominantly by synchrotron radiation, the contribution of LBLs dominates the diffuse neutrino intensity. We suggest that nearby LBLs may well be observable with future high-sensitivity TeV γ-ray telescopes.

Monte Carlo simulations of photohadronic processes in astrophysics

Abstract A new Monte Carlo program for photohadronic interactions of relativistic nucleons with an ambient photon radiation field is presented. The event generator is designed to fulfill typical astrophysical requirements, but can also be used for radiation and background studies at high energy colliders such as LEP2 and HERA, as well as for simulations of photon induced air showers. We consider the full photopion production cross section from the pion production threshold up to high energies. It includes resonance excitation and decay, direct single pion production and diffractive and non-diffractive multiparticle production. The cross section of each individual process is calculated by fitting experimental data, while the kinematics is determined by the underlying particle production process. We demonstrate that our model is capable of reproducing known accelerator data over a wide energy range.

A proton synchrotron blazar model for flaring in Markarian 501

Abstract The spectral energy distribution (SED) of gamma-ray (γ-ray) loud BL Lac objects typically has a double-humped appearance usually interpreted in terms of synchrotron self-Compton models. In proton blazar models, the SED is instead explained in terms of acceleration of protons and subsequent cascading. We discuss a variation of the synchrotron proton blazar model, first proposed by Mucke and Protheroe (Proc. Workshop GeV–TeV Astrophysics: Toward a Major Atmospheric Cherenkov Telescope VI, Snowbird, Utah, submitted for publication), in which the low energy part of the SED is mainly proton synchrotron radiation by electrons co-accelerated with protons, which produce the high energy part of the SED mainly as synchrotron radiation. As an approximation, we assume non-relativistic shock acceleration which could apply if the bulk of the plasma in the jet frame were non-relativistic. Our results may therefore change if a relativistic equation of state was used. We consider the case where the maximum energy of the accelerated protons is above the threshold for pion photoproduction interactions on the synchrotron photons of the low energy part of the SED. Using a Monte Carlo/numerical technique to simulate the interactions and subsequent cascading of the accelerated protons, we are able to fit the high-energy γ-ray portion of the observed SED of Markarian 501 during the April 1997 flare. We find that the emerging cascade spectra initiated by γ-rays from π0 decay and by e± from μ± decay turn out to be relatively featureless. Synchrotron radiation produced by μ± from π± decay, and even more importantly by protons, and subsequent synchrotron-pair cascading, is able to reproduce well the high energy part of the SED. For this fit, we find that synchrotron radiation by protons dominates the TeV emission, pion photoproduction being less important with the consequence that we predict a lower neutrino flux than in other proton blazar models.

Propagation of ultrahigh energy protons in the nearby universe

We present a new calculation of the propagation of protons with energies above 10{sup 19} eV over distances of up to several hundred Mpc. The calculation is based on a Monte Carlo approach using the event generator SOPHIA for the simulation of hadronic nucleon-photon interactions and a realistic integration of the particle trajectories in a random extragalactic magnetic field. Accounting for the proton scattering in the magnetic field affects noticeably the nucleon energy as a function of the distance to their source and allows us to give realistic predictions on arrival energy, time delay, and arrival angle distributions and correlations as well as secondary particle production spectra.

Photohadronic Processes in Astrophysical Environments

We discuss the first applications of our newly developed Monte Carlo event generator SOPHIA to multiparticle photoproduction of relativistic protons with thermal and power-law radiation fields. The measured total cross section is reproduced in terms of excitation and decay of baryon resonances, direct pion production, diffractive scattering, and non-diffractive multiparticle production. Non-diffractive multiparticle production is described using a string fragmentation model. We demonstrate that the widely used Delta-approximation for the photoproduction cross section is reasonable only for a restricted set of astrophysical applications. The relevance of this result for cosmic ray propagation through the microwave background and hadronic models of active galactic nuclei and gamma-ray bursts is brie°y discussed.

Modeling the April 1997 flare of Mkn 501

The April 1997 giant flare of Mkn 501 is modelled by a stationary Synchrotron-Proton-Blazar model. Our derived model parameters are consistent with X-ray-to-TeV-data in the flare state and diffusive shock acceleration of e− and p in a Kolmogorov/Kraichnan turbulence spectrum. While the merging pair-synchrotron cascade spectra initiated by photons from π0-decay and electrons from π±→μ±→e±-decay turn out to be relatively featureless, μ± and p synchrotron radiation and their cascade radiation produce a double-humped spectral energy distribution. For the present model we find p synchrotron radiation to dominate the TeV emission, while the contribution from the synchrotron radiation of the pairs, produced by the high energy hump, is only minor.

Application of the synchrotron proton blazar model to BL Lac objects

We apply the synchrotron proton blazar (SPB) model to the April 1997 flare of Markarian 501 and find we are able to fit the observed spectral energy distribution. We explore the effect of target photon density on the high energy part of the spectral energy distribution (SED) for fixed assumed magnetic field, emission region size and Doppler factor and find that the luminosity and peak frequency of the high energy part of the SED may depend on the luminosity of the low energy part of the SED in high-frequency peaked BL Lac objects (HBL).