We present the spectral signatures of the Bethe-Heitler pair production ($pe$) process on the spectral energy distribution (SED) of blazars, in scenarios where the hard $gamma$-ray emission is of photohadronic origin. If relativistic protons interact
with the synchrotron blazar photons producing $gamma$ rays through photopion processes, we show that, besides the $2-20$ PeV neutrino emission, the typical blazar SED should have an emission feature due to the synchrotron emission of $pe$ secondaries that bridges the gap betweeen the low-and high-energy humps of the SED, namely in the energy range 40 keV-40 MeV. We first present analytical expressions for the photopion and $pe$ loss rates in terms of observable quantities of blazar emission. For the $pe$ loss rate in particular, we derive a new approximate analytical expression for the case of a power-law photon distribution, which has an excellent accuracy with the numerically calculated exact one, especially at energies above the threshold for pair production. We show that for typical blazar parameters, the photopair synchrotron emission emerges in the hard X-ray/soft $gamma$-ray energy range with a characteristic spectral shape and non negligible flux, which may be even comparable to the hard $gamma$-ray flux produced through photopion processes. We argue that the expected $pe$ bumps are a natural consequence of leptohadronic models, and as such, they may indicate that blazars with a three-hump SED are possible emitters of high-energy neutrinos.
We present a time-dependent approach to the one-zone hadronic model in the case where the photon spectrum is produced by ultrarelativistic protons interacting with soft photons that are produced from protons and low magnetic fields. Assuming that pro
tons are injected at a certain rate in a homogeneous spherical volume containing a magnetic field, the evolution of the system can be described by five coupled kinetic equations, for protons, electrons, photons, neutrons, and neutrinos. Photopair and photopion interactions are modelled using the results of Monte-Carlo simulations and, in particular from the SOPHIA code for the latter. The coupling of energy losses and injection introduces a self-consistency in our approach and allows the study of the comparative relevancy of processes at various conditions, the efficiency of the conversion of proton luminosity to radiation, the resulting neutrino spectra, and the effects of time variability on proton injection, among other topics. We present some characteristic examples of the temporal behaviour of the system and show that this can be very different from the one exhibited by leptonic models. Furthermore, we argue that, contrary to the wide-held belief, there are parameter regimes where the hadronic models can become quite efficient. However, to keep the free parameters at a minimum and facilitate an in-depth study of the system, we have only concentrated on the case where protons are injected; i.e., we did not consider the effects of a co-accelerated leptonic component.
We study the expected variability patterns of blazars within the two-zone acceleration model putting special emphasis on flare shapes and spectral lags. We solve semi-analytically the kinetic equations which describe the particle evolution in the acc
eleration and radiation zone. We then perturb the solutions by introducing Lorentzian variations in its key parameters and examine the flaring behavior of the system. We apply the above to the X-ray observations of blazar 1ES 1218+304 which exhibited a hard lag behavior during a flaring episode and discuss possibilities of producing it within the context of our model. The steady-state radio to X-rays emission of 1ES 1218+304 can be reproduced with parameters which lie well within the ones generally accepted from blazar modeling. Additionally, we find that the best way to explain its flaring behavior is by varying the rate of particles injected in the acceleration zone.
Aims: Drawing an analogy with Active Galactic Nuclei, we investigate the one-zone SSC model of Gamma Ray Bursts afterglows in the presence of electron injection and cooling both by synchrotron and SSC losses. Methods: We solve the spatially averaged
kinetic equations which describe the simultaneous evolution of particles and photons, obtaining the multi-wavelength spectrum as a function of time. We back up our numerical calculations with analytical solutions of the equations using various profiles of the magnetic field evolution under certain simplifying assumptions. Results: We apply the model to the afterglow evolution of GRBs in a uniform density environment and examine the impact various parameters have on the multiwavelength spectra. We find that in cases where the electron injection and/or the ambient density is high, the losses are dominated by SSC and the solutions depart significantly from the ones derived in the synchrotron standard cases.
The Supercritical Pile is a very economical GRB model that provides for the efficient conversion of the energy stored in the protons of a Relativistic Blast Wave (RBW) into radiation and at the same time produces - in the prompt GRB phase, even in th
e absence of any particle acceleration - a spectral peak at energy $sim 1$ MeV. We extend this model to include the evolution of the RBW Lorentz factor $Gamma$ and thus follow its spectral and temporal features into the early GRB afterglow stage. One of the novel features of the present treatment is the inclusion of the feedback of the GRB produced radiation on the evolution of $Gamma$ with radius. This feedback and the presence of kinematic and dynamic thresholds in the model are sources of potentially very rich time evolution which we have began to explore. In particular, one can this way obtain afterglow light curves with steep decays followed by the more conventional flatter afterglow slopes, while at the same time preserving the desirable features of the model, i.e. the well defined relativistic electron source and radiative processes that produce the proper peak in the $ u F_{ u}$ spectra. In this note we present the results of a specific set of parameters of this model with emphasis on the multiwavelength prompt emission and transition to the early afterglow.
Aims: We investigate the one-zone SSC model of TeV blazars in the presence of electron acceleration. In this picture electrons reach a maximum energy where acceleration saturates from a combination of synchrotron and inverse Compton scattering losses
. Methods: We solve the spatially averaged kinetic equations which describe the simultaneous evolution of particles and photons, obtaining the multi-wavelength spectrum as a function of time. Results: We apply the model to the rapid flare of Mrk 501 of July 9, 2005 as this was observed by the MAGIC telescope and obtain the relevant parameters for the pre-flare quasi steady state and the ones during the flare. We show that a hard lag flare can be obtained with parameters which lie well within the range already accepted for this source. Especially the choice of a high value of the Doppler factor seems to be necessary.
Diffuse VHE gamma radiation from the Galactic Centre ridge observed by the H.E.S.S. telescope has been convincingly linked with the propagation of recently accelerated cosmic rays that interact with molecular hydrogen clouds during their diffusion. T
hrough a series of time-dependent simulations of that diffusion for different propagation parameters we have obtained the most probable values of the diffusion coefficient for the Galactic Centre region. Assuming that the diffusion coefficient is of the form kappa(E) = kappa_0*(E/E_0)^delta, then for different optimal combinations of kappa_0 and delta its value is obtained for cosmic rays originating from a central point (possibly Sgr A East) 10 kyr ago.
We examine the prompt and afterglow emission within the context of the Supercritical Pile model for GRBs. For this we have performed self-consistent calculations, by solving three time-dependent kinetic equations for protons, electrons and photons in
addition to the usual mass and energy conservation equations. We follow the evolution of the RBW as it sweeps up circumstellar matter and assume that the swept-up electrons and protons have energies equal to the Lorentz factor of the flow. While the electrons radiate their energies through synchrotron and inverse Compton radiation on short timescales, the protons, at least initially, start accumulating without any dissipation. As the accumulated mass of relativistic protons increases, however, they can become supercritical to the `proton-photon pair-production - synchrotron radiation network, and, as a consequence, they transfer explosively their stored energy to secondary electron-positron pairs and radiation. This results in a burst which has many features similar to the ones observed in GRB prompt emission. We have included in our calculations the radiation drag force exerted on the flow from the scattered radiation of the prompt emission on the circumstellar material. We find that this can decelerate the flow on timescales which are much faster than the ones related to the usual adiabatic/radiative ones. As a result the emission exhibits a steep drop just after the prompt phase, in agreement with the Swift afterglow observations.
It is well accepted today that diffusive acceleration in shocks results to the cosmic ray spectrum formation. This is in principle true for non-relativistic shocks, since there is a detailed theory covering a large range of their properties and the r
esulting power-law spectrum, which is nevertheless not as efficient to reach the very high energies observed in the cosmic ray spectrum. On the other hand, the cosmic ray maximum energy and the resulting spectra from relativistic shocks, are still under investigation and debate concerning their contribution to the features of the cosmic ray spectrum and the measured, or implied, cosmic ray radiation from candidate astrophysical sources. Here, we discuss the efficiency of the first order Fermi (diffusive) acceleration mechanism up to relativistic shock speeds, presenting Monte Carlo simulations.
We present the spectral and temporal radiative signatures expected within the Supercritical Pile model of Gamma Ray Bursts (GRB). This model is motivated by the need for a process that provides the dissipation necessary in GRB and presents a well def
ined scheme for converting the energy stored in the relativistic protons of the Relativistic Blast Waves (RBW) associated with GRB into radiation; at the same time it leads to spectra which exhibit a peak in the burst $ u F_{ u}$ distribution at an energy $E_p simeq 1$ MeV in the observers frame, in agreement with observation and largely independent of the Lorentz factor $Gamma$ of the associated relativistic outflow. Futhermore, this scheme does not require (but does not preclude) acceleration of particles at the shock other than that provided by the isotropization of the flow bulk kinetic energy on the RBW frame. In the present paper we model in detail the evolution of protons, electrons and photons from a RBW to produce detailed spectra of the prompt GRB phase as a function of time from across a very broad range spanning roughly $4 log_{10} Gamma$ decades in frequency. The model spectra are in general agreement with observations and provide a means for the delineating of the model parameters through direct comparison with trends observed in GRB properties.
