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Register a new userA time-dependent particle acceleration and emission model: Understanding the particle spectral evolution and blazar flares
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The jets of blazars are renowned for their multi-wavelength flares and rapid extreme variability; however, there are still some important unanswered questions about the physical processes responsible for these spectral and temporal changes in emission properties. In this paper, we develop a time-dependent particle evolution model for the time-varying emission spectrum of blazars. In the model, we introduce time-dependent electric and magnetic fields, which consistently include the variability of relevant physical quantities in the transport equation. The evolution on the electron distribution is numerically solved from a generalized transport equation that contains the terms describing the electrostatic, first-order and second-order emph{Fermi} acceleration, escape of particles due to both advection and spatial diffusion, as well as energy losses due to the synchrotron emission and inverse-Compton scattering of both synchrotron and external ambient photon fields. We find that the light curve profiles of blazars are consistent with the particle spectral evolution resulting from time-dependent electric and magnetic fields, rather than the effects of the acceleration or the cooling processes. The proposed model is able to simultaneously account for the variability of both the energy spectrum and the light curve profile of the BL Lac object Mrk 421 with reasonable assumptions about the physical parameters. The results strongly indicate that the magnetic field evolution in the dissipated region of a blazar jet can account for the variabilities.
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We present a new time-dependent leptonic code that we developed to model the varying multi-wavelength (MWL) emission during blazar flares. In our modeling, we assume that the blazar emission originates from a plasma blob located in the jet, and that relativistic electrons are injected into the blob and may undergo stochastic (Fermi II) or shock (Fermi I) acceleration. We numerically solve the kinetic equation for electron evolution in the blob, taking into account particle injection, escape, acceleration and radiative cooling. In order to calculate the spectral energy distribution (SED) of the blob emission we assume a synchrotron self-Compton (SSC) scenario, including also synchrotron self absorption and gamma-gamma absorption processes. Our code computes the evolution of the electron spectrum and of the associated broad-band SED. As a first application, we attempt to connect the continuous, steady-state emission from the blazar Mrk 421 with a flare observed in February 2010, using a minimal number of free parameters in a two-zone scenario in which a turbulent region is present around the emitting zone. Mrk 421 is a high-synchrotron-peaked (HSP) BL Lac, and one of the brightest extragalactic gamma-ray sources in the Very High Energy (VHE) gamma-ray band. It is also the closest TeV emitting blazar to the Earth (redshift z=0.031).
Observations of minute-scale flares in TeV Blazars place constraints on particle acceleration mechanisms in those objects. The implications for a variety of radiation mechanisms have been addressed in the literature; in this paper we compare four different acceleration mechanisms: diffusive shock acceleration, second-order Fermi, shear acceleration and the converter mechanism. When the acceleration timescales and radiative losses are taken into account, we can exclude shear acceleration and the neutron-based converted mechanism as possible acceleration processes in these systems. The first-order Fermi process and the converter mechanism working via SSC photons are still practically instantaneous, however, provided sufficient turbulence is generated on the timescale of seconds. We propose stochastic acceleration as a promising candidate for the energy-dependent time delays in recent gamma-ray flares of Markarian 501.
There are still some important unanswered questions about the detailed particle acceleration and escape occurring during the quiescent epoches. As a result, the particle distribution that is adopted in the blazar quiescent spectral model have numerous unconstrained shapes. To help remedy this problem, we introduce a analytical particle transport model to reproduce quiescent broadband spectral energy distribution of blazar. In this model, the exact electron distribution is solved from a generalized transport equation that contains the terms describing first-order and secondary-order emph{Fermi} acceleration, escape of particle due to both the advection and spatial diffusion, energy losses due to synchrotron emission and inverse-Compton scattering of an assumed soft photon field. We suggest that the advection is a significant escape mechanism in blazar jet. We find that in our model the advection process tends to harden the particle distribution, which enhances the high energy components of resulting synchrotron and synchrotron self-Comptom spectrum from jet. Our model is able to roughly reproduce the observed spectra of extreme BL Lac object 1ES 0414+009 with reasonable assumptions about the physical parameters.
We simulate time-dependent particle acceleration in the blast wave of a young supernova remnant (SNR), using a Monte Carlo approach for the diffusion and acceleration of the particles, coupled to an MHD code. We calculate the distribution function of the cosmic rays concurrently with the hydrodynamic evolution of the SNR, and compare the results with those obtained using simple steady-state models. The surrounding medium into which the supernova remnant evolves turns out to be of great influence on the maximum energy to which particles are accelerated. In particular, a shock going through a $rho propto r^{-2}$ density profile causes acceleration to typically much higher energies than a shock going through a medium with a homogeneous density profile. We find systematic differences between steady-state analytical models and our time-dependent calculation in terms of spectral slope, maximum energy, and the shape of the cut-off of the particle spectrum at the highest energies. We also find that, provided that the magnetic field at the reverse shock is sufficiently strong to confine particles, cosmic rays can be easily re-accelerated at the reverse shock.
According to the most popular model for the origin of cosmic rays (CRs), supernova remnants (SNRs) are the site where CRs are accelerated. Observations across the electromagnetic spectrum support this picture through the detection of non-thermal emission that is compatible with being synchrotron or inverse Compton radiation from high energy electrons, or pion decay due to proton-proton interactions. These observations of growing quantity and quality promise to unveil many aspects of CRs acceleration and require more and more accurate tools for their interpretation. Here, we show how multi-dimensional MHD models of SNRs, including the effects on shock dynamics due to back-reaction of accelerated CRs and the synthesis of non-thermal emission, turned out to be very useful to investigate the signatures of CRs acceleration and to put constraints on the acceleration mechanism of high energy particles. These models have been used to interpret accurately observations of SNRs in various bands (radio, X-ray and $gamma$-ray) and to extract from them key information about CRs acceleration.
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