Stella S. Boula
Blazars are a sub-category of radio-loud Active Galactic Nuclei having their jet pointing towards us and are known for their emission covering practically all frequencies of the electromagnetic spectrum. These sources, in some cases, exhibit a correlation between gamma-ray and radio emission, especially during flaring episodes. Adopting the hypothesis that high energy photons are emitted by relativistic electrons close to the central black hole, we study the evolution of this population of particles as they move down the jet and lose energy by radiation and adiabatic expansion. In this scenario, gamma-rays are produced early on, when the electrons are still very energetic, while radio emission at a later time when the electrons have cooled and the emission region becomes optically thin to synchrotron self-absorption due to expansion. We calculate the emitted spectrum by solving the kinetic equations of particles and photos using a numerical code which takes into account the expansion of the source. We will discuss the parameters entering our calculations (like the magnetic field strength, the density of relativistic electrons, etc) in connection to the observational data.
Blazars are the most extreme subclass of AGN, having their relativistic jets pointing towards the observer. Their spectral properties are characterized by non-thermal emission over the entire electromagnetic spectrum, rapid variability, high optical polarization, and apparent superluminal motion. One of the characteristic features of blazar jet emission is the shape of its spectral energy distribution (SED). It usually has two components: a low-energy component extending from radio to UV/soft X-rays and a high-energy component lying between hard X rays and TeV γ-rays. In this work, we are seeking to find the localization of blazars emission and the correlations of multiwavelength flares.
Model Setup: We assume an emitting region (a blob of radiative relativistic electrons with a power-law distribution) that expands. This region is assumed to be spherical with radius R(t), which increases linearly with the time, in its comoving frame, and moves with highly relativistic speed. The magnetic field and electrons' luminosity depend on the blob’s radius. The characteristic time scale of the problem is the initial crossing time of the source. The relativistic electrons interact and radiate through the following processes: synchrotron radiation, inverse Compton scattering, photon-photon absorption, synchrotron self-absorption, and adiabatic losses. We have developed a numerical code based on Mastichiadis & Kirk, 1995, to calculate the temporal evolution of the electrons and photons distribution function. Our code solves two integro-differential equations, each describing the losses/sinks and injection of relativistic electrons and photons in the emitting region.
The role of synchrotron self-absorption: The frequency below which the synchrotron radiation is absorbed can be derived by conditioning the optical depth to be equal to one (which depends on the time, and it is calculated by multiplying the absorption coefficient with the blob’s radius). The synchrotron self-absorption frequency is a function of the blob's radius through its dependence on the magnetic field and electron number density. A blob at small distances from the jet base is optically thick to synchrotron radiation, but it becomes optically thin as it expands and moves to further distances.
The structure of a blazar SED: 1. We assume that blobs with the same initial properties are continuously created at a distance from the central engine. This is equivalent to a conical flow with a half-opening angle φ. The distance z traveled by a blob since its "birth", as measured in the black hole's rest frame, is related to its radius. 2. We integrate along the line of sight the SED to reproduce the total steady-state spectrum of the source, which is observed. 3. We compare our results with the observational data of the source Mrk421.
Flaring Episodes: We assume a re-acceleration episode in a distance z from the central engine. The shape of this pulse could be related to a Lorentzian distribution. In this case, we reproduce symmetrical and extended flares, depending on the parameters set. Furthermore, we can investigate the multiwavelength correlations (e.g. time-lags between photons with different energies).
Conclusions: 1.The development of a new one-zone expanding numerical leptonic code. 2.The prediction of the localization of radio emission depending on the basic physical quantities of the source. 3. Flares in radio and γ-rays may be produced by re-acceleration of electrons at a large distance from the central engine. 4. Close to the central engine, radio flares could not be produced.