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Internal shocks occurring in blazars may accelerate both thermal and non-thermal electrons. While the non-thermal tail fills the higher end of the electron energy distribution (EED), thermal electrons populate the lowest energies of the shock-accelerated particles. In this paper, we examine the consequences that such a hybrid (thermal-non-thermal) EED has on the spectrum of blazars. Since the thermal component of the EED may extend to very low energies, the synchrotron emission of ultrarelativistic electrons may not be sufficiently accurate to compute blazar spectra. Thus, we replace the standard synchrotron process by the more general magneto-bremsstrahlung (MBS) mechanism encompassing the discrete emission of harmonics in the cyclotron regime, the transition from the discrete to continuum and the continuum emission in the synchrotron realm. In the γ -ray band, an EED of mostly thermal particles displays significant differences with respect to the one dominated by non-thermal particles. A thermally dominated EED produces a synchrotron self-Compton (SSC) peak extending only up to a few MeV, and the valley separating the MBS and the SSC peaks is much deeper than if the EED is dominated by non-thermal particles. The combination of these effects modifies the Compton dominance of a blazar, suggesting that the vertical scatter in the distribution of FSRQs and BL Lacs in the peak synchrotron frequency-Compton dominance parameter space could be attributed to different proportions of thermal/non-thermal particles in the EED of blazars. [ABSTRACT FROM AUTHOR]

Radiatively inefficient accretion flow models have been shown to accurately account for the spectrum and luminosity observed from Sgr A* in the X-ray regime down to mm wavelengths. However, observations at a fewGHz cannot be explained by thermal electrons alone but require the presence of an additional non-thermal particle population. Here, we propose a model for the origin of such a population in the accretion flow via means of a pulsar orbiting the supermassive black hole in our Galaxy. Interactions between the relativistic pulsar wind with the disc lead to the formation of a bowshock in thewind. During the pulsar,s transit through the accretion disc, relativistic pairs, accelerated at the shock front, are injected into the disc. The radio-emitting particles are long lived and remain within the disc long after the pulsar's transit. Periodic pulsar transits through the disc result in regular injection episodes of non-thermal particles. We show that for a pulsar with spin-down luminosity Lsd ~ 3 × 1035 erg s-1 and a wind Lorentz factor of γ w ~ 104 a quasi-steady synchrotron emission is established with luminosities in the 1-10 GHz range comparable to the observed one. [ABSTRACT FROM AUTHOR]

We explore the signature imprinted by dynamically relevant magnetic fields on the spectral energy distribution (SED) of blazars. It is assumed that the emission from these sources originates from the collision of cold plasma shells, whose magnetohydrodynamic evolution we compute by numerically solving Riemann problems. We compute the SEDs including the most relevant radiative processes and scan a broad parameter space that encompasses a significant fraction of the commonly accepted values of not directly measurable physical properties. We reproduce the standard double-hump SED found in blazar observations for unmagnetized shells, but show that the prototype double-hump structure of blazars can also be reproduced if the dynamical source of the radiation field is very ultrarelativistic both, in a kinematically sense (namely, if it has Lorentz factors ≳50) and regarding its magnetization (e.g. with flow magnetizations σ ≃ 0.1). A fair fraction of the blazar sequence could be explained as a consequence of shell magnetization: negligible magnetization in flat-spectrum radio quasars, and moderate or large (and uniform) magnetization in BL Lacs. The predicted photon spectral indices (Γph) in the γ-ray band are above the observed values (Γph, obs ≲ 2.6 for sources with redshifts 0.4 ≤ z ≤ 0.6) if the magnetization of the sources is moderate (σ ≃ 10−2). [ABSTRACT FROM AUTHOR]

Context. The temporal and spatial spectral evolution of the jets of active galactic nuclei (AGN) can be studied with multi-frequency, multi-epoch very-long-baseline-interferometry (VLBI) observations. The combination of both morphological (kinematical) and spectral parameters can be used to derive source-intrinsic physical properties, such as the magnetic field and the nonthermal particle density. Such a study is of special interest during the high states of activity in AGNs, since VLBI observations can provide estimates of the location of the flaring site. Furthermore, we can trace the temporal variations in the source-intrinsic parameters during the flare, which may reflect the interaction between the underlying plasma and a traveling shock wave. The source CTA102 exhibited such a radio flare around 2006. Aims. In the first two papers of this series (Papers I and II), we analyzed the single-dish light curves and the VLBI kinematics of the blazar CTA102 and suggested a shock-shock interaction between a traveling and a standing shock wave as a possible scenario to explain the observed evolution of the component associated to the 2006 flare. In this paper we investigate the core shift and spectral evolution to test our hypothesis of a shock-shock interaction. Methods. We used eight multi-frequency Very Long Baseline Array (VLBA) observations to analyze the temporal and spatial evolution of the spectral parameters during the flare. We observed CTA102 between May 2005 and April 2007 using the VLBA at six different frequencies spanning from 2 GHz up to 86GHz. After the calibrated VLBA images were corrected for opacity, we performed a detailed spectral analysis.We developed methods for aligning the images and extracting the uncertainties in the spectral parameters. From the derived values we estimated the magnetic field and the density of the relativistic particles and combined those values with the kinematical changes provided from the long-term VLBA monitoring (Paper II) and single-dish measurements (Paper I). Results. The detailed analysis of the opacity shift reveals that the position of the jet core is proportional to ʋ-1 with some temporal variations. The value suggests possible equipartition between magnetic field energy and particle kinetic energy densities at the most compact regions. From the variation in the physical parameters we deduced that the 2006 flare in CTA102 is connected to the ejection of a new traveling feature (tej = 2005.9) and to the interaction between this shock wave and a stationary structure (interpreted as a recollimation shock) around 0.1 mas from the core (de-projected 18 pc at a viewing angle of φ = 2.6°). The source kinematics, together with the spectral and structural variations, can be described by helical motions in an overpressured jet. [ABSTRACT FROM AUTHOR]

ABSTRACT Common models of blazars and gamma-ray bursts assume that the plasma underlying the observed phenomenology is magnetized to some extent. Within this context, radiative signatures of dissipation of kinetic and conversion of magnetic energy in internal shocks of relativistic magnetized outflows are studied. We model internal shocks as being caused by collisions of homogeneous plasma shells. We compute the flow state after the shell interaction by solving Riemann problems at the contact surface between the colliding shells, and then compute the emission from the resulting shocks. Under the assumption of a constant flow luminosity, we find that there is a clear difference between the models where both shells are weakly magnetized (σ≲ 10−2) and those where, at least, one shell has σ≳ 10−2. We obtain that the radiative efficiency is largest for models in which, regardless of the ordering, one shell is weakly and the other strongly magnetized. Substantial differences between weakly and strongly magnetized shell collisions are observed in the inverse-Compton part of the spectrum, as well as in the optical, X-ray and 1-GeV light curves. We propose a way to distinguish observationally between weakly magnetized and strongly magnetized internal shocks by comparing the maximum frequency of the inverse-Compton part and synchrotron part of the spectrum to the ratio of the inverse-Compton to synchrotron fluence. Finally, our results suggest that low-frequency peaked blazars (LBL) may correspond to barely magnetized flows, while high-frequency peaked blazars (HBL) could correspond to moderately magnetized ones. Indeed, by comparing with actual blazar observations, we conclude that the magnetization of typical blazars is σ≲ 0.01 for the internal shock model to be valid in these sources. [ABSTRACT FROM AUTHOR]

We use high-resolution relativistic magnetohydrodynamics simulations coupled with a radiative transfer code to compute multiwavelength afterglow light curves of magnetized ejecta of gamma-ray bursts interacting with a uniform circumburst medium. The aim of our study is to determine how the magnetization of the ejecta at large distance from the central engine influences the afterglow emission, and to assess whether observations can be reliably used to infer the strength of the magnetic field. We find that, for typical parameters of the ejecta, the emission from the reverse shock peaks for magnetization of the flow, and that it is greatly suppressed for higher σ0. The emission from the forward shock shows an achromatic break shortly after the end of the burst marking the onset of the self-similar evolution of the blast wave. Fitting the early afterglow of GRBs 990123 and 090102 with our numerical models we infer respective magnetizations of and for these bursts. We argue that the lack of observed reverse shock emission from the majority of the bursts can be understood if , since we obtain that the luminosity of the reverse shock decreases significantly for . For ejecta with our models predict that there is sufficient energy left in the magnetic field, at least during an interval of times the burst duration, to produce a substantial emission if the magnetic energy can be dissipated (for instance, due to resistive effects) and radiated away. [ABSTRACT FROM AUTHOR]