P. Mimica

SPECTRAL EVOLUTION OF SUPERLUMINAL COMPONENTS IN PARSEC-SCALE JETS

We present numerical simulations of the spectral evolution and emission of radio components in relativistic jets. We compute jet models by means of a relativistic hydrodynamics code. We have developed an algorithm (SPEV) for the transport of a population of nonthermal electrons including radiative losses. For large values of the ratio of gas pressure to magnetic field energy density, αB ~ 6 × 104, quiescent jet models show substantial spectral evolution, with observational consequences only above radio frequencies. Larger values of the magnetic field (αB ~ 6 × 102), such that synchrotron losses are moderately important at radio frequencies, present a larger ratio of shocked-to-unshocked regions brightness than the models without radiative losses, despite the fact that they correspond to the same underlying hydrodynamic structure. We also show that jets with a positive photon spectral index result if the lower limit γmin of the nonthermal particle energy distribution is large enough. A temporary increase of the Lorentz factor at the jet inlet produces a traveling perturbation that appears in the synthetic maps as a superluminal component. We show that trailing components can be originated not only in pressure matched jets, but also in overpressured ones, where the existence of recollimation shocks does not allow for a direct identification of such features as Kelvin-Helmholtz modes, and its observational imprint depends on the observing frequency. If the magnetic field is large (αB ~ 6 × 102), the spectral index in the rarefaction trailing the traveling perturbation does not change much with respect to the same model without any hydrodynamic perturbation. If the synchrotron losses are considered the spectral index displays a smaller value than in the corresponding region of the quiescent jet model.

On the dynamic efficiency of internal shocks in magnetized relativistic outflows

We study the dynamic efficiency of conversion of kinetic-to-thermal/magnetic energy of internal shocks in relativistic magnetized outflows. We model internal shocks as being caused by collisions of shells of plasma with the same energy flux and a non-zero relative velocity. The contact surface, where the interaction between the shells takes place, can break up either into two oppositely moving shocks (in the frame where the contact surface is at rest), or into a reverse shock and a forward rarefaction. We find that for moderately magnetized shocks (magnetization σ ≃ 0.1), the dynamic efficiency in a single two-shell interaction can be as large as 40 per cent. Thus, the dynamic efficiency of moderately magnetized shocks is larger than in the corresponding unmagnetized two-shell interaction. If the slower shell propagates with a sufficiently large velocity, the efficiency is only weakly dependent on its Lorentz factor. Consequently, the dynamic efficiency of shell interactions in the magnetized flow of blazars and gamma-ray bursts is effectively the same. These results are quantitatively rather independent on the equation of state of the plasma. The radiative efficiency of the process is expected to be a fraction f r < I of the estimated dynamic one, the exact value of f r depending on the particularities of the emission processes which radiate away the thermal or magnetic energy of the shocked states.

Internal shocks in relativistic outflows: collisions of magnetized shells

Aims. We study the collision of magnetized irregularities (shells) in relativistic outflows in order to explain the origin of the generic phenomenology observed in the non-thermal emission of both blazars and gamma-ray bursts. We focus on the influence of the magnetic field on the collision dynamics, and we investigate how the properties of the observed radiation depend on the strength of the initial magnetic field and on the initial internal energy density of the flow. Methods. The collisions of magnetized shells and the radiation resulting from these collisions are calculated using the ID relativistic magnetohydrodynamics code MRGENESIS. The interaction of the shells with the external medium prior to their collision is also determined using an exact solver for the corresponding 1D relativistic magnetohydrodynamic Riemann problem. In both cases we assume that the magnetic field is oriented perpendicular to the flow direction. Results. Our simulations show that two magnetization parameters - the ratio of magnetic energy density and thermal energy density, α B , and the ratio of magnetic energy density and mass-energy density, σ - play an important role in the pre-collision phase, while the dynamics of the collision and the properties of the light curves depend mostly on the magnetization parameter σ. Comparing synthetic light curves computed from hydrodynamic and magnetohydrodynamic models we find that the assumption commonly made in the former models that the magnetization parameter α B is constant and uniform, holds rather well, if α B < 0.01. The interaction of the shells with the external medium changes the flow properties at their edges prior to the collision. For sufficiently dense shells moving at large Lorentz factors (≥25) these properties depend only on the magnetization parameter σ. Internal shocks in GRBs may reach maximum efficiencies of conversion of kinetic into thermal energy between 6% and 10%, while in case of blazars, the maximum efficiencies are ∼ 2%.

XMM-Newton timing mode observations of Mrk 421

We present the results of a detailed temporal analysis of the bright BL Lac object Mrk 421 using the three available long timing mode observations by the EPIC PN camera. This detector mode is characterized by its long life time and is largely free of photon pile-up problems. The source was found in different intensity and variability states differing by up to more than a factor of three in count rate. A time resolved cross correlation analysis between the soft and hard energy bands revealed that the characteristics of the correlated emission, with lags of both signs, change on time scales of a few 10 3 s. Individual spectra, resolved on time scales of ∼100 s, can be quite well fitted by a broken power law and we find significant spectral variations on time scales as short as ∼500–1000 s. Both the hard and the soft band spectral indices show a non-linear correlation with the source flux. A simple power law model of the form Γ ∝ flux −a with ahard ∼ 0.13 and asoft ∼ 0.22 describes rather well the observed trend of decreasing Γ values with increasing flux, which appear to “saturate” at the same limiting value of Γ ∼ 1. 8a t the highest flux levels. A comparison of the observed light curves with numerical results from relativistic hydrodynamic computer simulations of the currently favored shock-in-jet models indicates that any determination of the jet’s physical parameters from “simple” emission models must be regarded with caution: at any time we are seeing the emission from several emission regions distinct in space and time, which are connected by the complex hydrodynamic evolution of the non-uniform jet.

RELATIVISTIC MAGNETOHYDRODYNAMICS: RENORMALIZED EIGENVECTORS AND FULL WAVE DECOMPOSITION RIEMANN SOLVER

We obtain renormalized sets of right and left eigenvectors of the flux vector Jacobians of the relativistic MHD equations, which are regular and span a complete basis in any physical state including degenerate ones. The renormalization procedure relies on the characterization of the degeneracy types in terms of the normal and tangential components of the magnetic field to the wave front in the fluid rest frame. Proper expressions of the renormalized eigenvectors in conserved variables are obtained through the corresponding matrix transformations. Our work completes previous analysis that present different sets of right eigenvectors for non-degenerate and degenerate states, and can be seen as a relativistic generalization of earlier work performed in classical MHD. Based on the full wave decomposition (FWD) provided by the renormalized set of eigenvectors in conserved variables, we have also developed a linearized (Roe-type) Riemann solver. Extensive testing against one- and two-dimensional standard numerical problems allows us to conclude that our solver is very robust. When compared with a family of simpler solvers that avoid the knowledge of the full characteristic structure of the equations in the computation of the numerical fluxes, our solver turns out to be less diffusive than HLL and HLLC, and comparable in accuracy to the HLLD solver. The amount of operations needed by the FWD solver makes it less efficient computationally than those of the HLL family in one-dimensional problems. However, its relative efficiency increases in multidimensional simulations.

On the existence of a reverse shock in magnetized gamma-ray burst ejecta

The role of magnetic fields in gamma-ray burst (GRB) flows remains controversial. The study of the early afterglow phases and, in particular, of the reverse shock dynamics and associated emission offers a promising probe of the magnetization of the ejecta. In this paper, we derive the conditions for the existence of a reverse shock in arbitrarily magnetized ejecta that decelerate and interact with the circumburst medium. Both constant and wind-like density profiles are considered. We show, in contrast to previous estimates, that ejecta with magnetization σ0 > 1 are not crossed by a reverse shock for a large fraction of the parameter space relevant to GRB flows. Allowing for shell spreading, there is always a relativistic or mildly relativistic reverse shock forming in σ0 < 0.3 ejecta. From this, we conclude that the paucity of optical flashes, believed to be a distinctive signature of a reverse shock, may be explained by the existence of dynamically important magnetic fields in the ejecta.

Synthetic X-ray light curves of BL Lacs from relativistic hydrodynamic simulations

We present the results of relativistic hydrodynamic simulations of the collision of two dense shells in a uniform external medium, as envisaged in the internal shock model for BL Lac jets. The non-thermal radiation produced by highly energetic electrons injected at the relativistic shocks is computed following their temporal and spatial evolution. The acceleration of electrons at the relativistic shocks is parametrized using two different models and the corresponding X-ray light curves are computed. We find that the interaction time scale of the two shells is influenced by an interaction with the external medium. For the chosen parameter sets, the efficiency of the collision in converting dissipated kinetic energy into the observed X-ray radiation is of the order of one percent.

Catching the radio flare in CTA 102 - III. Core-shift and spectral analysis

The temporal and spatial spectral evolution of the jets of AGN can be studied with multi-frequency, multi-epoch VLBI observations. The combination of both, morphological and spectral parameters can be used to derive source intrinsic physical properties such as the magnetic field and the non-thermal particle density. In the first two papers of this series, we analyzed the single-dish light curves and the VLBI kinematics of the blazar CTA 102 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. We used 8 multi-frequency VLBA observations to analyze the temporal and spatial evolution of the spectral parameters during the flare. We observed CTA 102 between May 2005 and April 2007 using the VLBA at six different frequencies spanning from 2 - 86 GHz. After the calibrated VLBA images were corrected for opacity, we performed a detailed spectral analysis. From the derived values we estimated the magnetic field and the density of the relativistic particles. The detailed analysis of the opacity shift reveals that the position of the jet core is proportional to nu^-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 of the physical parameters we deduced that the 2006 flare in CTA 102 is connected to the ejection of a new traveling feature (t=2005.9) and the interaction between this shock wave and a stationary structure around 0.1 mas from the core. The source kinematics together with the spectral and structural variations can be described by helical motions in an over-pressured jet.

OBSERVATIONAL EFFECTS OF ANOMALOUS BOUNDARY LAYERS IN RELATIVISTIC JETS

Recenttheoreticalworkhaspointedoutthatthetransitionlayerbetweenajetandthemediumsurroundingitmay be more complex than previously thought. Under physically realizable conditions, the transverse profile of the Lorentz factorintheboundarylayercanbenonmonotonic,displayingtheabsolutemaximumwheretheflowisfasterthanat the jet spine, followed by a steep falloff. Likewise, the rest-mass density reaches an absolute minimum (coincident withthe maximumin Lorentz factor)andthengrowsuntilitreachesthe external medium value.Such behavior isin contrasttothe standardmonotonic decline oftheLorentzfactor(froma maximumvalueatthejetcentralspine)and the corresponding increase of the rest-mass density (from the minimum reached at the jet core). We study the emission properties of the aforementioned anomalous shear layer structures in kiloparsec-scale jets, aiming to show observable differences with respect to conventional monotonic and smooth boundary layers. Subject headingg acceleration of particles — galaxies: jets — methods: numerical — MHD — radiation mechanisms: nonthermal — X-rays: general

Which physical parameters can be inferred from the emission variability of relativistic jets

We present results of a detailed numerical study and theoretical analysis of the dynamics of internal shocks in relativistic jets and the non-thermal flares associated with these shocks. In our model internal shocks result from collisions of density inhomogeneities (shells) in relativistic jet flows. We find that the merged shell resulting from the inelastic collision of shells has a complicated internal structure due to the non-linear dynamics of the interaction. Furthermore, the instantaneous efficiency for converting kinetic energy into thermal energy is found to be almost twice as high as theoretically expected during the period of significant emission. The Lorentz factors of the internal shocks are correlated with the initial inertial masses of the shells. 
Because of the complexity of the non-linear evolution the merged shell becomes very inhomogeneous and simple one-zone models are inadequate to extract physical parameters of the emitting region from the resulting light curves. In order to improve on these one-zone approximations, we propose a novel way of analyzing the space-time properties of the emission. Based on these properties we construct an analytic model of non-thermal flares which can be used to constrain some (unobservable) physical parameters of the internal shocks. These are the ratio of the Lorentz factors between the forward and the reverse shock (caused by the shell collision), and the shell crossing times of these shocks. The analytic model is validated by applying it to the synthetic light curves computed from our models. It can equally well be applied to observations.