Numerical Simulations of Blazar Jets and their Non-thermal Radiation

In the past years relativistic (magneto)hydrodynamic simulations have been used extensively to study the time-dependent hydrodynamic properties of extra galactic jets. While these simulations have been very successful in studying the formation, collimation, propagation and termination of relativistic jets, the models used to compute synthetic images from the hydrodynamic properties were relatively simple. On the other hand, there exist several theoretical models which assume a very simple hydrodynamic evolution, but treat the non-thermal particles and their emitted radiation with great detail. It was the aim of this work to include a detailed treatment of the non-thermal particles and their synchrotron radiation in high-resolution shock-capturing relativistic hydrodynamic (RHD) simulations. To achieve this goal we have developed a transport scheme for the non-thermal particles by treating them as "tracer" fluids in the RHD equations. Their temporal evolution is calculated using an analytic kinetic equation solver, and their synchrotron radiation is computed in a time-dependent manner taking into account the relevant relativistic effects, (e.g., light travel times to the observer). The energy density of a dynamically negligible magnetic field is assumed to be a fraction of the energy density of the thermal fluid. Two models have been developed for the parameterization of the acceleration of non-thermal particles at relativistic shocks: A type-E model where only the strength of the shock influences the number of accelerated particles and a type-N model where the shock strength only influences their energy distribution. We have demonstrated that our numerical method is able to capture the essentials of the temporal and spatial evolution of the non-thermal particles and the observed synchrotron radiation with a reasonable accuracy when applied to subparsec scale relativistic jets. Understanding the physical processes connected to the observed X-ray blazar light curves has been the main object of research with our new numerical tool. For the first time, the hydrodynamic evolution and the synchrotron radiation of a blazar jet was simulated consistently. We have simulated collisions of density inhomogeneities (shells) within a blazar jet. The results have shown that the efficiency of the observed synchrotron radiation varies with the relative velocities of the shells as well as with the amount of initially available mass. The surrounding medium plays an important role, because it heats up the shells prior to the collision, a fact which is neglected in simpler models. Assuming that the observed radiation results from the interaction of shells within a blazar jet, we have developed an analytic model which enables the determination of the unobservable parameters of the jets (i.e., length and velocity of the shells) from the light curve. The parameters predicted by the model have been compared to results of our simulations and we find that the agreement is surprisingly good, given the simplicity of the model. In addition, several long-term simulations of collisions of many shells have shown that a model of an intermittently working central engine seems to produce light curves more similar the observed ones than a model in which the central engine ejects a continuous outflow.In der vorliegenden Arbeit wurde die Synchrotronstrahlung kollimierter relativistischer Stroemungen (Jets) aus aktiven galaktischen Kernen mit Hilfe numerischer Simulationen untersucht. Bisherige Publikationen zu diesem Thema befassten sich mit hochaufloesenden (magneto)hydrodynamischen Simulationen in zwei oder drei Dimensionen, die zwar die hydrodynamische Entwicklung der Jets detailliert untersuchten, aber die Synchrotronstrahlung dieser Jets nur sehr grob modellierten. Auf der anderen Seite gibt es Studien, die die Synchrotronstrahlung der Jets sehr detailliert beschreiben, die aber ihre hydrodynamische Entwicklung nur auf sehr einfache Weise beruecksichtigen. In diesen Modellen wird zum Beispiel statt eines kontinuierlichen Jets, eine Reihe von diskreten Massenschalen betrachtet, die miteinander kollidieren. Der Einfluss der Umgebungsmaterie wird dabei komplett vernachlaessigt. In dieser Arbeit wurde nun erstmals in konsistenter Weise sowohl die hydrodynamische Entwicklung als auch die Synchrotronstrahlung eines relativistischen Jets simuliert. Das dazu entwickelte Programm wurde auf Blazare angewandt, die eine besonders interessante Unterklasse der aktiven galaktischen Kerne bilden. Eine charakeristische Eigenschaft von Blazaren ist ihre starke Variabilitaet im Roentgenwellenbereich, die dadurch modelliert wurde, dass Zusammenstoesse von Teilen eines Jets (Massenschalen) simuliert wurde und die daraus resultierende Synchrotronstrahlung berechnet wurde. Die Ergebnisse zeigen, dass die Kollisionen unterschiedliche Effizienzen in Hinblick auf die emittierte Synchrotronstrahlung haben, je nachdem wie gross die Relativgeschwindigkeit der kollidiernden Schalen ist. Auch der Einfluss der Umgebungsmaterie spielt eine wichtige Rolle, da sich die Schalen noch vor dem eigentlichen Zusammenstoss erhitzen, eine Tatsache die in einfachen Modellen nicht beruecksichtigt wird. Ein analytisches Modell, das die Variabilitaet der Roentgenstrahlung der Blazar-Jets beschreibt, wurde entwickelt. Die Parameter des Modells sind physikalische Groessen, die nicht direkt beobachtbar sind (z.B. die Geschwindigkeit und die Dicke der Schalen). Wendet man das analytische Model auf die Ergebnisse der Simulationen (wo die Geschwindigkeit und die Dicke bekannt ist) an, so findet man, dass Modell und die Simulationsergebnisse gut uebereinstimmen. Daher kann man das Modell, im Prinzip, dazu verwenden, aus den Beobachtungsdaten von Blazaren etwas ueber deren physikalische Eigenschaften zu lernen

Numerical simulations of the internal shock model in magnetized relativistic jets of blazars

The internal shocks scenario in relativistic jets is used to explain the variability of the blazar emission. Recent studies have shown that the magnetic field significantly alters the shell collision dynamics, producing a variety of spectral energy distributions and light-curves patterns. However, the role played by magnetization in such emission processes is still not entirely understood. In this work we numerically solve the magnetohydodynamic evolution of the magnetized shells collision, and determine the influence of the magnetization on the observed radiation. Our procedure consists in systematically varying the shell Lorentz factor, relative velocity, and viewing angle. The calculations needed to produce the whole broadband spectral energy distributions and light-curves are computationally expensive, and are achieved using a high-performance parallel code.Comment: 7 pages, 5 figures, proceeding of the "Swift: 10 Years of Discovery" conference (December 2014, Rome, Italy

Numerical study of broadband spectra caused by internal shocks in magnetized relativistic jets of blazars

The internal-shocks scenario in relativistic jets has been used to explain the variability of blazars' outflow emission. Recent simulations have shown that the magnetic field alters the dynamics of these shocks producing a whole zoo of spectral energy density patterns. However, the role played by magnetization in such high-energy emission is still not entirely understood. With the aid of \emph{Fermi}'s second LAT AGN catalog, a comparison with observations in the $\gamma$-ray band was performed, in order to identify the effects of the magnetic field.Comment: Proceedings of the meeting The Innermost Regions of Relativistic Jets and Their Magnetic Fields, June 10-14, 2013, Granada (Spain), 4 pages, 3 figure

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 [email protected]; [email protected]

On the influence of a hybrid thermal-non-thermal distribution in the internal shocks model for blazars

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