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

Numerical treatment of radiation processes in the internal shocks of magnetized relativistic outflows

Los blázares son un tipo de núcleo activo de galaxia (AGN, por sus siglas en inglés) que se encuentran entre los objetos astrofísicos más energéticos y violentos, a la par de los brotes de rayos gamma (GRB por sus siglas en inglés). Los procesos físicos y, en particular, el escenario del chorro relativista en el que se genera la radiación ultraenergética detectada por observatorios terrestres y en órbita, han conseguido atraer la atención e interés de los astrónomos y astrofísicos desde su descubrimiento. En la presente tesis investigamos el modelo de choques internos (IS) cuya hipótesis consta del choque de dos capas de plasma con geometría cilíndrica, formando dos ondas de choque que atraviesan las antedichas capas acelerando electrones a su paso: tanto térmicos como no térmicos. Dichos electrones interaccionan con el campo magnético presente en el chorro produciendo, de acuerdo con las observaciones, emisión magnetobremsstrahlung (MBS). En este modelo consideramos también que el chorro se encuentra envuelto en un ambiente de fotones monocromático, que equivaldría a la región de banda ancha (BLR) de un AGN. Ambos tipos de fotones, los del medio externo y los producidos in situ, eventualmente interaccionan con los electrones acelerados mediante la dispersión Compton inversa (IC). El objetivo básico de la presente tesis ha sido la búsqueda de algún indicio que pudiera revelar las huellas dejadas tanto por la magnetización de las capas como por la distribución energética de los electrones (EED) inyectados en el frente de choque en la distribución espectral de energía (SED). Nuestro enfoque ha sido numérico, lo cual significa que se desarrollaron herramientas numéricas sofisticadas, ampliando las ya existentes desarrolladas previamente y creando nuevas, que hemos usado sistemáticamente para simular el modelo de IS y reproducir las SEDs espectralmente amplias de los blázares. Entre estas mejoras se encuentra la manipulación adecuada de distribuciones híbridas (térmicas-no térmicas) de electrones, así como también el cómputo correcto de la emisión MBS de los electrones inmersos en el campo magnético amplificado en las regiones del plasma que las ondas de choque han atravesado. Datos observacionales fueron empleados para corroborar dichas simulaciones y delimitar el espacio de parámetros para, de esta forma, conseguir que nuestras SEDs sintéticas estuviesen en concordancia con las observaciones. Mostramos a partir de simulaciones que, si examinamos la dominancia Compton y el índice espectral de fotones en la banda de rayos gamma, una parte comsiderable de la secuencia de los blázares podría ser explicada por la magnetización de las capas; siendo las menos magnetizadas las que se encuentran en la región de los radiocuásares de espectro plano (FSRQs), mientras que las capas medianamente magnetizadas caen en la región de los objetos BL Lacertae (BL Lac). Por otra parte, al incluir electrones térmicos en la población inyectada y agregar la herramienta numérica que nos permite reproducir la emisión MBS de electrones poco energéticos, encontramos que el valle que separa las componentes sincrotrón e IC se hace más "profundo" cuando las distributiones inyectadas en el frente de choque son dominadas por electrones térmicos. Para estos casos descubrimos que el pico sincrotrón varía ligeramente entre modelos (10^11-10^13 Hz), al contrario de una componente IC sensible a la variación de parámetros. Este efecto induce una dispersión vertical en el plano dominancia Compton-pico sincrotrón, sugiriendo que quizá la proporción de electrones térmicos sobre los no térmicos está relacionada con la posición de los blázares en ese plano.Blazars are a type of active galactic nuclei (AGNs) which are among the most energetic and violent astrophysical objects, alongside γ-ray bursts (GRBs). The physical processes, and, in particular, the relativistic jet itself in which the high energy radiation detected by the terrestrial and space observatories is generated, has been attracting the attention and interest of astronomers and astrophysicists since their discovery. In the present thesis, we investigate the internal shock (IS) model in which two magnetized shells of plasma, with cylindrical geometry, collide forming shock waves, which propagate throughout the plasma accelerating electrons (thermal and nonthermal) in their wake. Those electrons interact with the magnetic field of the jet producing magnetobremsstrahlung emission, which is detected by observations. In this model we also consider that the surroundings of the jet in which this collision takes place are filled with a monochromatic photon field, which emulates the more complex broad line region (BLR) of the AGN. Both photons from the external field and those produced in situ are Compton upscattered by the accelerated electrons. The main work of the present thesis has been the search of signatures imprinted on the double bump spectral energy distribution (SED) of blazars that may uncover the degree of the shell magnetization and the profile of the electrons energy distribution (EED) injected at the shock front. We have approached the problem numerically, so that a fair fraction of the work has consisted on improving already existing sophisticated numerical tools or developing new ones from scratch. Among those improvements are the handling of hybrid distributions (thermal-nonthermal) of electrons, and the accurate computation of the magnetobremsstrahlung emission of the electrons immersed in the amplified magnetic field in the shocked region of the plasma shells. We have used these numerical tools to simulate the IS model and reproduce broadband SEDs of blazars. To validate our methodology and put bounds on the parameters of our model, observational data has been analyzed so that the generated SEDs are able to reproduce generic observational data and inferred physical trends. From the Compton dominance and the spectral index of γ-ray photons obtained in our models, we infer that a fair fraction of the blazar sequence could be explained by the shells magnetization; the negligibly magnetized models describing the Flat Spectrum Radio Quasars (FSRQs) region, whereas moderately magnetized shells fall into the BL Lacertae object (BL Lac) region. On the other hand, by including thermal electrons into the population of injected particles and using a numerical tool which reproduces the low energy region of the magnetobremsstrahlung (MBS) emission, we have found that the valley which separates the synchrotron and inverse-Compton (IC) components grows deeper when thermal dominated distributions are injected at the shock front. A slightly varying synchrotron peak between 1011–1013 Hz, in contrast with a parameters dependent IC component. These effects induce a scattering in the vertical direction of the Compton dominance-synchrotron peak plane. From this clear fact, we cautiously suggest that the proportions of the thermal/nonthermal electrons have a prominent role explaining the location of blazars in that plane

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