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Register a new userTurbulent Extreme Multi-Zone Model for Multi-Waveband Variations of Blazars
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Authors
A. P. Marscher
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The author is developing a numerical code with thousands of emission zones to simulate the time-dependent multi-waveband emission from blazars. The code is based on a model in which turbulent plasma flowing at a relativistic speed down a jet crosses a standing conical collimation shock that accelerates electrons to maximum energies in the 5-100 GeV range. This paper reports early results produced by the model. The simulated light curves and time profiles of the degree and position angle of polarization have a number of features in common with the observational data of blazars. Maps of the polarized intensity structure can be compared with those of blazars observed with very long baseline interferometry at short millimeter wavelengths.
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The author presents a model for variability of the flux and polarization of blazars in which turbulent plasma flowing at a relativistic speed down a jet crosses a standing conical shock. The shock compresses the plasma and accelerates electrons to energies up to gamma(max) > 1E4 times their rest-mass energy, with the value of gamma(max) determined by the direction of the magnetic field relative to the shock front. The turbulence is approximated in a computer code as many cells, each with a uniform magnetic field whose direction is selected randomly. The density of high-energy electrons in the plasma changes randomly with time in a manner consistent with the power spectral density of flux variations derived from observations of blazars. The variations in flux and polarization are therefore caused by continuous noise processes rather than by singular events such as explosive injection of energy at the base of the jet. Sample simulations illustrate the behavior of flux and linear polarization versus time that such a model produces. The variations in gamma-ray flux generated by the code are often, but not always, correlated with those at lower frequencies, and many of the flares are sharply peaked. The mean degree of polarization of synchrotron radiation is higher and its time-scale of variability shorter toward higher frequencies, while the polarization electric vector sometimes randomly executes apparent rotations. The slope of the spectral energy distribution exhibits sharper breaks than can arise solely from energy losses. All of these results correspond to properties observed in blazars.
Multi-wavelength light curves of bright gamma-ray blazars (e.g., 3C 454.3) reveal strong correlations across wavebands, yet striking dissimilarities in the details. This conundrum can be explained if the variable flux and polarization result from both (1) modulation in the magnetic field and relativistic electron content imparted at the jet input and (2) turbulence in the flow. In the Turbulent Extreme Multi-Zone (TEMZ) model being developed by the author, much of the optical and high-energy radiation in a blazar is emitted near the 43 GHz core of the jet as seen in VLBA images, parsecs from the central engine, as indicated by observations of a number of blazars. The model creates simulated light curves through numerical calculations that approximate the behavior of turbulent plasma - modulated by random fluctuations of the jet flow - crossing a cone-shaped standing shock system that compresses the plasma and accelerates electrons to highly relativistic energies. A standing shock oriented transverse to the jet axis (Mach disk) at the vertex of the conical shock can create a variable nonthermal seed photon field that is highly blueshifted in the frame of the faster jet plasma, leading to highly luminous, rapidly variable gamma-ray emission.
We are leading a comprehensive multi-waveband monitoring program of 34 gamma-ray bright blazars designed to locate the emission regions of blazars from radio to gamma-ray frequencies. The maps are anchored by sequences of images in both total and polarized intensity obtained with the VLBA at an angular resolution of ~ 0.1 milliarcseconds. The time-variable linear polarization at radio to optical wavelengths and radio to gamma-ray light curves allow us to specify the locations of flares relative to bright stationary features seen in the images and to infer the geometry of the magnetic field in different regions of the jet. Our data reveal that some flares occur simultaneously at different wavebands and others are only seen at some of the frequencies. The flares are often triggered by a superluminal knot passing through the stationary core on the VLBA images. Other flares occur upstream or even parsecs downstream of the core.
Context: Runaway stars produce bowshocks that are usually observed at infrared (IR) wavelengths. Non-thermal radio emission has been detected so far only from the bowshock of BD+43{deg}3654, whereas the detection of non-thermal radiation from these bowshocks at high energies remains elusive. Aims: We aim at characterising in detail the radio, X-ray, and gamma-ray emission from stellar bowshocks accounting for the structure of the region of interaction between the stellar wind and its environment. Methods: We develop a broadband-radiative, multi-zone model for stellar bowshocks that takes into account the spatial structure of the emitting region and the observational constraints. The model predicts the evolution and the emission of the relativistic particles accelerated and streaming together with the shocked flow. Results: We present broadband non-thermal spectral energy distributions for different scenarios, synthetic radio-cm synchrotron maps that reproduce the morphology of BD+43{deg}3654, and updated predictions in X-ray and gamma-ray energy ranges. We also compare the results of the multi-zone model applied in this work with those of a refined one-zone model. Conclusions: A multi-zone model provides better constraints than a one-zone model on the relevant parameters, namely the magnetic field intensity and the amount of energy deposited in non-thermal particles. However, one-zone models can be improved by carefully characterising the intensity of the IR dust photon field and the escape rate of the plasma from the shocked region. Finally, comparing observed radio maps with those obtained from a multi-zone model enables constraints to be obtained on the direction of stellar motion with respect to the observer.
We present a newly developed time-dependent three-dimensional multi-zone hadronic blazar emission model. By coupling a Fokker-Planck based lepto-hadronic particle evolution code 3DHad with a polarization-dependent radiation transfer code, 3DPol, we are able to study the time-dependent radiation and polarization signatures of a hadronic blazar model for the first time. Our current code is limited to parameter regimes in which the hadronic $gamma$-ray output is dominated by proton synchrotron emission, neglecting pion production. Our results demonstrate that the time-dependent flux and polarization signatures are generally dominated by the relation between the synchrotron cooling and the light crossing time scale, which is largely independent of the exact model parameters. We find that unlike the low-energy polarization signatures, which can vary rapidly in time, the high-energy polarization signatures appear stable. As a result, future high-energy polarimeters may be able to distinguish such signatures from the lower and more rapidly variable polarization signatures expected in leptonic models.
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