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Register a new userRelativistic Jets in Active Galactic Nuclei und Microquasars
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Gustavo Romero
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Collimated outflows (jets) appear to be a ubiquitous phenomenon associated with the accretion of material onto a compact object. Despite this ubiquity, many fundamental physics aspects of jets are still poorly understood and constrained. These include the mechanism of launching and accelerating jets, the connection between these processes and the nature of the accretion flow, and the role of magnetic fields; the physics responsible for the collimation of jets over tens of thousands to even millions of gravitational radii of the central accreting object; the matter content of jets; the location of the region(s) accelerating particles to TeV (possibly even PeV and EeV) energies (as evidenced by gamma-ray emission observed from many jet sources) and the physical processes responsible for this particle acceleration; the radiative processes giving rise to the observed multi-wavelength emission; and the topology of magnetic fields and their role in the jet collimation and particle acceleration processes. This chapter reviews the main knowns and unknowns in our current understanding of relativistic jets, in the context of the main model ingredients for Galactic and extragalactic jet sources. It discusses aspects specific to active Galactic nuclei (especially blazars) and microquasars, and then presents a comparative discussion of similarities and differences between them.
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Magnetic reconnection is often invoked to explain the non-thermal radiation of relativistic outflows, including jets of active galactic nuclei (AGN). Motivated by the largely unknown plasma composition of AGN jets, we study reconnection in the unexplored regime of electron-positron-proton (pair-proton) plasmas with large-scale two-dimensional particle-in-cell simulations. We cover a wide range of pair multiplicities (lepton-to-proton number ratio $kappa=1-199$) for different values of the all-species plasma magnetization ($sigma=1,3$ and 10) and electron temperature ($Theta_eequiv kT_e/m_ec^2=0.1-100$). We focus on the dependence of the post-reconnection energy partition and lepton energy spectra on the hot pair plasma magnetization $sigma_{e,h}$ (i.e., the ratio of magnetic to pair enthalpy densities). We find that the post-reconnection energy is shared roughly equally between magnetic fields, pairs, and protons for $sigma_{e,h}gtrsim 3$. We empirically find that the mean lepton Lorentz factor in the post-reconnection region depends on $sigma, Theta_e$, and $sigma_{e,h}$ as $langle gamma_e-1rangle approx sqrt{sigma}(1+4Theta_e)left(1+sigma_{e,h}/30right)$, for $sigmage1$. The high-energy part of the post-reconnection lepton energy distributions can be described by a power law, whose slope is mainly controlled by $sigma_{e,h}$ for $kappa gtrsim 3-6$, with harder power laws obtained for higher magnetizations. We finally show that reconnection in pair-proton plasmas with multiplicities $kappa sim 1-20$, magnetizations $sigma sim 1-10$, and temperatures $Theta_e sim 1-10$ results in particle power law slopes and average electron Lorentz factors that are consistent with those inferred in leptonic models of AGN jet emission.
This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of radio-loud Active Galactic Nuclei. For a summary, we refer to the paper.
We present deep Near-infrared (NIR) images of a sample of 19 intermediate-redshift ($0.3<z<1.0$) radio-loud active galactic nuclei (AGN) with powerful relativistic jets ($L_{1.4GHz} >10^{27}$ WHz$^{-1}$), previously classified as flat-spectrum radio quasars. We also compile host galaxy and nuclear magnitudes for blazars from literature. The combined sample (this work and compilation) contains 100 radio-loud AGN with host galaxy detections and a broad range of radio luminosities $L_{1.4GHz} sim 10^{23.7} - 10^{28.3}$~WHz$^{-1}$, allowing us to divide our sample into high-luminosity blazars (HLBs) and low-luminosity blazars (LLBs). The host galaxies of our sample are bright and seem to follow the $mu_{e}$-$R_{eff}$ relation for ellipticals and bulges. The two populations of blazars show different behaviours in the mnuc - mbulge plane, where a statistically significant correlation is observed for HLBs. Although it may be affected by selection effects, this correlation suggests a close coupling between the accretion mode of the central supermassive black hole and its host galaxy, that could be interpreted in terms of AGN feedback. Our findings are consistent with semi--analytical models where low--luminosity AGN emit the bulk of their energy in the form of radio jets, producing a strong feedback mechanism, and high--luminosity AGN are affected by galaxy mergers and interactions, which provide a common supply of cold gas to feed both nuclear activity and star formation episodes.
Energy deposition by active galactic nuclei jets into the ambient medium can affect galaxy formation and evolution, the cooling of gas flows at the centres of galaxy clusters, and the growth of the supermassive black holes. However, the processes that couple jet power to the ambient medium and determine jet morphology are poorly understood. For instance, there is no agreement on the cause of the well-known Fanaroff-Riley (FR) morphological dichotomy of jets, with FRI jets being shorter and less stable than FRII jets. We carry out global 3D magnetohydrodynamic simulations of relativistic jets propagating through the ambient medium. We show that the flat density profiles of galactic cores slow down and collimate the jets, making them susceptible to the 3D magnetic kink instability. We obtain a critical power, which depends on the galaxy core mass and radius, below which jets become kink-unstable within the core, stall, and inflate cavities filled with relativistically-hot plasma. Jets above the critical power stably escape the core and form powerful backflows. Thus, the kink instability controls the jet morphology and can lead to the FR dichotomy. The model-predicted dependence of the critical power on the galaxy optical luminosity agrees well with observations.
Shocks in jets and hot spots of Active Galactic Nuclei (AGN) are one prominent class of possible sources of very high energy cosmic ray particles (above 10^18eV). Extrapolating their spectrum to their plausible injection energy from some shock, implies an enormous hidden energy for a spectrum of index ~-2. Some analyzes suggest the particles injection spectrum at source to be as steep as -2.4 to -2.7, making the problem much worse, by a factor of order 10^6. Nevertheless, it seems implausible that more than at the very best 1/3 of the jet energy, goes into the required flux of energetic particles thus, one would need to allow for the possibility that there is an energy problem, which we would like to address in this work. Sequences of consecutive oblique shock features, or conical shocks, have been theorized and eventually observed in many AGN jets. Based on that, we use by analogy the Comptonisation effect and we propose a scenario of a single injection of particles which are accelerated consecutively by several oblique shocks along the axis of an AGN jet. We use detailed test-particle approximation Monte Carlo simulations in order to calculate particle spectra by acceleration at such a shock pattern while monitoring the efficiency of acceleration, calculating differential spectra. We find that the first shock of a sequence of oblique shocks, establishes a low energy power-law spectrum with ~E^-2.7. The consecutive shocks push the spectrum up in energy, rendering flatter distributions with steep cut-offs and characteristic depletion at low energies, an effect which could explain the puzzling apparent extra source power as well as the flat or inverted spectra from distant flaring sources.
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