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Register a new userRelativistic Magnetic Reconnection in Electron-Positron-Proton Plasmas: Implications for Jets of Active Galactic Nuclei
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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.
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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.
Hot collisionless accretion flows, such as the one in Sgr A$^{*}$ at our Galactic center, provide a unique setting for the investigation of magnetic reconnection. Here, protons are non-relativistic while electrons can be ultra-relativistic. By means of two-dimensional particle-in-cell simulations, we investigate electron and proton heating in the outflows of trans-relativistic reconnection (i.e., $sigma_wsim 0.1-1$, where the magnetization $sigma_w$ is the ratio of magnetic energy density to enthalpy density). For both electrons and protons, we find that heating at high $beta_{rm i}$ (here, $beta_{rm i}$ is the ratio of proton thermal pressure to magnetic pressure) is dominated by adiabatic compression (adiabatic heating), while at low $beta_{rm i}$ it is accompanied by a genuine increase in entropy (irreversible heating). For our fiducial $sigma_w=0.1$, the irreversible heating efficiency at $beta_{rm i}lesssim 1$ is nearly independent of the electron-to-proton temperature ratio $T_{rm e}/T_{rm i}$ (which we vary from $0.1$ up to $1$), and it asymptotes to $sim 2%$ of the inflowing magnetic energy in the low-$beta_{rm i}$ limit. Protons are heated more efficiently than electrons at low and moderate $beta_{rm i}$ (by a factor of $sim7$), whereas the electron and proton heating efficiencies become comparable at $beta_{rm i}sim 2$ if $T_{rm e}/T_{rm i}=1$, when both species start already relativistically hot. We find comparable heating efficiencies between the two species also in the limit of relativistic reconnection ($sigma_wgtrsim 1$). Our results have important implications for the two-temperature nature of collisionless accretion flows, and may provide the sub-grid physics needed in general relativistic MHD simulations.
Using fully kinetic simulations, we study the scaling of the inflow speed of collisionless magnetic reconnection from the non-relativistic to ultra-relativistic limit. In the anti-parallel configuration, the inflow speed increases with the upstream magnetization parameter $sigma$ and approaches the light speed when $sigma > O(100)$, leading to an enhanced reconnection rate. In all regimes, the divergence of pressure tensor is the dominant term responsible for breaking the frozen-in condition at the x-line. The observed scaling agrees well with a simple model that accounts for the Lorentz contraction of the plasma passing through the diffusion region. The results demonstrate that the aspect ratio of the diffusion region remains $sim 0.1$ in both the non-relativistic and relativistic limits.
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 large scale 3D particle-in-cell (PIC) simulations to examine particle energization in magnetic reconnection of relativistic electron-positron (pair) plasmas. The initial configuration is set up as a relativistic Harris equilibrium without a guide field. These simulations are large enough to accommodate a sufficient number of tearing and kink modes. Contrary to the non-relativistic limit, the linear tearing instability is faster than the linear kink instability, at least in our specific parameters. We find that the magnetic energy dissipation is first facilitated by the tearing instability and followed by the secondary kink instability. Particles are mostly energized inside the magnetic islands during the tearing stage due to the spatially varying electric fields produced by the outflows from reconnection. Secondary kink instability leads to additional particle acceleration. Accelerated particles are, however, observed to be thermalized quickly. The large amplitude of the vertical magnetic field resulting from the tearing modes by the secondary kink modes further help thermalizing the non-thermal particles generated from the secondary kink instability. Implications of these results for astrophysics are briefly discussed.
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