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Register a new userElectron and proton heating in trans-relativistic magnetic reconnection
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Michael E. Rowan n Harvard
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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.
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The plasma in low-luminosity accretion flows, such as the one around the black hole at the center of M87 or Sgr A* at our Galactic Center, is expected to be collisioness and two-temperature, with protons hotter than electrons. Here, particle heating is expected to be controlled by magnetic reconnection in the transrelativistic regime $sigma_{w}sim 0.1$-$1$, where the magnetization $sigma_{w}$ is the ratio of magnetic energy density to plasma enthalpy density. By means of large-scale 2D particle-in-cell simulations, we explore for a fiducial $sigma_w=0.1$ how the dissipated magnetic energy gets partitioned between electrons and protons, as a function of $beta_{rm i}$ (the ratio of proton thermal pressure to magnetic pressure) and of the strength of a guide field $B_{rm g}$ perpendicular to the reversing field $B_0$. At low $beta_{rm i};(lesssim 0.1)$, we find that the fraction of initial magnetic energy per particle converted into electron irreversible heat is nearly independent of $B_{rm g}/B_0$, whereas protons get heated much less with increasing $B_{rm g}/B_0$. As a result, for large $B_{rm g} /B_{0}$, electrons receive the overwhelming majority of irreversible particle heating (${sim}93%$ for $B_{rm g} /B_{0}=6$). This is significantly different than the antiparallel case $B_{rm g}/B_0=0$, in which electron irreversible heating accounts for only ${sim}18%$ of the total particle heating. At $beta_{rm i} sim 2$, when both species start already relativistically hot (for our fiducial $sigma_w=0.1$), electrons and protons each receive ${sim}50%$ of the irreversible particle heating, regardless of the guide field strength. Our results provide important insights into the plasma physics of electron and proton heating in hot accretion flows around supermassive black holes.
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Cosmic sources of gamma-ray radiation in the GeV range are often characterized by violent variability, in particular this concerns blazars, gamma-ray bursts, and the pulsar wind nebula Crab. Such gamma-ray emission requires a very efficient particle acceleration mechanism. If the environment, in which such emission is produced, is relativistically magnetized (i.e., that magnetic energy density dominates even the rest-mass energy density of matter), then the most natural mechanism of energy dissipation and particle acceleration is relativistic magnetic reconnection. Basic research into this mechanism is performed by means of kinetic numerical simulations of various configurations of collisionless relativistic plasma with the use of the particle-in-cell algorithm. Such technique allows to investigate the details of particle acceleration mechanism, including radiative energy losses, and to calculate the temporal, spatial, spectral and angular distributions of synchrotron and inverse Compton radiation. The results of these simulations indicate that the effective variability time scale of the observed radiation can be much shorter than the light-crossing time scale of the simulated domain.
Magnetic reconnection, especially in the relativistic regime, provides an efficient mechanism for accelerating relativistic particles and thus offers an attractive physical explanation for nonthermal high-energy emission from various astrophysical sources. I present a simple analytical model that elucidates key physical processes responsible for reconnection-driven relativistic nonthermal particle acceleration (NTPA) in the large-system, plasmoid-dominated regime in two dimensions. The model aims to explain the numerically-observed dependencies of the power-law index $p$ and high-energy cutoff $gamma_c$ of the resulting nonthermal particle energy spectrum $f(gamma)$ on the ambient plasma magnetization $sigma$, and (for $gamma_c$) on the system size $L$. In this self-similar model, energetic particles are continuously accelerated by the out-of-plane reconnection electric field $E_{rm rec}$ until they become magnetized by the reconnected magnetic field and eventually trapped in plasmoids large enough to confine them. The model also includes diffusive Fermi acceleration by particle bouncing off rapidly moving plasmoids. I argue that the balance between electric acceleration and magnetization controls the power-law index, while trapping in plasmoids governs the cutoff, thus tying the particle energy spectrum to the plasmoid distribution.
Particle acceleration and heating at mildly relativistic magnetized shocks in electron-ion plasma are investigated with unprecedentedly high-resolution two-dimensional particle-in-cell simulations that include ion-scale shock rippling. Electrons are super-adiabatically heated at the shock, and most of the energy transfer from protons to electrons takes place at or downstream of the shock. We are the first to demonstrate that shock rippling is crucial for the energization of electrons at the shock. They remain well below equipartition with the protons. The downstream electron spectra are approximately thermal with a limited supra-thermal power-law component. Our results are discussed in the context of wakefield acceleration and the modelling of electromagnetic radiation from blazar cores.
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