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Register a new userComptonization by Reconnection Plasmoids in Black Hole Coronae I: Magnetically Dominated Pair Plasma
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We perform two-dimensional particle-in-cell simulations of reconnection in magnetically dominated electron-positron plasmas subject to strong Compton cooling. We vary the magnetization $sigmagg1$, defined as the ratio of magnetic tension to plasma inertia, and the strength of cooling losses. Magnetic reconnection under such conditions can operate in magnetically dominated coronae around accreting black holes, which produce hard X-rays through Comptonization of seed soft photons. We find that the particle energy spectrum is dominated by a peak at mildly relativistic energies, which results from bulk motions of cooled plasmoids. The peak has a quasi-Maxwellian shape with an effective temperature of $sim 100$ keV, which depends only weakly on the flow magnetization and the strength of radiative cooling. The mean bulk energy of the reconnected plasma is roughly independent of $sigma$, whereas the variance is larger for higher magnetizations. The spectra also display a high-energy tail, which receives $sim 25$% of the dissipated reconnection power for $sigma=10$ and $sim 40$% for $sigma=40$. We complement our particle-in-cell studies with a Monte-Carlo simulation of the transfer of seed soft photons through the reconnection layer, and find the escaping X-ray spectrum. The simulation demonstrates that Comptonization is dominated by the bulk motions in the chain of Compton-cooled plasmoids and, for $sigmasim 10$, yields a spectrum consistent with the typical hard state of accreting black holes.
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Basic properties of relativistic magnetic reconnection in electron-positron pair plasmas are investigated by using a particle-in-cell (PIC) simulation. We first revisit a problem by Hesse & Zenitani (2007), who examined the kinetic Ohms law across the X line. We formulate a relativistic Ohms law by decomposing the stress-energy tensor. Then, the role of the new term, called the heat-flow inertial term, is examined in the PIC simulation data. We further evaluate the energy balance in the reconnection system. These analyses demonstrate physically transparent ways to diagnose relativistic kinetic data.
96 -
N. Nishizuka Institute of Space andn Astronautical Science
, Japan Aerospace Exploration Agency
2013
We propose the particle acceleration model coupled with multiple plasmoid ejections in a solar flare. Unsteady reconnection produces plasmoids in a current sheet and ejects them out to the fast shocks, where particles in a plasmoid are reflected upstream the shock front by magnetic mirror effect. As the plasmoid passes through the shock front, the reflection distance becomes shorter and shorter driving Fermi acceleration, until it becomes proton Larmor radius. The fractal distribution of plasmoids may also have a role in naturally explaining the power-law spectrum in nonthermal emissions.
Magnetic reconnection in strongly magnetized astrophysical plasma environments is believed to be the primary process for fast energy release and particle energization. Currently there is strong interest in relativistic magnetic reconnection, in that it may provide a new explanation for high-energy particle acceleration and radiation in strongly magnetized astrophysical systems. We review recent advances in particle acceleration and reconnection physics in the magnetically-dominated regime. More discussion is focused on the physics of particle acceleration, power-law formation as well as the reconnection rate problem. In addition, we provide an outlook for studying reconnection acceleration mechanisms and kinetic physics in the next step.
Magnetic energy around compact objects often dominates over plasma rest mass, and its dissipation can power the object luminosity. We describe a dissipation mechanism which works faster than magnetic reconnection. The mechanism involves two strong Alfven waves with anti-aligned magnetic fields $boldsymbol{B}_1$ and $boldsymbol{B}_2$ that propagate in opposite directions along background magnetic field $boldsymbol{B}_0$ and collide. The collision forms a thin current sheet perpendicular to $boldsymbol{B}_0$, which absorbs the incoming waves. The current sheet is sustained by electric field $boldsymbol{E}$ breaking the magnetohydrodynamic condition $E<B$ and accelerating particles to high energies. We demonstrate this mechanism with kinetic plasma simulations using a simple setup of two symmetric plane waves with amplitude $A=B_1/B_0=B_2/B_0$ propagating in a uniform $boldsymbol{B}_0$. The mechanism is activated when $A>1/2$. It dissipates a large fraction of the wave energy, $f=(2A-1)/A^2$, reaching $100%$ when $A=1$. The plane geometry allows one to see the dissipation process in a one-dimensional simulation. We also perform two-dimensional simulations, enabling spontaneous breaking of the plane symmetry by the tearing instability of the current sheet. At moderate $A$ of main interest the tearing instability is suppressed. Dissipation transitions to normal, slower, magnetic reconnection at $Agg 1$. The fast dissipation described in this paper may occur in various objects with perturbed magnetic fields, including magnetars, jets from accreting black holes, and pulsar wind nebulae.
Plasmoids -- magnetized quasi-circular structures formed self-consistently in reconnecting current sheets -- were previously considered to be the graveyards of energetic particles. In this paper, we demonstrate the important role of plasmoids in shaping the particle energy spectrum in relativistic reconnection (i.e., with upstream magnetization $sigma_{rm up} gg 1$). Using two dimensional particle-in-cell simulations in pair plasmas with $sigma_{rm up}=10$ and $100$, we study a secondary particle energization process that takes place inside compressing plasmoids. We demonstrate that plasmoids grow in time, while their interiors compress, amplifying the internal magnetic field. The magnetic field felt by particles injected in an isolated plasmoid increases linearly with time, which leads to particle energization as a result of magnetic moment conservation. For particles injected with a power-law distribution function, this energization process acts in such a way that the shape of the injected power law is conserved, while producing an additional non-thermal tail $f(E)propto E^{-3}$ at higher energies followed by an exponential cutoff. The cutoff energy, which increases with time as $E_{rm cut}proptosqrt{t}$, can greatly exceed $sigma_{rm up} m_e c^2$. We analytically predict the secondary acceleration timescale and the shape of the emerging particle energy spectrum, which can be of major importance in certain astrophysical systems, such as blazar jets.
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