Stochastic Particle Acceleration in Turbulence Generated by Magnetorotational Instability

Abstract

We investigate stochastic particle acceleration in accretion flows. It is believed that magnetorotational instability (MRI) generates turbulence inside accretion flows and that cosmic rays (CRs) are accelerated by the turbulence. We calculate equations of motion for CRs in the turbulent fields generated by MRI with the shearing box approximation and without back reaction to the field. Our results show that the CRs randomly gain or lose their energy through interaction with the turbulent fields. The CRs diffuse in the configuration space anisotropically: the diffusion coefficient in the direction of the unperturbed flow is about 20 times higher than the Bohm coefficient, while those in the other directions are only a few times higher than the Bohm. The momentum distribution is isotropic and its evolution can be described by the diffusion equation in momentum space where the diffusion coefficient is a power-law function of the CR momentum. We show that the shear acceleration works efficiently for energetic particles. We also cautiously note that in the shearing box approximation, particles that cross the simulation box many times along the radial direction undergo unphysical runaway acceleration by the Lorentz transformation, which needs to be taken into account with special care.

The Interplay of Magnetically Dominated Turbulence and Magnetic Reconnection in Producing Nonthermal Particles

“…In the stochastic acceleration range, the energy diffusion coefficient scales as D γ ∝ γ 2 (compare with the dashed black lines in the bottom panels). A similar dependence on the particle energy was also found in Lynn et al (2014); Kimura et al (2016Kimura et al ( , 2019; Wong et al (2019), and is consistent with particle acceleration by non-resonant and/or broadened resonant interactions with the turbulent fluctuations (e.g. Skilling 1975;Blandford & Eichler 1987;Schlickeiser 1989;Chandran 2000;Cho & Lazarian 2006;Lemoine 2019).…”

“…Micha lek & Ostrowsky 1996;Arzner et al 2006;Fraschetti & Melia 2008;O'Sullivan et al 2009;Teraki & Asano 2019) or it was provided by turbulent fields obtained from MHD simulations (e.g. Ambrosiano et al 1988;Dmitruk et al 2004;Kowal et al 2012;Dalena et al 2014;Lynn et al 2014;Kimura et al 2016;Beresnyak & Li 2016;Isliker et al 2017;González et al 2017;Kimura et al 2019). These approaches offer a useful strategy to study the problem of particle acceleration with relatively inexpensive computational simulations.…”

Acceleration and escape processes of high-energy particles in turbulence inside hot accretion flows

“…However, each astronomical object has a different driving mechanism of turbulence, which may lead to a different behaviour of the CR particles (see Roh et al 2016 for supernova remnants and Porth et al 2016 for pulsar wind nebulae). Kimura et al (2016) performed test-particle simulations in the MRI turbulence using the shearing box approximation (Hawley et al 1995). However, the shearing box approximation has a few inconsistencies with the hot accretion flows, such as geometrical thickness and non-negligible advection cooling (Narayan & Yi 1994).…”

Ultrahigh-energy cosmic-ray nuclei from black hole jets: Recycling galactic cosmic rays through shear acceleration

“…54]. Both the turbulence and magnetic field are likely to be strong in the jet, and the diffusion process in the strong turbulence is likely to be the Bohm limit [49,55]. Thus, we use the Bohm limit there, λ i,jet = E/(Z i eB jet ), where B jet is the magnetic field strength in the jet.…”