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

Abstract

We investigate acceleration and propagation processes of high-energy particles inside hot accretion flows. The magnetorotational instability (MRI) creates turbulence inside accretion flows, which triggers magnetic reconnection and may produce non-thermal particles. They can be further accelerated stochastically by the turbulence. To probe the properties of such relativistic particles, we perform magnetohydrodynamic simulations to obtain the turbulent fields generated by the MRI, and calculate orbits of the high-energy particles using snapshot data of the MRI turbulence. We find that the particle acceleration is described by a diffusion phenomenon in energy space with a diffusion coefficient of the hard-sphere type: D ǫ ∝ ǫ 2 , where ǫ is the particle energy. Eddies in the largest scale of the turbulence play a dominant role in the acceleration process. On the other hand, the stochastic behaviour in configuration space is not usual diffusion but superdiffusion: the radial displacement increases with time faster than that in the normal diffusion. Also, the magnetic field configuration in the hot accretion flow creates outward bulk motion of high-energy particles. This bulk motion is more effective than the diffusive motion for higher energy particles. Our results imply that typical active galactic nuclei that host hot accretion flows can accelerate CRs up to ǫ ∼ 0.1 − 10 PeV.

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.…”

Numerical models of neutrino and gamma-ray emission from magnetic reconnection in the core of radio-galaxies

“…According to the analytical calculations of [15], the acceleration rate by magnetic reconnection is balanced by CR energy losses at ∼ 3 × 10 17 eV for parameters corresponding to Cen A (see their Fig.3). On the other hand, [38] show that a similar acceleration mechanism driven by reconnection can accelerate CRs up to 10 PeV in typical active galactic nuclei hosting hot accretion flows. We then choose the intermediate value of ε cut = 5 × 10 16 eV for the cut-off energy in our models.…”

PIC methods in astrophysics: simulations of relativistic jets and kinetic physics in astrophysical systems

“…In all these simulations no shock has been found comparing with reflected and injected jet simulations. More advanced kinetic instabilities are found in magnetorotational instability (MRI) (e.g., Hoshino 2015;Kunz et al 2016;Hirabayashi and Hoshino 2017;Kimura et al 2019). Hirabayashi and Hoshino (2017) show a series of stratified-shearing-box simulations of collisionless accretion disks in the recently developed framework of kinetic MHD, which can handle finite non-gyrotropy of a pressure tensor.…”