Magnetically Arrested Disks in Quiescent Black-Hole Binaries: Formation Scenario, Observable Signatures, and Potential PeVatrons

TL;DR

This paper proposes that magnetically arrested disks (MADs) in quiescent black-hole binaries explain their multiwavelength emission and could be the sources of cosmic rays near the knee energy, involving accretion physics and particle acceleration.

Contribution

It introduces a formation scenario for MADs in quiescent BH binaries and links them to observable signatures and cosmic-ray production, a novel connection in astrophysics.

Findings

Broadband emission consistent with optical and X-ray data

Protons accelerated to PeV energies and escape as cosmic rays

MADs can dominate cosmic-ray spectrum around the knee

Abstract

We propose magnetically arrested disks (MADs) in quiescent black-hole (BH) binaries as the origin of the multiwavelength emission, and argue that this class of sources can dominate the cosmic-ray spectrum around the knee. X-ray luminosities of Galactic BH binaries in the quiescent state are far below the Eddington luminosity, and thus, radiatively inefficient accretion flows (RIAFs) are formed in the inner region. Strong thermal and turbulent pressures in RIAFs produce outflows, which can create large-scale poloidal magnetic fields. These fields are carried to the vicinity of the BH by the rapid inflow motion, forming a MAD. Inside the MAD, non-thermal protons and electrons are naturally accelerated by magnetic reconnections or stochastic acceleration by turbulence. Both thermal and non-thermal electrons emit broadband photons via synchrotron emission, which are broadly consistent with the optical and X-ray data of the quiescent BH X-ray binaries. Moreover, protons are accelerated up to PeV energies and diffusively escape from these MADs, which can account for the cosmic-ray intensity around the knee energy.