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

Abstract

Abstract We propose magnetically arrested disks (MADs) in quiescent (low-luminosity) black hole (BH) binaries as the origin of 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, nonthermal protons and electrons are naturally accelerated by magnetic reconnections or stochastic acceleration by turbulence. Both thermal and nonthermal 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.