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

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.

Ultrahigh-energy diffuse gamma-ray emission from cosmic-ray interactions with medium surrounding acceleration sources

“…Some fresh sources in such a region may explain the excess (Bi et al 2009;Guo et al 2016a). Very recently, the DGE in the Galactic plane above 100 TeV energies was for the first time measured by the Tibet-ASγ experiment (Amenomori et al 2021), which has attracted wide attention for possible physical discussion (Kimura et al 2021;Dzhatdoev 2021;Fang & Murase 2021;Liu & Wang 2021;Qiao et al 2021;Huentemeyer 2021;Esmaili & Serpico 2021;Koldobskiy et al 2021;Bouyahiaoui et al 2021;Dzhappuev et al 2021;Li & Ma 2021;Tibaldo et al 2021;Nath Maity et al 2021). The Tibet-ASγ fluxes are higher than the prediction of the conventional CR propagation model, and additional components or modification of the conventional propagation framework may be needed (Amenomori et al 2021;Liu & Wang 2021;Qiao et al 2021).…”

Constraining the position of the knee in the galactic cosmic ray spectrum with ultra-high-energy diffuse $γ$-rays

“…The Galactic diffuse γ-ray emission (DGE) is expected to be produced by interactions between CRs and the interstellar medium (ISM) as well as the interstellar radiation field (ISRF), during the propagation of CRs in the Milky Way. Justly recently, the DGE in the Galactic plane at 957 TeV energies was for the first time measured by the Tibet-ASγ experiment [8], which has attracted wide attention for possible physical discussion [9][10][11][12][13][14][15][16][17][18][19][20][21]. This measurement shed new light to study the individual nuclear spectrum.…”