Supermassive Black Holes as Possible Sources of Ultrahigh-energy Cosmic Rays

Abstract

The production and acceleration mechanisms of ultrahigh-energy cosmic rays (UHECRs) of energy >1020 eV, clearly beyond the GZK cutoff limit, remain unclear, which points to the exotic nature of the phenomena. Recent observations of extragalactic neutrinos may indicate that the source of UHECRs is an extragalactic supermassive black hole (SMBH). We demonstrate that ultraefficient energy extraction from a rotating SMBH driven by the magnetic Penrose process (MPP) could indeed fit the bill. We envision ionization of neutral particles, such as neutron beta decay, skirting close to the black hole horizon that energizes protons to over 1020 eV for an SMBH of mass 109 M ⊙ and magnetic field 104 G. Applied to the Galactic center SMBH, we have a proton energy of order ≈1015.6 eV that coincides with the knee of the cosmic-ray spectra. We show that large γ z factors of high-energy particles along the escaping directions occur only in the presence of an induced charge of the black hole, which is known as the Wald charge in the case of a uniform magnetic field. It is remarkable that the process requires neither an extended acceleration zone nor fine-tuning of accreting-matter parameters. Further, this leads to certain verifiable constraints on the SMBH’s mass and magnetic field strength as the source of UHECRs. This clearly makes the ultraefficient regime of the MPP one of the most promising mechanisms for fueling the UHECR powerhouse.

Influence of Cosmic Repulsion and Magnetic Fields on Accretion Disks Rotating around Kerr Black Holes

“…The Lorentz factors obtained in this way can be of order of 10 even for mediate values of the magnetic parameter magnitude, B ∼ 1, as demonstrated in [33]. However, for elementary particles in the vicinity of realistic magnetized black holes, the magnetic parameter magnitude can be by many orders higher, implying much more efficient acceleration of elementary particles by the electromagnetic field, much larger energy of these particles and related Lorentz factor reaching many orders of 10 [39]. The ionization of Keplerian disks by irradiation, or other ways as neutron decay on electrons and protons, can be treated as a magnetic Penrose process (MPP) that in combination with the chaotic scattering can be considered as a very simple model of creation of jets in the close vicinity of the horizon of magnetized black holes-in the MPP, the energy of the orbital motion E 0 can be enormous due to the action of the electric part of the electromagnetic field governed by the potential component A t , and the chaotic scattering near the black hole horizon can transform the whole energy of the E 0 mode into the energy of the translational motion E z , giving escaping particles with Lorentz factor γ >> 1.…”

“…On the other hand, the closed section corresponding to bound orbits (case a) governs the regular epicyclic motion, if the charge particle energy is low enough that the approximation of linear harmonic oscillations can be relevant, or chaotic motion, if the energy is high enough. The second case of the chaotic motion can be for very high energies shifted to the process of chaotic scattering [33] causing transformation from the circular regular motion to a final state of regular motion along the magnetic field lines demonstrating potentially an extremely efficient version of the MPP [39]. We briefly expose both the epicyclic motion, and the chaotic scattering.…”

“…First, we discuss the role of the external magnetic fields in the charged test particle motion and its connections to the astrophysical processes, namely in the fate of ionized Keplerian disks and its possible relation to the high-frequency quasiperiodic oscillations (HF QPOs) observed in microquasars or some active galactic nuclei [22,31,32]. We emphasize the possible high importance of the Magnetic Penrose Process (MPP) [80] in the case of ionized Keplerian disks and related extreme acceleration of charged particles in the vicinity of the Kerr black hole horizon [10,33,39,81] influenced by radiation-reaction due to synchrotron radiation of escaping particles [36]. 4 Second, we discuss the influence of the external magnetic fields on the special charged toroidal structures levitating above (below) the black hole equatorial plane, complementary to the toroidal charged structures located in the equatorial plane.…”

“…All the possible realistic ionization scenarios imply a simple model of ionized Keplerian disks based on the MPP process: a neutral particle becomes charged while its mechanical momentum remains conserved and starts to feel the influence of the electromagnetic field. This model was introduced in [33] where particle escape velocities and structure of escape zones were explored that were later studied also in [10,34,39]. This ionization model was also used to study the regular regime of the epicyclic oscillations [32] and its relation to the HF QPOs [22], or the fate of the Keplerian disks in dependence on the inclination angle in the case of non-rotating black holes [10,35].…”

“…The matter forming an electrically neutral (Keplerian) accretion disk orbiting a magnetized black hole can get ionized, e.g., by an irradiation or some instability processes, and starts to feel the external electromagnetic field (the black hole rotation induces in the vicinity of the horizon an electric field related to the asymptotically uniform magnetic field [23]). Under the influence of the electromagnetic field, the original purely circular motion of the electrically neutral matter has to be transformed into one of the following regimes of the motion of created charged particles: (a) regular oscillatory motion possibly reflecting the high-frequency X-ray quasiperiodic oscillations observed in microquasars [22], (b) destruction of the ionized region of the disk due to the radial infall into the black hole, (c) chaotic motion governing transformation of the Keplerian disk into thick toroidal structure, in combination with the creation of winds, (d) relativistic jets for the case of rotating black holes [33] and charged particles that could have even ultra-high energy [38,39].…”