Production of intense episodic Alfvén pulses: GRMHD simulation of black hole accretion discs

The episodic dynamics of the magnetic eruption of spinning black hole (BH) accretion discs and the associated intense shape-up of their jets are studied via three-dimensional general-relativistic magnetohydrodynamics (GRMHD). The embedded magnetic fields in the disc are amplified by a magnetorotational instability (MRI) so large as to cause an eruption of the magnetic field (reconnection) and large chunks of matter accrete episodically toward the roots of the jets upon such an event. We also find that the eruption events produce intensive Alfvén pulses, which propagate through the jets. After the eruption, the disc returns to the weakly magnetic state. Such disc activities cause short-time variabilities in mass accretion rate at the event horizon, as well as electromagnetic luminosity inside the jet. Since the dimensionless strength parameter a₀ = eE/m_eωc of these Alfvén wave pulses is extremely high for a substantial fraction of Eddington accretion rate accretion flows on to supermassive black holes, the Alfvén shocks turn into ultrarelativistic (a₀ ≫ 1) bow wake acceleration, manifesting as ultra-high-energy cosmic rays and electrons, which finally emit gamma-rays. Since our GRMHD model has universality in its spatial and temporal scales, it is applicable to a wide range of astrophysical objects, ranging from active galactic nuclei (AGNs, the primary target of this research) to micro-quasars. Properties such as the time variabilities of blazar gamma-ray flares and the spectrum observed by the Fermi Gamma-ray Observatory are explained well by linear acceleration of electrons by a bow wake.