Production of intense episodic Alfv\'en pulses: GRMHD simulation of black hole accretion disks

TL;DR

This paper uses 3D GRMHD simulations to study episodic magnetic eruptions in black hole accretion disks, revealing intense Alfvén pulses that can accelerate particles to ultra-high energies, explaining observed gamma-ray variabilities.

Contribution

It introduces a novel simulation of episodic magnetic eruptions producing intense Alfvén pulses, linking disk dynamics to ultra-high energy cosmic rays and gamma-ray emissions.

Findings

Episodic magnetic eruptions generate intense Alfvén pulses in jets.

Alfvén pulses can accelerate particles to ultra-relativistic energies.

Model explains gamma-ray variability in blazars observed by Fermi.

Abstract

The episodic dynamics of the magnetic eruption of a spinning black hole (BH) accretion disks and its associated intense shapeup of their jets is studied via three-dimensional general-relativistic magnetohydrodynamics (GRMHD). The embedded magnetic fields in the disk get amplified by the magnetorotational instability (MRI) so large as to cause an eruption of magnetic field (recconection) and large chunks of matter episodically accrete toward the roots of the jets upon such an event. We also find that the eruption events produce intensive Alfv\'en pulses, which propagate through the jets. After the eruption, the disk backs to the weakly magnetic states. Such disk 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 $a_0=eE/m_e \omega c$ of these Alfv\'en wave pulses is extremely high for a substantial fraction of Eddington accretion rate accretion flow onto a supermassive black hole, the Alfv\'en shocks turn into ultrarelativistic $(a_0\gg 1)$ bow wake acceleration, manifesting into the 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 those of AGN (which is the primary target of this research), to micro-quasars. Properties such as time variabilities of blazar gamma-ray flares and spectrum observed by {\it Fermi} Gamma-ray Observatory are well explained by linear acceleration of electrons by the bow wake.