Abstract
Magnetic field can be amplified and twisted near a supermassive black hole residing in a galactic nucleus. At the same time magnetic null points develop near the horizon. We examine a large-scale oblique magnetic field near a rotating (Kerr) black hole as an origin of magnetic layers, where the field direction changes abruptly in the ergosphere region. In consequence of this, magnetic null points can develop by purely geometrical effects of the strong gravitational field and the frame-dragging mechanism. We identify magnetic nulls as possible sites of magnetic reconnection and suggest that particles may be accelerated efficiently by the electric component. The situation we discuss is relevant for starving nuclei of some galaxies which exhibit episodic accretion events, namely, Sagittarius A* black hole in our Galaxy.
Highlights
Most galaxies including the Milky Way are believed to host a supermassive black hole in the centre [13, 31]
We show that antiparallel field lines are brought into mutual contact, within the low-density conditions, by the frame-dragging action of the rotating black hole (BH)
We considered the influence of the black hole rotation acting onto the ordered magnetic field in the physical frame of a star orbiting a black hole, or plunging down to it
Summary
Most galaxies including the Milky Way are believed to host a supermassive black hole in the centre [13, 31]. We begin this investigation by assuming an organised (ordered) largescale magnetic field This is obviously a crude starting point, but a sensible one, representing the field generated by sources distant from the BH (astrophysical black holes are practically uncharged and possess no intrinsic dipole-like magnetic fields). One expects that a dissipation region develops where the magnetic field structure changes abruptly across a separatrix curve, so these spots, occurring just above the ISCO, can act as places where particles are energised [11] Ingredients necessary for this scenario to work – i.e. an ordered magnetic field due to external sources plus the diluted plasma environment of disturbed stars – are naturally present near Sgr A* black hole. Thereby the electric component is capable of accelerating the charged matter once it is injected in the area of the magnetic null
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