Black hole magnetosphere with small-scale flux tubes

Abstract

There is observational evidence that the X-ray continuum source that creates the broad fluorescent emission lines in some Seyfert Galaxies may be compact and located at a few gravitational radii above the black hole. We consider the possibility that this compact source may be powered by small scale flux tubes near the black hole that are attached to the orbiting accretion disk. As a first step, this paper investigates the salient features of black hole magnetospheres that contain small scale, disk-hole linking "closed" flux tubes, using the force-free approximation in an axisymmetric setting. We find that the extent of the closed zone is a result of the balance between the black hole spin induced twist in the closed zone and the confinement pressure of the external (open) field of the disk. The maximal extent of the closed zone, for a typical external confinement, is usually a few gravitational radii. The pressure competition between the closed zone and the external confinement could in principle lead to interesting dynamics and dissipation relevant for the compact X-ray corona.

Striped Blandford/Znajek jets from advection of small-scale magnetic field

“…fields connect the tip of the AD to the BH in the plunging region, it is more likely to open up the magnetic field lines or to develop vertical, ordered magnetic structures. In order to validate our own results, we have reproduced the numerical setup suggested by Yuan et al (2019a) in appendix C, and elaborate on it employing our GRFFE code. For that, we have run a set of ancillary models in which we use an AD setup, which combines the essential magnetospheric structure of Yuan et al (2019a) with our co-rotating models (Figure 11).…”

“…In order to validate our own results, we have reproduced the numerical setup suggested by Yuan et al (2019a) in appendix C, and elaborate on it employing our GRFFE code. For that, we have run a set of ancillary models in which we use an AD setup, which combines the essential magnetospheric structure of Yuan et al (2019a) with our co-rotating models (Figure 11). Especially, we have focused on the establishment of an equilibrium of loops for α = 2 with different loop-length l. These loop lengths have been chosen to bracket the critical loop length l crit = 3.2r g obtained by Parfrey et al (2015).…”

“…Our numerical models, backed up by the ancillary simulations employed to compare to Yuan et al (2019a), suggest that it is necessary to allow for several tens of rotational periods of the central object in order to build up inclined structures of twisted magnetic field lines (see Figures 11 and C2). Thus, there exists an additional relevant timescale to set the efficiency of the BZ process in our setup.…”

“…Prescribing the surface resistivity R of a disc-shaped central membrane (representing the BH) of radius r 1 and angular velocity Ω BH , they are able to mimic different degrees of co-rotation Figure C1. Snapshots from toy-models employed to compare our results to those of Yuan et al (2019a). The models consist of discshaped membranes (located at z = 0) emulating both the central object (a BH or generically a star) surrounded by a thin disc.…”

X-Ray Fluorescence from Super-Eddington Accreting Black Holes

“…The line profile has a dependence on the BH spin because the innermost edge of a thin disk is given by the innermost stable circular orbit (ISCO) of the BH, which is smaller for a BH with larger prograde spin. X-ray reverberation has also been used to study the nature of coronal formation, for which a full understanding has not yet been achieved (Blandford et al 2017;Yuan et al 2019). Therefore, various coronal geometries have been investigated.…”