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Register a new userBlack hole magnetosphere with small scale flux tubes--II. Stability and dynamics
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In some Seyfert Galaxies, the hard X-rays that produce fluorescent emission lines are thought to be generated in a hot corona that is compact and located at only a few gravitational radii above the supermassive black hole. We consider the possibility that this X-ray source may be powered by small scale magnetic flux tubes attached to the accretion disk near the black hole. We use three dimensional, time dependent force-free simulations in a simplified setting to study the dynamics of such flux tubes as they get continuously twisted by the central compact star/black hole. We find that, the dynamical evolution of the flux tubes connecting the central compact object and the accretion disk is strongly influenced by the confinement of the surrounding field. Although differential rotation between the central object and the disk tends to inflate the flux tubes, strong confinement from surrounding field quenches the formation of a jet-like outflow, as the inflated flux tube becomes kink unstable and dissipates most of the extracted rotational energy relatively close to the central object. Such a process may be able to heat up the plasma and produce strong X-ray emission. We estimate the energy dissipation rate and discuss its astrophysical implications.
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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.
Recent observations of SgrA* by the GRAVITY instrument have astrometrically tracked infrared flares (IR) at distances of $sim 10$ gravitational radii ($r_g$). In this paper, we study a model for the flares based on 3D general relativistic magnetohydrodynamic (GRMHD) simulations of magnetically arrested accretion disks (MADs) which exhibit violent episodes of flux escape from the black hole magnetosphere. These events are attractive for flare modeling for several reasons: i) the magnetically dominant regions can resist being disrupted via magneto-rotational turbulence and shear, ii) the orientation of the magnetic field is predominantly vertical as suggested by the GRAVITY data, iii) magnetic reconnection associated with the flux eruptions could yield a self-consistent means of particle heating/acceleration during the flare events. In this analysis we track erupted flux bundles and provide distributions of sizes, energies and plasma parameter. In our simulations, the orbits tend to circularize at a range of radii from $sim 5-40 r_g$. The magnetic energy contained within the flux bundles ranges up to $sim10^{40}$ erg, enough to power IR and X-ray flares. We find that the motion within the magnetically supported flow is substantially sub-Keplerian, in tension with the inferred period-radius relation of the three GRAVITY flares.
We discuss stationary and axisymmetric trans-magnetosonic outflows in the magnetosphere of a rotating black hole (BH). Ejected plasma from the plasma source located near the BH is accelerated far away to form a relativistic jet. In this study, the plasma acceleration efficiency and conversion of fluid energy from electromagnetic energy are considered by employing the trans-fast magnetosonic flow solution derived by Takahashi & Tomimatsu (2008). Considering the parameter dependence of magnetohydrodynamical flows, we search for the parameters of the trans-magnetosonic outflow solution to the recent M87 jet observations and obtain the angular velocity values of the magnetic field line and angular momentum of the outflow in the magnetized jet flow. Therefore, we estimate the locations of the outer light surface, Alfven surface, and separation surface of the flow. We also discuss the electromagnetic energy flux from the rotating BH (i.e., the Blandford-Znajek process), which suggests that the energy extraction mechanism is effective for the M87 relativistic jet.
We discuss the issues of stability of accretion disks that may undergo the limit-cycle oscillations due to the two main types of thermal-viscous instabilities. These are induced either by the domination of radiation pressure in the innermost regions close to the central black hole, or by the partial ionization of hydrogen in the zone of appropriate temperatures. These physical processes may lead to the intermittent activity in AGN on timescales between hundreds and millions of years. We list a number of observational facts that support the idea of the cyclic activity in high accretion rate sources. We conclude however that the observed features of quasars may provide only indirect signatures of the underlying instabilities. Also, the support from the sources with stellar mass black holes, whose variability timescales are observationally feasible, is limited to a few cases of the microquasars. Therefore we consider a number of plausible mechanisms of stabilization of the limit cycle oscillations in high accretion rate accretion disks. The newly found is the stabilizing effect of the stochastic viscosity fluctuations.
In a companion paper [1], we have presented a cross-correlation approach to near-horizon physics in which bulk dynamics is probed through the correlation of quantities defined at inner and outer spacetime hypersurfaces acting as test screens. More specifically, dynamical horizons provide appropriate inner screens in a 3+1 setting and, in this context, we have shown that an effective-curvature vector measured at the common horizon produced in a head-on collision merger can be correlated with the flux of linear Bondi-momentum at null infinity. In this paper we provide a more sound geometric basis to this picture. First, we show that a rigidity property of dynamical horizons, namely foliation uniqueness, leads to a preferred class of null tetrads and Weyl scalars on these hypersurfaces. Second, we identify a heuristic horizon news-like function, depending only on the geometry of spatial sections of the horizon. Fluxes constructed from this function offer refined geometric quantities to be correlated with Bondi fluxes at infinity, as well as a contact with the discussion of quasi-local 4-momentum on dynamical horizons. Third, we highlight the importance of tracking the internal horizon dual to the apparent horizon in spatial 3-slices when integrating fluxes along the horizon. Finally, we discuss the link between the dissipation of the non-stationary part of the horizons geometry with the viscous-fluid analogy for black holes, introducing a geometric prescription for a slowness parameter in black-hole recoil dynamics.
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