Effect of the toroidal magnetic field on the runaway instability of relativistic tori

Runaway instability operates in fluid tori around black holes. It affects systems close to the critical (cusp overflowing) configuration. The runaway effect depends on the radial profile l(R) of the angular momentum distribution of the fluid, on the dimension-less spin a of the central black hole, and other factors, such as self-gravity. Previously it was demonstrated that, for the power-law dependence of the radial angular momentum profile, non-magnetized tori always become runaway stable for a sufficiently high positive value of q. Here we discuss the role of runaway instability within a framework of an axially symmetric model of perfect fluid endowed with a purely toroidal magnetic field. The gradual accretion of material over the cusp transfers the mass and angular momentum into the black hole, thereby changing the intrinsic parameters of the Kerr metric. We studied the effect of the ratio of gas to magnetic pressure and other parameters of the model on the evolution of critical configurations that are just on the verge of cusp overflow. We show that the toroidal magnetic component inside an accretion torus does not change the frequency of its oscillations significantly. We identify these oscillations as the radial epicyclic mode. These weak effects can trigger the runaway instability even in situations when the purely hydrodynamical regime of the torus is stable. On the other hand, in most cases the stable configuration remains unaffected, and the initial deviations gradually decay after several orbital periods. We show examples of the torus evolution depending on the initial magnetization beta, the slope q, and the spin a. The toroidal magnetic field plays a more important role in the early phases of the accretion process until the perturbed configuration finds a new equilibrium or disappears because of the runaway instability.Comment: Astronomy and Astrophysics accepte

Galactic Center Minispiral: Interaction Modes of Neutron Stars

Streams of gas and dust in the inner parsec of the Galactic center form a distinct feature known as the Minispiral, which has been studied in radio waveband as well as in the infrared wavebands. A large fraction of the Minispiral gas is ionized by radiation of OB stars present in the Nuclear Star Cluster (NSC). Based on the inferred mass in the innermost parsec ($\sim 10^6$ solar masses), over $\sim 10^3$ -- $10^4$ neutron stars should move in the sphere of gravitational influence of the SMBH. We estimate that a fraction of them propagate through the denser, ionized medium concentrated mainly along the three arms of the Minispiral. Based on the properties of the gaseous medium, we discuss different interaction regimes of magnetised neutron stars passing through this region. Moreover, we sketch expected observational effects of these regimes. The simulation results may be applied to other galactic nuclei hosting NSC, where the expected distribution of the interaction regimes is different across different galaxy types.Comment: 12 pages, 17 figures, published in Acta Polytechnic

Dust-enshrouded star near supermassive black hole: predictions for high-eccentricity passages near low-luminosity galactic nuclei

Supermassive black holes reside in cores of galaxies, where they are often surrounded by a nuclear cluster and a clumpy torus of gas and dust. Mutual interactions can set some stars on a plunging trajectory towards the black hole. We model the pericentre passage of a dust-enshrouded star during which the dusty envelope becomes stretched by tidal forces and is affected by the interaction with the surrounding medium. In particular, we explore under which conditions these encounters can lead to periods of enhanced accretion activity. We discuss different scenarios for such a dusty source. To this end, we employed a modification of the Swift integration package. Elements of the cloud were modelled as numerical particles that represent the dust component that interacts with the optically thin gaseous environment. We determine the fraction of the total mass of the dust component that is diverted from the original path during the passages through the pericentre at $\sim 10^3$ Schwarzschild radii and find that the main part of the dust ($\gtrsim 90\%$ of its mass) is significantly affected upon the first crossing. The fraction of mass captured at the second passage generally decreases to very low values. As an example, we show predictions for the dusty source evolution assuming the current orbital parameters of the G2 cloud (also known as Dusty S-Cluster Object, DSO) in our Galactic centre. Encounter of a core-less cloud with a supermassive black hole is, most likely, a non-repeating event: the cloud is destroyed. However, in the case of a dust-enshrouded star, part of the envelope survives the pericentre passage. We discuss an offset of $\lesssim 0.3$ arcsec between the centre of mass of the diverted part and the star along the eccentric orbit. Finally, we examine an interesting possibility of a binary star embedded within a common wind envelope that becomes dispersed at the pericentre passage.Comment: 18 pages, 15 figures, Astronomy and Astrophysics accepte

Multiple accretion events as a trigger for Sgr A* activity

Gas clouds are present in the Galactic centre, where they orbit around the supermassive black hole. Collisions between these clumps reduce their angular momentum, and as a result some of the clumps are set on a plunging trajectory. Constraints can be imposed on the nature of past accretion events based on the currently observed X-ray reflection from the molecular clouds. We discuss accretion of clouds in the context of enhanced activity of Sgr A* during the past few hundred years. We put forward a scenario according to which gas clouds bring material close to the horizon of the black hole on <~0.1 parsec scale. We have modelled the source intrinsic luminosity assuming that multiple events occur at various moments in time. These events are characterized by the amount of accreted material and the distribution of angular momentum. We parameterized the activity in the form of a sequence of discrete events, followed the viscous evolution, and calculated the luminosity of the system from the time-dependent accretion rate across the inner boundary. Accreting clumps settle near a circularization radius, spread there during the viscous time, and subsequently feed the black hole over a certain period. A significant enhancement (by factor of ten) of the luminosity is only expected if the viscous timescale of the inflow is very short. On the other hand, the increase in source activity is expected to be much less prominent if the latter timescale is longer and a considerable fraction of the material does not reach the centre. A solution is obtained under two additional assumptions: (i) the radiative efficiency is a decreasing function of the Eddington ratio; (ii) the viscous decay of the luminosity proceeds somewhat faster than the canonical L(t)~t^{-5/3} profile. We applied our scheme to the case of G2 cloud in the Galactic centre to obtain constraints on the core-less gaseous cloud model.Comment: Astronomy and Astrophysics accepte

Magnetized black holes: the role of rotation, boost, and accretion in twisting the field lines and accelerating particles

Combined influence of rotation of a black hole and ambient magnetic fields creates conditions for powerful astrophysical processes of accretion and outflow of matter which are observed in many systems across the range of masses; from stellar-mass black holes in binary systems to supermassive black holes in active galactic nuclei. We study a simplified model of outflow of electrically charged particles from the inner region of an accretion disk around a spinning (Kerr) black hole immersed in a large-scale magnetic field. In particular, we consider a non-axisymmetric magnetosphere where the field is misaligned with the rotation axis. In this contribution we extend our previous analysis of acceleration of jet-like trajectories of particles escaping from bound circular orbits around a black hole. While we have previously assumed the initial setup of prograde (co-rotating) orbits, here we relax this assumption and we also consider retrograde (counter-rotating) motion. We show that the effect of counter-rotation may considerably increase the probability of escape from the system, and it allows more efficient acceleration of escaping particles to slightly higher energies compared to the co-rotating disk.Comment: 11 pages, 6 figures; to appear in proceedings of 16th Marcel Grossmann Meeting, Session PT5 "Dragging is never draggy: MAss and CHarge flows in GR'' (id. #393), 5-10 July 202

Hot-spot model for accretion disc variability as random process - II. Mathematics of the power-spectrum break frequency

We study some general properties of accretion disc variability in the context of stationary random processes. In particular, we are interested in mathematical constraints that can be imposed on the functional form of the Fourier power-spectrum density (PSD) that exhibits a multiply broken shape and several local maxima. We develop a methodology for determining the regions of the model parameter space that can in principle reproduce a PSD shape with a given number and position of local peaks and breaks of the PSD slope. Given the vast space of possible parameters, it is an important requirement that the method is fast in estimating the PSD shape for a given parameter set of the model. We generated and discuss the theoretical PSD profiles of a shot-noise-type random process with exponentially decaying flares. Then we determined conditions under which one, two, or more breaks or local maxima occur in the PSD. We calculated positions of these features and determined the changing slope of the model PSD. Furthermore, we considered the influence of the modulation by the orbital motion for a variability pattern assumed to result from an orbiting-spot model. We suggest that our general methodology can be useful in for describing non-monotonic PSD profiles (such as the trend seen, on different scales, in exemplary cases of the high-mass X-ray binary Cygnus X-1 and the narrow-line Seyfert galaxy Ark 564). We adopt a model where these power spectra are reproduced as a superposition of several Lorentzians with varying amplitudes in the X-ray-band light curve. Our general approach can help in constraining the model parameters and in determining which parts of the parameter space are accessible under various circumstances.Comment: Astronomy and Astrophysics accepte