152 research outputs found
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 (
solar masses), over -- 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
Schwarzschild radii and find that the main part of the dust ( 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 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
- …