16 research outputs found
Relaxation of dark matter halos: how to match observational data?
We show that moderate energy relaxation in the formation of dark matter halos
invariably leads to profiles that match those observed in the central regions
of galaxies. The density profile of the central region is universal and
insensitive to either the seed perturbation shape or the details of the
relaxation process. The profile has a central core; the multiplication of the
central density by the core radius is almost independent of the halo mass, in
accordance with observations. In the core area the density distribution behaves
as an Einasto profile with low index (); it has an extensive region
with at larger distances. This is exactly the shape that
observations suggest for the central region of galaxies. On the other hand,
this shape does not fit the galaxy cluster profiles. A possible explanation of
this fact is that the relaxation is violent in the case of galaxy clusters;
however, it is not violent enough when galaxies or smaller dark matter
structures are considered. We discuss the reasons for this.Comment: 9 pages, 4 figures, accepted to Astronomy & Astrophysic
Why does Einasto profile index occur so frequently?
We consider the behavior of spherically symmetric Einasto halos composed of
gravitating particles in the Fokker-Planck approximation. This approach allows
us to consider the undesirable influence of close encounters in the N-body
simulations more adequately than the generally accepted criteria. The Einasto
profile with index is a stationary solution of the Fokker-Planck
equation in the halo center. There are some reasons to believe that the
solution is an attractor. Then the Fokker-Planck diffusion tends to transform a
density profile to the equilibrium one with the Einasto index . We
suggest this effect as a possible reason why the Einasto index
occurs so frequently in the interpretation of N-body simulation results. The
results obtained cast doubt on generally accepted criteria of N-body simulation
convergence.Comment: 7 pages, 2 figures, Accepted to JCA
Accretion of a massive magnetized torus on a rotating black hole
We present numerical simulations of the axisymmetric accretion of a massive
magnetized plasma torus on a rotating black hole. We use a realistic equation
of state, which takes into account neutrino cooling and energy loss due to
nucleus dissociations. We simulated various magnetic field configurations and
torus models, both optically thick and thin for neutrinos. It is shown that the
neutrino cooling does not significantly change either the structure of the
accretion flow or the total energy release of the system. The calculations
evidence heating of the wind surrounding the collapsar by the shock waves
generated at the jet-wind border. This mechanism can give rise to a hot corona
around the binary system like SS433.
Angular momentum of the accreting matter defines the time scale of the
accretion. Due to the absence of the magnetic dynamo in our calculations, the
initial strength and topology of the magnetic field determines magnetization of
the black hole, jet formation properties and the total energy yield. We
estimated the total energy transformed to jets as {ergs}
which was sufficient to explain hypernova explosions like GRB 980425 or GRB
030329.Comment: 11 pages, 9 figures, submitted to MNRA
Dark matter annihilation in the gravitational field of a black hole
In this paper we consider dark matter particle annihilation in the
gravitational field of black holes. We obtain exact distribution function of
the infalling dark matter particles, and compute the resulting flux and spectra
of gamma rays coming from the objects. It is shown that the dark matter density
significantly increases near a black hole. Particle collision energy becomes
very high affecting relative cross-sections of various annihilation channels.
We also discuss possible experimental consequences of these effects.Comment: 9 pages, 1 figur
The real and apparent convergence of N-body simulations of the dark matter structures: Is the Navarro–Frenk–White profile real?
While N-body simulations suggest a cuspy profile in the centra of the dark matter halos of galaxies, the majority of astronomical observations favor a relatively soft cored density distribution of these regions. The routine method of testing the convergence of N-body simulations (in particular, the negligibility of two-body scattering effect) is to find the conditions under which formed structures is insensitive to numerical parameters. The results obtained with this approach suggest a surprisingly minor role of the particle collisions: the central density profile remains untouched and close to the Navarro–Frenk–White shape, even if the simulation time significantly exceeds the collisional relaxation time . In order to check the influence of the unphysical test body collisions we use the Fokker–Planck equation. It turns out that a profile where is an attractor: the Fokker–Planck diffusion transforms any reasonable initial distribution into it in a time shorter than , and then the cuspy profile should survive much longer than τrτr, since the Fokker–Planck diffusion is self-compensated if . Thus the purely numerical effect of test body scattering may create a stable NFW-like pseudosolution. Moreover, its stability may be mistaken for the simulation convergence. We present analytical estimations for this potential bias effect and call for numerical tests. For that purpose, we suggest a simple test that can be performed as the simulation progresses and would indicate the magnitude of the collisional influence and the veracity of the simulation results