The galaxy halo formation in the absence of violent relaxation and a universal density profile of the halo center

While N-body simulations testify for a cuspy profile of the central region of the dark matter haloes, observations favor a shallow, cored density profile of the central region of, at least, some spiral galaxies and dwarf spheroidals. We show that a central profile, very close to the observed one, inevitably forms in the center of dark matter haloes if we make a supposition about a moderate energy relaxation of the system during the halo formation. If we assume the energy exchange between dark matter particles during the halo collapse to be not too intensive, the profile is universal: it depends almost not at all on the properties of the initial perturbation and is very akin, but not identical, to the Einasto profile with small Einasto index $n\sim 0.5$. We estimate the size of the 'central core' of the distribution, i.e., the extent of the very central region with a respectively gentle profile, and show that the cusp formation is unlikely, even if the dark matter is cold. The obtained profile is in a good agreement with observational data for, at least, some types of galaxies, but clearly disagrees with N-body simulations.Comment: 8 pages, 4 figure

Electric charge estimation of a new-born black hole

Though a black hole can theoretically possess a very big charge ($Q/(\sqrt{G} M) \simeq 1$), the charge of the real astrophysical black holes is usually considered to be negligible. This supposition is based on the fact that an astrophysical black hole is always surrounded by some plasma, which is a very good conductor. However, it disregards that the black holes have usually some angular momentum, which can be interpreted as its rotation of a sort. If in the plasma surrounding the hole there is some magnetic field, it leads to the electric field creation and, consequently, to the charge separation. In this article we estimate the upper limit of the electric charge of stellar mass astrophysical black holes. We have considered a new black hole formation process and shown that the charge of a new-born black hole can be significant ($\sim 10^{13}$ {Coulombs}). Though the obtained charge of an astrophysical black hole is big, the charge to mass ratio is small $Q/(\sqrt{G} M) \sim 10^{-7}$, and it is not enough to affect significantly either the gravitational field of the star or the dynamics of its collapse.Comment: 11 pages, 1 figure, accepted to International Journal of Modern Physics

Extragalactic dark matter and direct detection experiments

Recent astronomical data strongly suggest that a significant part of the dark matter, composing the Local Group and Virgo Supercluster, is not incorporated into the galaxy haloes and forms diffuse components of these galaxy clusters. Apparently, a portion of the particles from these components may penetrate into the Milky Way and make an extragalactic contribution to the total dark matter containment of our Galaxy. We find that the particles of the diffuse component of the Local Group are apt to contribute $\sim 12$ to the total dark matter density near the Earth. The particles of the extragalactic dark matter stand out because of their high speed ($\sim 600$ {km/s}), i.e. they are much faster than the galactic dark matter. In addition, their speed distribution is very narrow ($\sim 20$ {km/s}). The particles have isotropic velocity distribution (perhaps, in contrast to the galactic dark matter). The extragalactic dark matter should give a significant contribution to the direct detection signal. If the detector is sensitive only to the fast particles ($v<450$ {km/s}), the signal may even dominate. The density of other possible types of the extragalactic dark matter (for instance, of the diffuse component of the Virgo Supercluster) should be relatively small and comparable with the average dark matter density of the Universe. However, these particles can generate anomaly high energy collisions in direct dark matter detectors.Comment: 5 pages, 2 figure

Dark matter annihilation at cosmological redshifts: possible relic signal from annihilation of weakly interacting massive particles

We discuss the possibility to observe the products of dark matter annihilation that was going on in the early Universe. Of all the particles that could be generated by this process we consider only photons, as they are both uncharged and easily detectable. The earlier the Universe was, the higher the dark matter concentration $n$ and the annihilation rate (proportional to $n^2$) were. However, the emission from the very early Universe cannot reach us because of the opacity. The main part of the signal was generated at the moment the Universe had just become transparent for the photons produced by the annihilation. Thus, the dark matter annihilation in the early Universe should have created a sort of relic emission. We obtain its flux and the spectrum. If weakly interacting massive particles (WIMPs) constitute dark matter, it is shown that we may expect an extragalactic gamma-ray signal in the energy range 0.5 - 20 {MeV} with a maximum near 8 {MeV}. We show that an experimentally observed excess in the gamma-ray background at 0.5 - 20 {MeV} could be created by the relic WIMPs annihilation only if the dark matter structures in the universe had appeared before the universe became transparent for the annihilation products ($z \simeq 300$). We discuss in more detail physical conditions whereby this interpretation could be possible.Comment: 8 pages, 3 figure

On collisions with unlimited energies in the vicinity of Kerr and Schwarzschild black hole horizons

Two particle collisions close to the horizon of the rotating nonextremal Kerr's and Schwarzschild black holes are analyzed. For the case of multiple collisions it is shown that high energy in the centre of mass frame occurs due to a great relative velocity of two particles and a large Lorentz factor. The dependence of the relative velocity on the distance to horizon is analyzed, the time of movement from the point in the accretion disc to the point of scattering with large energy as well as the time of back movement to the Earth are calculated. It is shown that they have reasonable order.Comment: 6 pages, 1 figure. arXiv admin note: significant text overlap with arXiv:1105.154

Light curve and neutrino spectrum emitted during the collapse of a nonrotating, supermassive star

The formation of a neutrino pulse emitted during the relativistic collapse of a spherical supermassive star is considered. The free collapse of a body with uniform density in the absence of rotation and with the free escape of the emitted neutrinos can be solved analytically by quadrature. The light curve of the collapsing star and the spectrum of the emitted neutrinos at various times are calculated.Comment: 17 pages, 2 figures, published in Astronomy Report