Search

We offer an exact analytical equation for the void central region. We show that the central density is solely determined by the amplitude of the initial perturbation. Our results suggest that N-body simulations somewhat overestimate the emptiness of voids: the majority of them should have the central underdensity |$\delta _\mathrm{ c} \gt -73{{\ \rm per\ cent}}$| , and there should be almost no voids with |$\delta _\mathrm{ c} \lt -88{{\ \rm per\ cent}}$|⁠. The central region of a void is a part of an open Friedmann's 'universe', and its evolution differs drastically from the Universe evolution: there is a long stage when the curvature term dominates that prevents the formation of galaxy clusters and massive galaxies inside voids. The density profile in the void centre should be very flat. We discuss some void models obtained by N-body simulations and offer some ways to improve them. We also show that the dark energy makes the voids less underdense. [ABSTRACT FROM AUTHOR]

Being generated, the relic neutrino background contained equal fractions of electron , muon , and taon neutrinos. We show that the gravitational field of our Galaxy and other nearby cosmic objects changes this composition near the Solar System. As a result, the relic background becomes enriched with taon and particularly muon neutrinos. The electron relic neutrinos are the rarest for a terrestrial observer. [ABSTRACT FROM AUTHOR]

The gravitational field of a galaxy group or cluster slows down the Hubble stream and turns its speed to zero at some radius R 0. We offer an exact analytical relation between R 0 and the mass of the group. [ABSTRACT FROM AUTHOR]

We suggest a possible mechanism of ultra diffuse galaxy formation: the UDGs may occur as a result of a central collision of galaxies. If the galaxies are young and contain a lot of gas, the collision may kick all the gas off the systems and thus strongly suppress any further star formation. As a result, the galaxies now have a very low surface brightness and other properties typical of the ultra diffuse galaxies. We use the Coma cluster (where numerous UDGs were recently discovered) to test the efficiency of the process. The mechanism works very well and can transform a significant fraction of the cluster population into ultra diffuse galaxies. The UDGs formed by the process concentrate towards the center of the cluster, and their globular cluster systems remain undamaged, in accordance with observational results. The projected surface density of UDGs in the cluster may help us to recognize the mechanism of UDG formation, or clarify relative contributions of several possible competitive mechanisms at work. [ABSTRACT FROM AUTHOR]

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 (n ~ 0.5); it has an extensive region with ρ ∝ r-2 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. [ABSTRACT FROM AUTHOR]

While N-body simulations testify to a cuspy profile of the central region of dark matter halos, 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 halos 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 is 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 a small Einasto index n ∼ 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 good agreement with observational data for at least some types of galaxies but clearly disagrees with N-body simulations. [ABSTRACT FROM AUTHOR]

Recent astronomical data strongly suggest that a significant part of the dark matter content of the Local Group and Virgo Supercluster is not incorporated into the galaxy halos and forms diffuse components of these galaxy clusters. A portion of the particles from these components may penetrate the MilkyWay 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 ~12% to the total dark matter density near Earth. The particles of the extragalactic dark matter stand out because of their high speed (~600 km s-1), i.e., they are much faster than the galactic dark matter. In addition, their speed distribution is very narrow (~20 km s-1). The particles have an isotropic velocity distribution (perhaps, in contrast to the galactic dark matter). The extragalactic dark matter should provide a significant contribution to the direct detection signal. If the detector is sensitive only to the fast particles (v > 450 km s-1), then 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. [ABSTRACT FROM AUTHOR]

ABSTRACT Contemporary cosmological conceptions suggest that the dark matter in haloes of galaxies and galaxy clusters has most likely a clumpy structure. If a stream of gas penetrates through it, a small-scale gravitational field created by the clumps disturbs the flow resulting in momentum exchange between the stream and the dark matter. In this article, we perform an analysis of this effect, based on the hierarchical halo model of the dark matter structure and Navarro-Frenk-White density profiles. We consider the clumps of various masses, from the smallest up to the highest ones M≥ 109 M⊙. It has been found that in any event the effect grows with the mass of the clump: not only the drag force acting on the clump but also its acceleration increases. We discuss various astrophysical systems. The mechanism proved to be ineffective in the case of galaxy or galaxy cluster collisions. On the other hand, it played an important role during the process of galaxy formation. As a result, the dark matter should have formed a more compact, oblate and faster rotating substructure in the halo of our Galaxy. We have shown that this thick disc should be more clumpy than the halo. This fact is very important for the indirect detection experiments since it is the clumps that give the main contribution to the annihilation signal. Our calculations show that the mechanism of momentum exchange between the dark and baryon matter is ineffective on the outskirts of the galactic halo. It means that the clumps from there were not transported to the thick disc, and this region should be more clumpy than the halo on the average. [ABSTRACT FROM AUTHOR]

ABSTRACT The velocity distribution of the dark matter particles on the outskirts of the Solar system remains unclear. We suggest to determine it using experimentally found properties of the oldest halo objects. Indeed, the oldest halo stars and globular clusters form a collisionless system, as well as dark matter particles do, and they evolved in the same gravitational field. If we accept this analogy, we can show that the velocity distribution of the dark matter particles should be highly anisotropic and have a sharp maximum near υ∼ 500 km s−1. The distribution is totally different from the Maxwell one. We analyse the influence of the distribution function on the results of dark matter detection experiments. It is found that the direct detection signal should differ noticeably from the one calculated from the Maxwell distribution with 〈υ〉≃ 220 km s−1, which is conventional for direct detection experiments (the ratio depends on the detector properties and typically falls within the range between 6 and 0.2). Moreover, the sharp distinction from the Maxwell distribution can be very essential to the observations of dark matter annihilation. [ABSTRACT FROM AUTHOR]

Abstract: 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. The calculation are performed in the ideal relativistic MHD approximation using an upwind conservative scheme that is based on a linear Riemann solver and the constrained transport method to evolve the magnetic field. The gravitational attraction of the black hole is introduced via the Kerr metric in the Kerr–Schild coordinates. We simulate various magnetic field configurations and torus models, both optically thick and thin for neutrinos. We have found an effect of alternation of the magnetic field orientation in the ultrarelativistic jet formed as a result of the collapse. The calculations show evidence for heating of the wind surrounding the collapsar by the shock waves generated at the jet–wind border. 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 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 the magnetization of the black hole, jet formation properties and the total energy yield. We estimate the total energy of accretion which transformed to jets as 1.3×1052 ergs which was sufficient to explain hypernova explosions like GRB 980425 or GRB 030329. [ABSTRACT FROM AUTHOR]

Abstract: The hypothesis is rapidly gaining popularity that the dark energy pervading our universe is extra-repulsive (). The density of such a substance (usually called phantom energy) grows with the cosmological expansion and may become infinite in a finite time producing a Big Rip. In this Letter we analyze the late stages of the universe evolution and demonstrate that the presence of the phantom energy in the universe is not enough in itself to produce the Big Rip. This singularity occurrence requires the fulfillment of some additional, rather strong conditions. A more probable outcome of the cosmological evolution is the decay of the phantom field into ‘normal’ matter. The second, more intriguing consequence of the presence of the phantom field is the possibility to introduce a cosmological scenario that does not contain a Big Bang. In the framework of this model the universe eternally expands, while its density and other physical parameters oscillate over a wide range, never reaching the Plank values. Thus, the universe evolution has no singularities at all. [Copyright &y& Elsevier]

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 the fact that black holes usually have some angular momentum, which can be interpreted as their rotation of a sort. If in the plasma surrounding the hole there is some magnetic field, it leads to electric field creation and, consequently, 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 newborn black hole can be significant (~ 1013C). 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. [ABSTRACT FROM AUTHOR]

We discuss the possibility of observing the products of the 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 younger the Universe was, the higher the dark matter concentration n and the annihilation rate (proportional to n2) 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 signal from the annihilation of WIMPs only if the dark matter structures in the Universe had appeared before the Universe became transparent for the annihilation products . We discuss in more detail physical conditions whereby this interpretation could be possible. [ABSTRACT FROM AUTHOR]

In this paper we consider dark matter particle annihilation in the gravitational field of black holes. We obtain the 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. [ABSTRACT FROM AUTHOR]

The phase dependence of the spectral properties of the X-ray source 4U0115+63 is analzed based on X-ray observations obtained with the Beppo-Sax observatory. The spectrum contains a feature that is interpreted as a cyclotron absorption line. Deep lines of the first three orders were always observed, while the fourth-order line was detected only at certain phases. The parameters of the cyclotron feature depend strongly on the phase, while the remaining spectrum varies only relatively weakly with phase. The cyclotron lines of various orders are appreciably non-equidistant, and the second-order line is nearly always deeper than the first-order line. Possible physical conditions in the active region of the source that could give rise to the observed spectrum are discussed. [ABSTRACT FROM AUTHOR]

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.© 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

The spectrum of cyclotron radiation produced by electrons with a strongly anisotropic velocity distribution is calculated taking into account higher harmonics. The motion of the electrons is assumed to be ultrarelativistic along the magnetic field and nonrelativistic across the field. One characteristic feature of the resulting spectrum is that harmonics of various orders are not equally spaced. The physical properties and observed spectra of four X-ray pulsars displaying higher cyclotron harmonics are analyzed. It was shown that at least in one of them, the cyclotron feature can apparently be only an emission line. Moreover, the observed harmonics are not equidistant, and display certain other properties characteristic of emission by strongly anisotropic ultrarelativistic electrons. In addition, there are indirect theoretical arguments that the electrons giving rise to cyclotron features in the spectra of X-ray pulsars are ultrarelativistic and characterized by strongly anisotropic distributions. As a result, estimates of the magnetic fields of X-ray pulsars (which are usually derived from the energies of cyclotron lines) and certain other physical parameters require substantial revision.© 2002 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

Supernova remnants (SNRs) are among the most important targets for γ -ray observatories. Being prominent non-thermal sources, they are very likely responsible for the acceleration of the bulk of Galactic cosmic rays (CRs). To firmly establish the SNR paradigm for the origin of cosmic rays, it should be confirmed that protons are indeed accelerated in, and released from, SNRs with the appropriate flux and spectrum. This can be done by detailed theoretical models which account for microphysics of acceleration and various radiation processes of hadrons and leptons. The current generation of Cherenkov telescopes has insufficient sensitivity to constrain theoretical models. A new facility, the Cherenkov Telescope Array (CTA), will have superior capabilities and may finally resolve this long standing issue of high-energy astrophysics. We want to assess the capabilities of CTA to reveal the physics of various types of SNRs in the initial 2000 years of their evolution. During this time, the efficiency to accelerate cosmic rays is highest. We perform time-dependent simulations of the hydrodynamics, the magnetic fields, the cosmic-ray acceleration, and the non-thermal emission for type Ia, Ic and IIP SNRs. We calculate the CTA response to the γ -ray emission from these SNRs for various ages and distances, and we perform a realistic analysis of the simulated data. We derive distance limits for the detectability and resolvability of these SNR types at several ages. We test the ability of CTA to reconstruct their morphological and spectral parameters as a function of their distance. Finally, we estimate how well CTA data will constrain the theoretical models. [ABSTRACT FROM AUTHOR]

Abstract  Some of the factors that could influence changes in phenology of the song thrush (Turdus philomelos) during spring migration have been analyzed in relation to the timing of their return to the south-east Baltic region over the last 40 years. These include wind direction and velocity, temperature and precipitation in the wintering areas and along the migratory route. In March, a significant correlation was found between the timing of passage in the Baltic region and both the air temperatures in wintering area and winds over the migratory route. In April, when mass migration of thrushes occurs, the timing of passage was correlated with temperature, winds and, partly, precipitation over the route of migration. In this month, the frequency of tailwinds increased significantly along the route of migration from south-west France to the Baltic region over the last 40 years. The tailwind frequency was correlated with the beginning, middle and end of spring passage, accounting for 51% of the variance of median dates of the passage. The higher ambient temperatures over the migratory route explain nearly 20% of the variance in the timing of passage. Our results suggest that the advance in the timing of spring migration is primarily due to (1) the increased frequency of tailwinds favorable for migratory flights over the migratory route, and (2) the earlier emergence of the conditions enhancing migration due to higher spring temperatures in the winter quarters and on the migratory route. [ABSTRACT FROM AUTHOR]