International Journal of Modern Physics D

The lateral distribution (LD) of particles in extensive air showers (EAS) of cosmic rays (CRs) broadens as the cascade propagates in the atmosphere. A universal relation between LD parameters and shower age can be used to estimate the age and/or maximum depth, X_max, basing on EAS measurements with surface arrays. We have simulated this relation using a Monte Carlo method implemented in the CORSIKA code. Our aim is to specify an algorithm applicable in particular to Yakutsk array data analysis.

The environs of supermassive black holes are among the universe's most extreme phenomena. Understanding the physical processes occurring in the vicinity of black holes may provide the key to answer a number of fundamental astrophysical questions including the detectability of strong gravity effects, the formation and propagation of relativistic jets, the origin of the highest energy gamma-rays and cosmic rays, and the nature and evolution of the central engine in active galactic nuclei (AGN). As a step towards this direction, this paper reviews some of the progress achieved in the field based on observations in the very high energy domain. It particularly focuses on nonthermal particle acceleration and emission processes that may occur in the rotating magnetospheres originating from accreting, supermassive black hole systems. Topics covered include direct electric field acceleration in the black hole's magnetosphere, ultra-high energy cosmic ray production, Blandford–Znajek mechanism, centrifugal acceleration and magnetic reconnection, along with the relevant efficiency constraints imposed by interactions with matter, radiation and fields. By way of application, a detailed discussion of well-known sources (Sgr A*; Cen A; M87; NGC1399) is presented.

We study the properties of a steady, multi-species, low angular momentum accretion flow around a Schwarzschild black hole. Each species is described by a relativistic equation of state. We find that the transonic properties depend strongly on the composition of the flow. We find that an electron-positron pair plasma is the least relativistic one. This flow produces only one sonic point very close to the event horizon and does not show multiple critical points for any angular momentum or energy. When the baryons are present, the number of critical points depend on the specific energy content. Since the number of critical points decide whether the flow will have nonlinearities or shock waves, our results imply that whether standing shocks will form or not depends on the flow composition. Thus, for instance, a pure electron-positron pair plasma will never undergo a shock transition, while mixing it with some baryons (common in outflows and jets, for example) as in a completely ionized gas, will have shocks. We study in detail how the baryon loading affects the shock properties and discuss the implications in astrophysical observations.

We consider the feasibility of using the Galileo Navigation Satellite System to constrain possible extensions or modifications to general relativity, by assessing the impact of the related additions to the Newtonian potential and comparing with the available observables: the relative frequency shift and the time-delay of light propagation. We address the impact of deviations from General Relativity based on the parametrized Post-Newtonian parameters due to the presence of a cosmological constant, of a constant acceleration like the putative Pioneer anomaly, a Yukawa potential term due to massive scalar fields and a power-law potential term, which can arise from Ungravity or f(R) theories.

In this paper we discussed the observational aspects of rotation in the Universe on different scales. We show dependence between the angular momentum of the structures and their size. The presented observational situation is that the galaxies, their pairs and compact groups have a non-vanishing angular momentum. In the structures of mass corresponding to groups of galaxies, this feature has not been found, while in the clusters and superclusters, alignment of galaxy orientation has been actually found. Also we know that galaxies have net angular momentum due to the fact that we actually measure the rotation curves of galaxies. These facts lead to the conclusion that theories which connect galaxy angular momentum with its surrounding structure are at some extend favored by data. We show that in the light of scenarios of galaxy formations this result could be interpreted as an effect of tidal force's mechanism, but it is also consistent with Li's model, in which galaxies form in the rotating universe. The theoretical and observational aspects of possible global rotation of the Universe were discussed as well.

In this paper we study the isotropic cases of static charged fluid spheres in general relativity. For this purpose we consider two different specializations and under these we solve the Einstein–Maxwell field equations in isotropic coordinates. The analytical solutions thus obtained are matched to the exterior Reissner–Nordström solutions which concern the values for the metric coefficients e^ν and e^μ. We derive the pressure, density and pressure-to-density ratio at the center of the charged fluid sphere and boundary R of the star. Our conclusion is that static charged fluid spheres provide a good connection to compact stars.

We identify the factors responsible for the appearance of energy density inhomogeneities in a self-gravitating fluid, and describe the evolution of those factors from an initially homogeneous distribution. It is shown that a specific combination of the Weyl tensor and/or local anisotropy of pressure and/or dissipative fluxes entails the formation of energy density inhomogeneities. Different cases are analyzed in detail and in the particular case of dissipative fluids, the role of relaxational processes as well as nonlocal effects are brought out.

We investigate a k-essence field localized on the brane evolving linearly with the cosmological time for some kinetic functions and consider the atypical k-essence model for linear and nonlinear k-fields in Friedmann–Robertson–Walker (FRW) cosmology. In the former case the k-field is driven by an inverse quadratic polynomial potential and the solutions exhibit several different behaviors which include expanding, contracting and bouncing universes as well as a model with a finite time span, some of them ending in a big crunch or a big rip. In the latter case we find the potential and show that the atypical k-essence model is dynamically incomplete. Particularly, by selecting the extended tachyonic kinetic functions we analyze the high and low energy limits of our model, obtaining the nearly power-law solution. We introduce a tachyon field with negative energy density and show that the universe evolves between two singularities.