Gamma-ray burst efficiency and possible physical processes shaping the early afterglow

The discovery by Swift that a good fraction of gamma-ray bursts (GRBs) have a slowly decaying X-ray afterglow phase led to the suggestion that energy injection into the blast wave takes place several hundred seconds after the burst. This implies that right after the burst the kinetic energy of the blast wave was very low and in turn the efficiency of production of γ-rays during the burst was extremely high, rendering the internal shocks model unlikely. We re-examine the estimates of kinetic energy in GRB afterglows and show that the efficiency of converting the kinetic energy into γ-rays is moderate and does not challenge the standard internal shock model. We also examine several models, including in particular energy injection, suggested to interpret this slow decay phase. We show that with proper parameters, all these models give rise to a slow decline lasting several hours. However, even those models that fit all X-ray observations, and in particular the energy injection model, cannot account self-consistently for both the X-ray and the optical afterglows of well-monitored GRBs such as GRB 050319 and GRB 050401. We speculate about a possible alternative resolution of this puzzle.     

The Hill stability of a binary or planetary system during encounters with a third inclined body

The dynamical interaction of a binary or planetary system and a third body moving on a parabolic orbit inclined to the system is discussed in terms of Hill stability for the full three-body problem. The situation arises in binary star disruption and exchange, in extrasolar planetary system disruption, exchange and capture. It is found that increasing the inclination of the third body decreases the Hill regions of stability. This makes exchange or disruption of the component masses more likely as does increasing the eccentricity of the binary.
The stability criteria are applied to determine possible disruption and capture distances for currently known extrasolar planetary systems.     

Clusters and groups of galaxies in the 2dF galaxy redshift survey: A new catalogue

We create a new catalogue of groups and clusters, applying the friends‐of‐friends method to the 2dF GRS final release. We investigate various selection effects due to the use of a magnitude limited sample. For this purpose we follow the changes in group sizes and mean galaxy number densities within groups when shifting nearby observed groups to larger distances. We study the distribution of sizes of dark matter haloes in N ‐body simulations and compare properties of these haloes and the 2dF groups. We show that at large distances from the observer luminous and intrinsically greater groups dominate, but in these groups only very bright members are seen, which form compact cores of the groups. These two effects almost cancel each other, so that the mean sizes and densities of groups do not change considerably with distance. Our final sample contains 10750 groups in the Northern part, and 14465 groups in the Southern part of the 2dF survey with membership N _gal ≥ 2. We estimate the total luminosities of our groups, correcting for group members fainter than the observational limit of the survey. The cluster catalogue is available at our web‐site ( http://www.aai.ee/∼maret/2dfgr.html ). (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

General relativistic analysis of structure and stability of charged fluid disks around compact objects

In this paper we give a detailed general relativistic formulation of the study of structure and stability of charged fluid disks around compact objects like black holes neglecting the self-gravitation of the disk itself. Having presented the general equations for equilibrium as well as for perturbations we solve explicitly the cases of rigidly and differentially rotating thin disks, with constant charge density and zero pressure, confined to the equatorial plane of the black hole. By using normal mode analysis we have analysed the stability of such disks under purely radial perturbations and find that the disks are generally stable. On leave of absence from Government College, Jagadalpur 494005     

On the global solution of the N-body problem

In connection with the publication (Wang Qiu-Dong, 1991) the Poincaré type methods of obtaining the maximal solution of differential equations are discussed. In particular, it is shown that the Wang Qiu-Dong'sglobal solution of the N-body problem has been obtained in Babadzanjanz (1979). First the more general results on differential equations have been published in Babadzanjanz (1978).     

Cosmological constraints from the X-ray gas mass fraction in relaxed lensing clusters observed with Chandra

We present precise measurements of the X-ray gas mass fraction for a sample of luminous, relatively relaxed clusters of galaxies observed with the Chandra observatory, for which independent confirmation of the mass results is available from gravitational lensing studies. Parametrizing the total (luminous plus dark matter) mass profiles using the model of Navarro, Frenk & White, we show that the X-ray gas mass fractions in the clusters asymptote towards an approximately constant value at a radius r ₂₅₀₀, where the mean interior density is 2500 times the critical density of the Universe at the redshifts of the clusters. Combining the Chandra results on the X-ray gas mass fraction and its apparent redshift dependence with recent measurements of the mean baryonic matter density in the Universe and the Hubble constant determined from the Hubble Key Project, we obtain a tight constraint on the mean total matter density of the Universe,     , and measure a positive cosmological constant,     . Our results are in good agreement with recent, independent findings based on analyses of anisotropies in the cosmic microwave background radiation, the properties of distant supernovae, and the large-scale distribution of galaxies.     

Formation and evolution of elliptical galaxies and QSO activity

We present the results of a numerical code that combines multi-zone chemical evolution with 1D hydrodynamics to follow in detail the evolution and radial behaviour of gas and stars during the formation of elliptical galaxies. We use the model to explore the links between the evolution and formation of elliptical galaxies and QSO activity. The knowledge of the radial gas flows in the galaxy allows us to trace metallicity gradients, and, in particular, the formation of a high-metallicity core in ellipticals. The high-metallicity core is formed soon enough to explain the metal abundances inferred in high-redshift quasars. The star formation rate and the subsequent feedback regulate the episodes of wind, outflow and cooling flow, thus affecting the recycling of the gas and the chemical enrichment of the intergalactic medium. The evolution of the galaxy shows several stages, some of which are characterized by a complex flow pattern, with inflow in some regions and outflow in other regions. All models, however, exhibit during their late evolution a galactic wind at the outer boundary and, during their early evolution, an inflow towards the galactic nucleus. The characteristics of the inner inflow could explain the bolometric luminosity of a quasar lodged at the galactic centre as well as the evolution of the optical luminosity of quasars.