4,512 research outputs found
On the Clustering of GRBs on the Sky
The two-point correlation of the 4th (current) BATSE catalog (2494 objects)
is calculated. It is shown to be consistent with zero at nearly all angular
scales of interest. Assuming that GRBs trace the large scale structure in the
universe we calculate the angular correlation function for the standard CDM
(sCDM) model. It is shown to be at if the
BATSE catalog is assumed to be a volume-limited sample up to .
Combined with the error analysis on the BATSE catalog this suggests that nearly
GRBs will be needed to make a positive detection of the two-point
angular correlation function at this angular scale.Comment: 5 pages, Latex with aipproc.sty, incl. 1 ps-Fig., Proc. of the 5th
Huntsville Gamma Ray Burst Symposium, Oct. 1999, ed. R.M. Kippen, AI
Inequality and Network Structure
This paper explores the manner in which the structure of a social network constrains the level of inequality that can be sustained among its members. We assume that any distribution of value across the network must be stable with respect to coalitional deviations, and that players can form a deviating coalition only if they constitute a clique in the network. We show that if the network is bipartite, there is a unique stable payoff distribution that is maximally unequal in that it does not Lorenz dominate any other stable distribution. We obtain a complete ordering of the class of bipartite networks and show that those with larger maximum independent sets can sustain greater levels of inequality. The intuition behind this result is that networks with larger maximum independent sets are more sparse and hence offer fewer opportunities for coalitional deviations. We also demonstrate that standard centrality measures do not consistently predict inequality. We extend our framework by allowing a group of players to deviate if they are all within distance k of each other, and show that the ranking of networks by the extent of extremal inequality is not invariant in k.inequality;networks;coalitional deviations;power;centrality
Primordial magnetic field limits from cosmological data
We study limits on a primordial magnetic field arising from cosmological
data, including that from big bang nucleosynthesis, cosmic microwave background
polarization plane Faraday rotation limits, and large-scale structure
formation. We show that the physically-relevant quantity is the value of the
effective magnetic field, and limits on it are independent of how the magnetic
field was generated.Comment: 7 pages, 6 figure
Contribution of Galaxies to the Background Hydrogen-Ionizing Flux
We estimate the evolution of the contribution of galaxies to the cosmic
background flux at by means of a semi-analytic model of galaxy
formation and evolution. Such a modelling has been quite successful in
reproducing the optical properties of galaxies. We assume hereafter the
high-redshift damped Lyman- (DLA) systems to be the progenitors of
present day galaxies, and we design a series of models which are consistent
with the evolution of cosmic comoving emissivities in the available near
infrared (NIR), optical, ultraviolet (UV), and far infrared (FIR) bands along
with the evolution of the neutral hydrogen content and average metallicity of
damped Lyman- systems (DLA). We use these models to compute the
galactic contribution to the Lyman-limit emissivity and background flux for . We take into account the absorption of Lyman-limit photons by
HI and dust in the interstellar medium (ISM) of the galaxies. We find that the
background Lyman-limit flux due to galaxies might dominate (or be comparable
to) the contribution from quasars at almost all redshifts if the absorption by
HI in the ISM is neglected. The ISM HI absorption results in a severe
diminishing of this flux--by almost three orders of magnitude at high redshifts
to between one and two orders at . Though the resulting galaxy flux
is completely negligible at high redshifts, it is comparable to the quasar flux
at .Comment: 14 pages, 5 figures, requires mn.sty, accepted for publication in
MNRA
Algebraic-eikonal approach to medium energy proton scattering from odd-mass nuclei
We extend the algebraic-eikonal approach to medium energy proton scattering
from odd-mass nuclei by combining the eikonal approximation for the scattering
with a description of odd-mass nuclei in terms of the interacting boson-fermion
model. We derive closed expressions for the transition matrix elements for one
of the dynamical symmetries and discuss the interplay between collective and
single-particle degrees of freedom in an application to elastic and inelastic
proton scattering from Pt.Comment: latex, 14 pages, 4 figures uuencoded, to be published in Physical
Review
Luminosity Function of GRBs
We attempt to constrain the luminosity function of Gamma Ray Bursts (GRBs) from the observed number count--flux relation and the afterglow redshift data. We assume three classes of luminosity functions for our analysis: (a) Log-normal distribution, (b) Schechter distribution, and (c) Scale-free distribution. We assume several models of the evolution of the GRB population for each luminosity function. Our analysis shows that: (a) log-normal is the only luminosity function that is compatible with both the observations. This result is independent of the GRB evolution model, (b) for log-normal function, the average photon luminosity and the width of the luminosity function that are compatible with both the observations fall in the range: 10^{55} sec^{-1} \la L_0 \la 10^{56} sec^{-1} and 2 \la \sigma \la 3, (c) the agreement of observations with other luminosity functions requires the GRB population to evolve more strongly than the evolution of the star-formation rate of the universe
Isotropic and Anisotropic Regimes of the Field-Dependent Spin Dynamics in Sr2IrO4: Raman Scattering Studies
A major focus of experimental interest in Sr2IrO4 has been to clarify how the
magnetic excitations of this strongly spin-orbit coupled system differ from the
predictions of anisotropic 2D spin-1/2 Heisenberg model and to explore the
extent to which strong spin-orbit coupling affects the magnetic properties of
iridates. Here, we present a high-resolution inelastic light (Raman) scattering
study of the low energy magnetic excitation spectrum of Sr2IrO4 and doped
Eu-doped Sr2IrO4 as functions of both temperature and applied magnetic field.
We show that the high-field (H>1.5 T) in-plane spin dynamics of Sr2IrO4 are
isotropic and governed by the interplay between the applied field and the small
in-plane ferromagnetic spin components induced by the Dzyaloshinskii-Moriya
interaction. However, the spin dynamics of Sr2IrO4 at lower fields (H<1.5 T)
exhibit important effects associated with interlayer coupling and in-plane
anisotropy, including a spin-flop transition at Hc in Sr2IrO4 that occurs
either discontinuously or via a continuous rotation of the spins, depending
upon the in-plane orientation of the applied field. These results show that
in-plane anisotropy and interlayer coupling effects play important roles in the
low-field magnetic and dynamical properties of Sr2IrO4.Comment: 8 pages, 4 figures, submitte
Spectroscopy of the recurrent nova V3890 Sagittarii 18 d after the 1990 outburst
The spectrum of the recurrent nova V3890 Sgr, obtained from the Vainu Bappu Observatory 18 d after the 1990 outburst maximum, is presented. The nova was in the coronal line phase. The spectrum is similar to that of the recurrent nova RS Oph. An extinction E(B – V) = 1.1 is derived from the B – V colours, and from Balmer and He I line ratios. From the maximum magnitude-rate-of-decline relations for a nova, M v= – 8.6 mag is estimated, which places the nova at a distance of about 5 kpc. Balmer line fluxes are used to derive the density, ∼ 10 9 cm -3, and the mass of the ejected ionized shell, ∼10 -7 M☉. The temperature and radius estimates for the ionizing source are 3 × 10 5 K and 0.06 R☉. A helium abundance of 0.23 is estimated
Wonder Gift of Nature: Spirulina
Humans have to keep themselves healthy and fit to have a progressive life. A nation with healthy people moves ahead. In this context, it will be apt to be aware of the fact that nature has given India a very precious gift in the form of Spirulina, a blue green alga. Spirulina is very rich in protein content. Its utility as a dietary supplement and a therapeutic substance, not only for animals but also for human beings, is well
recognised
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