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The upcoming Ooty Wide Field Array (OWFA) will operate at $326.5 \, {\rm MHz}$ which corresponds to the redshifted 21-cm signal from neutral hydrogen (HI) at z = 3.35. We present two different prescriptions to simulate this signal and calculate the visibilities expected in radio-interferometric observations with OWFA. In the first method we use an input model for the expected 21-cm power spectrum to directly simulate different random realizations of the brightness temperature fluctuations and calculate the visibilities. This method, which models the HI signal entirely as a diffuse radiation, is completely oblivious to the discrete nature of the astrophysical sources which host the HI. While each discrete source subtends an angle that is much smaller than the angular resolution of OWFA, the velocity structure of the HI inside the individual sources is well within reach of OWFA's frequency resolution and this is expected to have an impact on the observed HI signal. The second prescription is based on cosmological N-body simulations. Here we identify each simulation particle with a source that hosts the HI, and we have the freedom to implement any desired line profile for the HI emission from the individual sources. Implementing a simple model for the line profile, we have generated several random realizations of the complex visibilities. Correlations between the visibilities measured at different baselines and channels provides an unique method to quantify the statistical properties of the \HI signal. We have used this to quantify the results of our simulations, and explore the relation between the expected visibility correlations and the underlying HI power spectrum., Comment: To appear in the Special Section of the JAA on the Ooty Wide Field Array

Tomographic intensity mapping of the HI using the redshifted 21 cm observations opens up a new window towards our understanding of cosmological background evolution and structure formation. This is a key science goal of several upcoming radio telescopes including the Square Kilometer Array (SKA). In this article we focus on the post-reionization signal and investigate the of cross correlating the 21 cm signal with other tracers of the large scale structure. We consider the cross-correlation of the post-reionization 21 cm signal with the Lyman-alpha forest, Lyman-break galaxies and late time anisotropies in the CMBR maps like weak lensing and the Integrated Sachs Wolfe effect. We study the feasibility of detecting the signal and explore the possibility of obtaining constraints on cosmological models using it., Comment: To appear in Journal of Astrophysics and Astronomy (JOAA) special issue on "Science with the SKA: an Indian perspective"

We consider the coupling of the magnons in both quantum ferromagnets and antiferromagnets to the longitudinal order-parameter fluctuations, and the resulting nonanalytic behavior of the longitudinal susceptibility. In classical magnets it is well known that long-range correlations induced by the magnons lead to a singular wave-number dependence of the form $1/k^{4-d}$ in all dimensions 2

We consider the problem of localizing a target taking the help of a set of anchor beacon nodes.A small number of beacon nodes are deployed at known locations in the area.The target can detect a beacon provided it happens to lie within the beacons's transmission range.Thus, the target contains a measurement vector containing the readings of the beacons: '1' corresponding to a beacon if it is able to detect the target and '0' if the beacon is not able to detect the target.The goal is two fold: to determine the location of the target based on the binary measurement vector at the target and to study the behavior of the localization uncertainty as a function of the beacon transmission range and the number of beacons deployed.Beacon transmission range means signal strength of the beacon to transmit and receive the signals which is called as Received Signal Strength.To localize the target, we propose a grid mapping based approach, where the readings corresponding to locations on a grid overlaid on a region of interest are used to localize a target.To study the behavior of the localization uncertainty as a function of the sensing radius and number of beacons,extensive simulations and numerical experiments are carried out.The results provide insights into an importance of optimally setting the sensing radius and the improvement obtainable with increasing number of beacons.

We show that the single-particle and transport relaxation rates in ferromagnetic metals, which determine the thermal and electrical conductivity, respectively, at asymptotically low temperature do not obey a power law as previously thought, but rather show an exponential temperature dependence. The reason is the splitting of the conduction band that inevitably results from a nonzero magnetization. At higher temperatures there is a sizable temperature window where the transport rate shows a T^2 temperature dependence, in accord with prior results. This window is separated from the asymptotic regime by a temperature scale that is estimated to range from tens of mK to tens of K for typical ferromagnets. We motivate and derive a very general effective theory for metallic magnets that we then use to derive these results. Comparisons with existing experiments are discussed, and predictions for future experiments at low temperatures are made., Comment: 14pp, 3 figs; expanded discussion

The assumption that the universe is homogeneous and isotropic on large scales is one of the fundamental postulates of cosmology. We have tested the large scale homogeneity of the galaxy distribution in the Sloan Digital Sky Survey Data Release One (SDSS-DR1) using volume limited subsamples extracted from the two equatorial strips which are nearly two dimensional (2D). The galaxy distribution was projected on the equatorial plane and we carried out a 2D multi-fractal analysis by counting the number of galaxies inside circles of different radii in the range 5 Mpc/h to 150 Mpc/h centred on galaxies. Different moments of the count-in-cells were analysed to identify a range of length-scales (60-70 Mpc/h to 150Mpc/h) where the moments show a power law scaling behaviour and to determine the scaling exponent which gives the spectrum of generalised dimension D_q. If the galaxy distribution is homogeneous, D_q does not vary with q and is equal to the Euclidean dimension which in our case is 2. We find that D_q varies in the range 1.7 to 2.2. We also constructed mock data from random, homogeneous point distributions and from LCDM N-body simulations with bias b=1, 1.6 and 2, and analysed these in exactly the same way. The values of D_q in the random distribution and the unbiased simulations show much smaller variations and these are not consistent with the actual data. The biased simulations, however, show larger variations in D_q and these are consistent with both the random and the actual data. Interpreting the actual data as a realisation of a biased LCDM universe, we conclude that the galaxy distribution is homogeneous on scales larger than 60-70 Mpc/h., Comment: 8 pages including 4 figures. Submitted to MNRAS