Overcomplete steerable pyramid filters and rotation invariance

A given (overcomplete) discrete oriented pyramid may be converted into a steerable pyramid by interpolation. We present a technique for deriving the optimal interpolation functions (otherwise called 'steering coefficients'). The proposed scheme is demonstrated on a computationally efficient oriented pyramid, which is a variation on the Burt and Adelson (1983) pyramid. We apply the generated steerable pyramid to orientation-invariant texture analysis in order to demonstrate its excellent rotational isotropy. High classification rates and precise rotation identification are demonstrated

Long term Ultra-Violet Variability of Seyfert galaxies

Flux variability is one of the defining characteristics of Seyfert galaxies, a class of active galactic nuclei (AGN). Though these variations are observed over a wide range of wavelengths, results on their flux variability characteristics in the ultra-violet (UV) band are very limited. We present here the long term UV flux variability characteristics of a sample of fourteen Seyfert galaxies using data from the International Ultraviolet Explorer acquired between 1978 and 1995. We found that all the sources showed flux variations with no statistically significant difference in the amplitude of UV flux variation between shorter and longer wavelengths. Also, the flux variations between different near-UV (NUV, 1850 - 3300 A) and far-UV (FUV, 1150 - 2000 A) passbands in the rest frames of the objects are correlated with no time lag. The data show indications of (i) a mild negative correlation of UV variability with bolometric luminosity and (ii) weak positive correlation between UV variability and black hole mass. At FUV, about 50% of the sources show a strong correlation between spectral indices and flux variations with a hardening when brightening behaviour, while for the remaining sources the correlation is moderate. In NUV, the sources do show a harder when brighter trend, however, the correlation is either weak or moderate.Comment: Accepted by Journal of Astrophysics and Astronom

Nanomagnetics with lasers

Both liquid and vapour phase pulsed laser deposition (PLD) techniques have been used to synthesize nanophase magnetic alloys of CoPt. While the liquid route results in soft phase (disordered fcc) nanoparticles of CoPt near equiatomic composition dispersed in a surfactant-polymer matrix, the conventional vapour phase PLD allows growth of high coercivity nanoscale structures of CoPt on (001) SrTiO3. The magnetization, M(T), dynamics of the colloidal particles is examined. Two distinct particle distributions are established from analysis of M(T) data, in conformity with results of electron microscopy. In vapour deposited films at low growth rate (~0.4 Å/s), morphology changes from a self-similar fractal to nanodots as the deposition temperature is raised from 700-800 °C. The large lattice mismatch between (001) SrTiO3 and the ac/bc plane of L10 ordered phase imparts tensile strain to the films whose morphological manifestations can be suppressed at high growth rates