A double peaked pulse profile observed in GX 1+4

The hard X-ray pulsar GX 1+4 was observed several times in the last few years with a pair of balloon-borne Xenon filled Multi-cell Proportional Counters (XMPC). In a balloon flight made on 22 March 1995, the source was detected in a bright state, the average observed source count rate being $8.0\pm0.2/s$ per detector. X-ray pulsations with a period of $121.9\pm0.1$ s were detected in the source with a broad double peak pulse feature. When observed in December 1993 with the same instrument, the pulse profile of GX 1+4 showed a single peak. This change in the pulse profile to a double pulse structure in about 15 months indicates either activation of the opposite pole of the neutron star if the magnetic field is asymmetric or possibly a change in the beam pattern, from a pencil beam to a fan beam. Assuming a fan beam configuration, the pulse profile is used to find the inclinations of the magnetic axis and the viewing axis with the spin axis. The derived angles support the GINGA observations of a dip in the pulse profile which was resolved to have a local maximum in one of the observations and was explained with resonance scattering of cyclotron line energy photons by the accretion column (Makishima et al., \markcite{maki1988}, Dotani et al., \markcite{dotani1989}.). Compared to our previous observation of the same source with the same telescope (Rao et al., \markcite{rao1994}) a period change rate of $0.72 \pm 0.40 s/yr$ is obtained which is the lowest rate of change of period for this source since its discovery. Average pulse fraction in the hard X-ray range is low (30%), consistent with its anti correlation with luminosity as reported by us earlier (Rao et al., \markcite{rao1994}) and the observed spectrum is very hard (power law photon index $1.67\pm0.12$).Comment: 10 pages, to appear in A&

The Onset of Stationary and Oscillatory Convection in a Horizontal Porous Layer Saturated with Viscoelastic Liquid Heated and Soluted From Below: Effect of Anisotropy

The onset of double diffusive stationary and oscillatory convection in a viscoelastic Oldroyd type fluid saturated in an anisotropic porous layer heated and soluted from below is studied. The flow is governed by the extended Darcy model for Oldroyd fluid. Stability analysis based on the method of perturbations of infinitesimal amplitude is performed using the normal mode technique. The analysis examines the effect of the Darcy Rayleigh number, the solutal Darcy the Rayleigh number, the relaxation time, the retardation time and the Lewis number. Important conclusions include the destabilizing effect of the relaxation time, the Darcy Rayleigh number and the Lewis number and the stabilizing effect of the solutal Darcy Rayleigh number, the retardation time and anisotropy parameter. Some of the results are generalization of the previous findings for isotropic porous medium

Influence of Microstructure on Mechanical Properties of Snow

Snow, being composed of ice grains of varying shapes, sizes, orientations, etc., has been treated as a particulate material. The mechanical properties of snow have been described in terms of microstructural parameters. A set of variables, which characterise the microstructure of snow at the granular level, has been chosen and quantified following the techniques of quantitative stereology for section plane. The data of quasi-static tests, e.g. constant strain-rate creep tests, have been analysed to determine the Young's modulus and compactive viscosity and the same have been correlated with the microstructural parameters. Inspite of scatter, definite trends are discernible. Considering the fact that deformation of snow is associated with translation and rotation of constituent grains in such a way as to attain the most stable configuration, the concept of fabric reconstruction, which is characterised by the concentration of normals (to the tangent plane at the point of grain contact) in the direction of the applied load, has been examined. The results demonstrated the occurrence of fabric reconstruction during the process of deformation. Finally, a dimensionless quantity, called the microstructural index (I), has been proposed to adequately represent the influence of microstructure