2001 (IPP)
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Item Photon propagation in a magnetized medium(2001-02-03) Konar, SushanUsing the real time formalism of the finite temperature field theory we calculate the 1-loop polarization tensor in the presence of a background magnetic field in a medium. The expression is obtained to linear order in the background field strength. We discuss the Faraday rotation as well as the photon absorption probabilities in this context.Item Torque decay in the pulsar (p,p) diagrom effects of crustal ohmic dissipation and alignment(2001-07-05) Tauris, T. M.; Konar, SushanWe investigate the evolution of pulsars in the (P, ˙ P) diagram. We first present analytical formulae to follow the evolution of a pulsar using simple exponential models for magnetic field decay and alignment. We then compare these evolutionary tracks with detailed model calculations using ohmic decay of crustal neutron star magnetic fields. We find that, after an initial phase with a small braking index, n, pulsars evolve with enhanced torque decay (n ≫ 3) for about 1 Myr. The long term evolution depends on the impurity parameter of the crust. If impurities are negligible in older isolated pulsars we expect their true age to be approximately equal to their observed characteristic age, τ = P/(2 ˙ P). It is not possible from data to constrain model parameters of the neutron star crust.Item Effective neutrino photon interaction in a magnetized medium(2001-05-01) Bhattacharya, Kaushik; Ganguly, Avijit K.; Konar, SushanNeutrino-photon processes, forbidden in vacuum, can take place in presence of a thermal medium or an external electro-magnetic field, mediated by the corresponding charged leptons (real or virtual). The effect of a medium or an electromagnetic field is two-fold - to induce an effective ν−γ vertex and to modify the dispersion relations of all the particles involved to render the processes kinematically viable. It has already been noted that in presence of a thermal medium such an electromagnetic interaction translates into the neutrino acquiring a small effective charge. In this work, we extend this concept to the case of a thermal medium in presence of an external magnetic field and calculate the effective charge of a neutrino in the limit of a weak magnetic field. We find that the effective charge is direction dependent which is a direct effect of magnetic field breaking the isotropy of the space.Item Diamagnetic screening of the magnetic field in accreting neutron stars(2001-09-20) Choudhuri, Arnab Rai; Konar, SushanA possible mechanism for screening of the surface magnetic field of an accreting neutron star, by the accreted material, is investigated. In particular, we investigate the nature of the evolution of the internal field configuration in the case of a) a polar cap accretion and b) a spherical accretion