IUCAA Preprints
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Item Evolution of multipolar magnetic field in isolated neutron stars(2015-03-13) Mitra, Dipanjan; Konar, Sushan; Bhattacharya, DipankarItem Magnetic field evolution of accreting neutron stars - III(2015-03-13) Konar, Sushan; Bhattacharya, DipankarThe evolutionary scenario of the neutron star magnetic field is examined assuming a spindown-induced expulsion of magnetic flux originally confined to the core, in which case the expelled flux undergoes ohmic decay. The nature of field evolution, for accreting neutron stars, is investigated incorporating the crustal microphysics and material movement due to accretion. This scenario explains the observed field strengths of neutron stars but only if the crustal lattice contains a large amount of impurity which is in direct contrast to the models that assume an original crustal field.Item Magnetic field evolution of accreting neutron stars- II(2015-03-13) Konar, Sushan; Bhattacharya, DipankarWe investigate the evolution of the magnetic field of isolated pulsars and of neutron stars in different kinds of binary systems, assuming the field to be originally confined to the crust. Our results for the field evolution in isolated neutron stars helps us to constrain the physical parameters of the crust. Modelling the full evolution of a neutron star in a binary system through several stages of interaction we compare the resulting final field strength with that observed in neutron stars in various types of binary systems. One of the interesting aspects of our result is a positive correlation between the rate of accretion and the final field strength, for which some observational indication already exists. Our results also match the overall picture of the field evolution in neutron stars derived from observations.Item Magnetic fields of neutron stars(2015-03-01) Konar, Sushan; Bhattacharya, DipankarThe evolution of the magnetic field is investigated for isolated as well as binary neutron stars. The overall nature of the field evolution is seen to be similar for an initial crustal field and an expelled flux. The major uncertainties of the present models of field evolution and the directions in which further investigation are required are also discussed in detail.Item Faraday effect : A field theoretical point of view(2015-03-01) Ganguly, Avijit K.; Konar, Sushan; Pal, Palash. B.We analyze the structure of the vacuum polarization tensor in the presence of a background electromagnetic field in a medium. The most general gauge invariant structure contains many form factors. We use various discrete symmetries and crossing symmetry to constrain these form factors. From these symmetry arguments, we show why the vacuum polarization tensor has to be even in the background field when there is no background medium. Taking then the background field to be purely magnetic, we evaluate the vacuum polarization to linear order in it. The result shows the phenomenon of Faraday rotation, i.e., the rotation of the plane of polarization of a plane polarized light passing through this background. Finally, we calculate the amount of Faraday rotation in different kinds of media - non-relativistic, completely degenerate, and ultra-relativistic.Item Whither strange pulsars ?(2015-03-01) Konar, SushanItem 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