Browsing by Author "Konar, Sushan"

Now showing 1 - 14 of 14

Absorption of Electro-magnetic Waves in a Magnetized Medium
(2000-07-04) Ganguly, Avijit K.; Konar, Sushan
In continuation to our earlier work, in which the structure of the vacuum polarisation tensor in a medium was analysed in presence of a background electro-magnetic ﬁeld, we discuss the absorptive part of the vacuum polarization tensor. Using the real time formalism of ﬁnite temperature ﬁeld theory we calculate the absorptive part of 1-loop vacuum polarisation tensor in the weak ﬁeld limit (eB < m2 ). Estimates of the absorption probability are also made for diﬀerent physical conditions of the background medium.
Diamagnetic screening of the magnetic field in accreting neutron stars
(2002-03-01) Konar, Sushan; Choudhuri, Arnab Rai
A 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.
Diamagnetic screening of the magnetic field in accreting neutron stars
(2001-09-20) Choudhuri, Arnab Rai; Konar, Sushan
A 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
Effective neutrino photon interaction in a magnetized medium
(2001-05-01) Bhattacharya, Kaushik; Ganguly, Avijit K.; Konar, Sushan
Neutrino-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.
Evolution of multipolar magnetic field in isolated neutron stars
(2015-03-13) Mitra, Dipanjan; Konar, Sushan; Bhattacharya, Dipankar
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.
Magnetic field evolution of accreting neutron stars - III
(2015-03-13) Konar, Sushan; Bhattacharya, Dipankar
The 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.
Magnetic field evolution of accreting neutron stars - IV effect of the curvature of space-time
(2000-09-26) Konar, Sushan
The evolution of the magnetic ﬁeld in an accreting neutron star is investigated using a fully general relativistic treatment and assuming that initially the currents supporting the ﬁeld are completely conﬁned to the crust. We ﬁnd that the ﬁeld decay slows down due to the inclusion of the curvature of space-time but the ﬁnal results do not diﬀer signiﬁcantly from those obtained assuming a ﬂat space-time. We also ﬁnd that such modiﬁcations are small compared to the uncertainties introduced by a lack of precise knowledge of the neutron star micro-physics.
Magnetic field evolution of accreting neutron stars- II
(2015-03-13) Konar, Sushan; Bhattacharya, Dipankar
We 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.
Magnetic fields of neutron stars
(2015-03-01) Konar, Sushan; Bhattacharya, Dipankar
The 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.
Neutrino propagation in a weakly magnetized medium
(2002-03-02) Konar, Sushan; Das, Subinoy
Neutrino-photon processes, forbidden in vacuum, can take place in the presence of a thermal medium and/or an external electro-magnetic ﬁeld, mediated by the corresponding charged leptons (real or virtual). Such interactions affect the propagation of neutrinos through a magnetized plasma. We investigate the neutrino-photon absorptive processes, at the one-loop level, for massless neutrinos in a weakly magnetized plasma. We ﬁnd that there is no correction to the absorptive part of the axial-vector–vector amplitude due to the presence of a magnetic ﬁeld to the linear order in the ﬁeld strength.
Photon propagation in a magnetized medium
(2001-02-03) Konar, Sushan
Using 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.
Torque decay in the pulsar (p,p) diagrom effects of crustal ohmic dissipation and alignment
(2001-07-05) Tauris, T. M.; Konar, Sushan
We 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.
Whither strange pulsars ?
(2015-03-01) Konar, Sushan