arXiv subjects

Sushan Konar

Publications and source records attributed to Sushan Konar.

48 records · Page 3Linked to original sources

arXiv2002

Effective Neutrino Photon Interaction in a Magnetized Medium

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.

hep-ph↗

arXiv2001

Photon Propagation in a Magnetized Medium

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.

hep-ph↗

arXiv2001

Torque Decay in the Pulsar (P,P_dot) Diagram. Effects of Crustal Ohmic Dissipation and Alignment

We investigate the evolution of pulsars in the (P,P-dot) 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, tau = P/(2*P-dot). It is not possible from data to constrain model parameters of the neutron star crust.

astro-ph↗

arXiv2000

Absorption of Electro-magnetic Waves in a Magnetized Medium

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 ($eB < m^2$). Estimates of the absorption probability are also made for different physical conditions of the background medium.

hep-ph↗

arXiv2000

Evolution of the Magnetic Field in Accreting Neutron Stars

There has been sufficient observational indication suggesting a causal connection between the binary history of neutron stars and the evolution of their magnetic field. In particular, it is believed that the generation of the low-field millisecond pulsars is a consequence of the processing of normal high-field neutron stars in binary systems. We try to understand the mechanism of field evolution in neutron stars that are members of binary systems with an aim to understand the problem of millisecond pulsar generation. To this end we have looked at four related problems : i. the effect of diamagnetic screening on the final field of a neutron star accreting material from its binary companion; ii. evolution of magnetic flux located in the crust of an accreting neutron star; iii. application of the above-mentioned model to real systems and a comparison with observations; iv. an investigation into the consequences of magnetic flux being initially located in the core of the star and its observational implications.

astro-ph↗

arXiv2000

Whither Strange Pulsars ?

Both neutron stars and strange stars are capable of supporting fast rotations observed in pulsars. On the basis of this it has been argued that some of the pulsars could be strange stars. We investigate whether strange stars can sustain characteristic pulsar magnetic fields ($10^8 - 10^{13.5} Gauss$) over astronomically significant time-scales. Furthermore, we check whether strange stars fit into the general scenario of field evolution of pulsars. It is found that as far as the evolution of the magnetic field is concerned the strange pulsar hypothesis runs into serious difficulties to explain the observational data.

astro-ph↗

arXiv1999

Magnetic Fields of Neutron Stars

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.

astro-ph↗

arXiv1999

Faraday effect : a field theoretical point of view

We analyze the structure of the vacuum polarization tensor in the presence of a background electromagnetic field in a medium. We use various discrete symmetries and crossing symmetry to constrain the form factors obtained for the most general case. 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. We find that the usual expression for Faraday rotation, which is derived for a non-degenerate plasma in the non-relativistic approximation, undergoes substantial modification if the background is degenerate and/or relativistic. We give explicit expressions for Faraday rotation in completely degenerate and ultra-relativistic media.

hep-ph↗

arXiv1999

Magnetic Field Evolution of Accreting Neutron Stars - III

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 may explain 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.

astro-ph↗

arXiv1999

Evolution of Multipolar Magnetic Field in Isolated Neutron Stars

The evolution of the multipolar structure of the magnetic field of isolated neutron stars is studied assuming the currents to be confined to the crust. We find that except for multipoles of very high order ($l\gsim 25$) the evolution is similar to that of a dipole. Therefore no significant evolution is expected in pulse shape of isolated radio pulsars due to the evolution of the multipole structure of the magnetic field.

astro-ph↗

arXiv1998

Magnetic Field Evolution of Accreting Neutron Stars - II

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.

astro-ph↗

arXiv1997

qq' -> qq': a second look at the IR divergences

The transport properties of the quark-gluon plasma is important both from the point of view of the astrophysical systems and the heavy ion collisions. Therefore, accurate calculations of the elastic scattering cross-sections of $qq' \to qq'$ and $ q\qbar' \to q\qbar'$ processes are required. At the tree-diagram level exchange of massless gluons leads to divergent cross sections for such processes. An effective way to remove the infrared divergence is to introduce a Debye mass as an infrared regulator into the gluon propagator for the thermal gluons. In this work we find that at the two-gluon exchange level such a technique fails to remove the divergences. In a chiral symmetry broken phase the mass could be introduced following the standard procedure which cures the divergence. But in a chirally symmetric phase chiral invariant mass of the fermions needs to be considered.

hep-ph↗