arXiv subjects
Publications and source records attributed to Sushan Konar.
There appears to be more than one statistically different populations in the energy distribution of glitches, strongly indicative of different mechanisms accessing different energy ranges responsible for them.
The super-massive white dwarf models proposed by Das & Mukhopadhyaya [Phys. Rev. D 86 042001 (2012)], based on modifying the equation of state by a super-strong magnetic field in the centre, are very far from equilibrium because of the neglect of Lorentz forces, as has recently been shown by Nityananda & Konar [arXiv:1306.1625].
This article briefly reviews our current understanding (or lack thereof) of the evolution of magnetic fields in neutron stars, with an emphasis on the binary systems. In particular, the significance of the newly emerging population of accreting millisecond pulsars (AMXP) is discussed.
In this thesis we address the question of {\em the evolution of the magnetic field in 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. Therefore, in this thesis 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 as described below : 1. the effect of diamagnetic screening on the final field of a neutron star accreting material from its binary companion; 2. evolution of magnetic flux located in the crust of an accreting neutron star; 3. application of the above-mentioned model to real systems and a comparison with observations; 4. an investigation into the consequences of magnetic flux being initially located in the core of the star and its observational implications.
Many of the characteristic properties of the millisecond pulsars found in globular clusters are markedly different from those in the Galactic disc. We find that one such physical parameter is the surface magnetic field strength. Even though the average spin-periods do not differ much the average surface magnetic field is 2-5 times larger in the globular cluster pulsars. This effect could be apparent, arising due to one or more of several biases. Alternatively, if future observations confirm this effect to be real, then this could be interpreted as a preferential recycling of pulsars in tight binaries where the mass transfer takes place at high accretion rates.
The single glitch observed in PSR B1821-24, a millisecond pulsar in M28, is unusual on two counts. First, the magnitude of this glitch is at least an order of magnitude smaller ($Δν/ ν\sim 10^{-11}$) than the smallest glitch observed to date. Secondly, all other glitching pulsars have strong magnetic fields with $B \gsim 10^{11} G$ and are young, whereas PSR B1821-24 is an old recycled pulsar with a field strength of $2.25\times10^9 G$. We have suggested earlier that some of the recycled pulsars could actually be strange quark stars. In this work we argue that the crustal properties of such a {\em strange} pulsar are just right to give rise to a glitch of this magnitude, explaining the scarcity of larger glitches in millisecond pulsars.
There has been strong observational evidence suggesting a causal connection between the binary history of neutron stars and the evolution of their magnetic field. In this article we discuss one of the plausible mechanisms proposed for the evolution of the surface magnetic field, that of the diamagnetic screening of the field by accreted material.
We discuss the possibility that high-frequency QPOs in neutron-star binary systems may result from forced resonant oscillations of matter in the innermost parts of the accretion disc, excited by gravitational perturbations coming from asymmetries of the neutron star or from the companion star. We find that neutron-star asymmetries could, in principle, be effective for inducing both radial and vertical oscillations of relevant amplitude while the binary companion might possibly produce significant radial oscillations but not vertical ones. Misaligned neutron-star quadrupole moments of a size advocated elsewhere for explaining limiting neutron star periods could be large enough also for the present purpose.
One interesting method of constraining the dense matter Equations of State is to measure the advancement of the periastron of the orbit of a binary radio pulsar (when it belongs to a double neutron star system). There is a great deal of interest on applicability of this procedure to the double pulsar system PSR J0737-3039 (A/B). Although the above method can be applied to PSR A in future within some limitations, for PSR B this method can not be applied. On the other hand, the study of genesis of PSR B might be useful in this connection and its low mass might be an indication that it could be a strange star.
It appears that there is a genuine shortage of radio pulsars with surface magnetic fields significantly smaller than $\sim 10^8$ Gauss. We propose that the pulsars with very low magnetic fields are actually strange stars locked in a state of minimum free energy and therefore at a limiting value of the magnetic field which can not be lowered by the system spontaneously.
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.
Millisecond pulsars, with magnetic fields weaker by three to four orders compared to those of ordinary pulsars, are presumed to be neutron stars spun up by binary accretion. We expect the magnetic field to get screened by the accreted material. Our simulation of this screening mechanism shows, for the first time, that the field decreases by a purely geometric factor $\sin^{-7/2} θ_{\rm P,i}$ before freezing to an asymptotic value, where $ θ_{\rm P,i}$ is the initial angular width of the polar cap. If $ θ_{\rm P,i}$ lies in the range $5^o$--$10^o$, then the magnetic field diminution factor turns out to be $\sim 10^3$--$10^4$ in conformity with observational data.
We discuss the nature of neutrino propagation in the presence of strong magnetic fields of Magnetars.
Recently, we have proposed a model for the screening of the magnetic field of an accreting neutron star by the accreted material flowing from the polar regions towards the equator and sinking there underneath the surface (Choudhuri & Konar). In this model it was assumed that the flow pattern remained stationary over time. However, as the surface magnetic field weakens, the accretion takes place over a wider region around the pole, making the flow more radial and isotropic. In the present work, we extend this two-dimensional model to include the time-dependence of the flow of the accreted material. The final radial flow is found to be less efficient in screening the magnetic field compared to the initial tangential flow. After an initial phase of rapid decay, the magnetic field slowly reaches an asymptotic value when the accretion becomes nearly isotropic and radial. Assuming the initial extent of the polar cap to be $\sim 5^0$--$10^0$, a simple geometric argument suggests that the magnetic field should decay by 3-4 orders of magnitude before stabilizing to an asymptotic value, consistent with the magnetic fields observed in millisecond pulsars.
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.
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.
A possible mechanism for screening of the surface magnetic field of an accreting neutron star, by the accreted material, is investigated. We model the material flow in the surface layers of the star by an assumed 2-D velocity field satisfying all the physical requirements. Using this model velocity we find that, in the absence of magnetic buoyancy, the surface field is screened (submergence of the field by advection) within the time scale of material flow of the top layers. On the other hand, if magnetic buoyancy is present, the screening happens over a time scale which is characteristic of the slower flow of the deeper (hence, denser) layers. For accreting neutron stars, this longer time scale turns out to be about $10^5$ years, which is of the similar order as the accretion time scale of most massive X-ray binaries.