Sushan Konar

Diamagnetic screening of the magnetic field in accreting neutron stars — II. The effect of polar cap widening

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. In this earlier 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 with 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 similar to5degrees-10degrees, 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.

Gravitational excitation of high frequency QPOs (Research Note)

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.

Strange pulsar hypothesis

It appears that there is a genuine shortage of radio pulsars with surface magnetic fields significantly smaller than ∼10 8 G. 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 cannot be lowered by the system spontaneously.

Neutrino propagation in a weakly magnetized medium

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.

Members of the double pulsar system PSR J0737-3039: Neutron stars or strange stars?

Abstract 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 cannot 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.

A divine surprise: The golden mean and the round-oil error

This article deals with an example of the systematic round-off error that can be encountered in numerical computations. The example is based on the recursion relation used for calculating higher powers of the golden mean. In the process, the link between the golden mean and the Fibonacci sequence also becomes apparent.

Gravity defied from potato asteroids to magnetised neutron stars: 2. The failed stars

Gravitation, the universal attractive force, acts upon all matter (and radiation) relentlessly. Stable extended structures can exist only when gravity is held off by other forces of nature. This series of articles explores this interplay, looking at objects that just missed being stars in this particular installment.