Department of Physics
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Item Magnetic field and EOS effects on axion emission from the non-rotating neutron stars(Elsevier, 2025-12) Sarkar, Tapomoy GuhaWe present here a comparative study of different axion emission mechanism from the core of highly magnetized neutron stars. In this work, it is assumed that neutron stars is being cooled as a consequence of the emission of neutrinos and axionic dark matter particles from the core of the neutron stars. We employ three different equation of states APR, FPS,SLY to solve the modified TOV equations, determine the mass and the pressure profiles by assuming that core of Neutron Stars to be made up of a hadronic matter. By employing above profiles, we obtain the cooling rate of spherically symmetric NSs with and without magnetic field to see the impact of magnetic field employing the different equation of states. The same is also studied with and without considering axion emission within the star core. Luminosities of axions as a dark matter candidates, neutrinos and photons have been plotted as a function of time with and without magnetic field. A fraction of these axions may undergo photon conversion in the powerful magnetic field within NSs magnetosphere, producing broadband radio or X-ray waves. Finally, we show the effect of magnetic field within the possible axion mass range on the actual observables such as total X-ray flux and conversion probability as a result of different axion to photon conversion mechanisms.Item Pulsars as Weber gravitational wave detectors(Elsevier, 2019-04) Das, ArpanA gravitational wave (GW) passing through a pulsar will lead to a variation in the moment of inertia of the pulsar affecting its rotation. This will affect the extremely accurately measured spin rate of the pulsar as well as its pulse profile (due to induced wobbling depending on the source direction). The effect will be most pronounced at resonance and should be detectable by accurate observations of the pulsar signal. The pulsar, in this sense, acts as a remotely stationed Weber detector of gravitational waves whose signal can be monitored on earth. With possible GW sources spread around in the universe, pulsars in their neighborhoods can provide us a family of remote detectors all of which can be monitored on earth. Even if GW are detected directly by earth based conventional detectors, such pulsar detectors can provide additional information for accurate determination of the source location. This can be of crucial importance for sources which do not emit any other form of radiation such as black hole mergers. For the GW events already detected by LIGO (and Virgo), we propose that one should look for specific pulsars which would have been disturbed by these events, and will transmit this disturbance via their pulse signals in any foreseeable future. One should be able to predict these future pulsar events with some accuracy so that a focused effort can be made to detect any possible changes in the signals of those specific pulsars.Item Probing phase transitions in a pulsar core through the observable effects on pulse profile modulation(Springer, 2024-07) Layek, BiswanathThere are compelling arguments in favour of various baryon-rich exotic QCD phases in the core of a pulsar. We suggested a technique to probe such phases by studying the effects of phase transition-induced density fluctuations on pulse profile modulation. Such density fluctuations cause the initial moment of inertia tensor (MI) of an oblate shape pulsar to get random additional contributions for each component. These contributions are assumed to be Gaussian of width , which characterizes the strength of density fluctuations. Using sample values of and the pulsar’s deformation parameter , we solve Euler’s equations for the rotational dynamics of the pulsar to observe the effects of wobbling through the modifications of pulse profiles. Our results show a specific pattern in the perturbed pulses. The wobbling of the pulsar dies away once the density fluctuations fade away. This feature distinguishes the transient pulse modulations from the pre-existing wobbling. The decay time of these modulations, being directly related to the relaxation time of density fluctuations, it provides valuable information about the nature of phase transition.Item Detecting superfluid transition in the pulsar core(OUP, 2024-07) Layek, BiswanathIt is believed that the core of a neutron star can be host to various novel phases of matter, from nucleon superfluid phase to exotic high baryon density quantum chromodynamics (QCD) phases. Different observational signals for such phase transitions have been discussed in the literature. Here, we point out a unique phenomenon associated with phase transition to a superfluid phase, which may be the nucleon superfluid phase or a phase like the colour-flavour locked phase, allowing for superfluid vortices. In any superfluid phase transition, a random network of vortices forms via the so-called Kibble–Zurek mechanism, which eventually mostly decays away, finally leaving primarily vortices arising from the initial angular momentum of the core. This transient, random vortex network can have a non-zero net angular momentum for the superfluid component, which will generally be oriented in an arbitrary direction. This is in contrast to the final vortices, which arise from initial rotation and hence have the initial angular momentum of the neutron star. The angular momentum of the random vortex network is balanced by an equal and opposite angular momentum in the normal fluid due to the conservation of angular momentum, thereby imparting an arbitrarily oriented angular momentum component to the outer shell of the neutron star. This will affect the pulse timing and pulse profile of a pulsar. These changes in the pulses will decay away in a characteristic manner that this as the random vortex network decays, obeying specific scaling laws leading to universal features for the detection of superfluid transitions occurring in a pulsar core.Item Glitches due to quasineutron-vortex scattering in the superfluid inner crust of a pulsar(APS, 2023-01) Layek, BiswanathWe revisit the mechanism of vortex unpinning caused by the neutron-vortex scattering [B. Layek and P. R. Yadav, Mon. Not. R. Astron. Soc. 499, 455 (2020)] in the inner crust of a pulsar. The strain energy released by the crustquake is assumed to be absorbed in some part of the inner crust and causes pair-breaking quasineutron excitations from the existing free neutron superfluid in the bulk of the inner crust. The scattering of these quasineutrons with the vortex core normal neutrons unpins a large number of vortices from the thermally affected regions and results in pulsar glitches. We consider the geometry of a cylindrical shell of the affected pinning region to study the implications of the vortex unpinning in the context of pulsar glitches. We find that a pulsar can release about ∼1011–1013 vortices by this mechanism. These numbers are equivalent to the glitch size of orders ∼10−11–10−9 for Vela-like pulsars with the characteristic age τ≃104 years. We also suggest a possibility of a vortex avalanche triggered by the movement of the unpinned vortices. A rough estimate of the glitch size caused by an avalanche shows an encouraging result.Item Vortex unpinning due to crustquake-initiated neutron excitation and pulsar glitches(Oxford, 2020-09) Layek, BiswanathPulsars undergoing crustquake release strain energy, which can be absorbed in a small region inside the inner crust of the star and excite the free superfluid neutrons therein. The scattering of these neutrons with the surrounding pinned vortices may unpin a large number of vortices and effectively reduce the pinning force on vortex lines. Such unpinning by neutron scattering can produce glitches for Crab-like pulsars and Vela pulsar of size in the range of ∼10−8–10−7 and ∼10−9–10−8, respectively. Although we discuss here the crustquake-initiated excitation, the proposal is very generic and equally applicable for any other sources, which can excite the free superfluid neutrons, or can be responsible for superfluid – normal phase transition of neutron superfluid in the inner crust of a pulsar.