Department of Physics
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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 Pulsar as a weber detector of gravitational waves and a probe to its internal phase transitions(World Scientific, 2024) Layek, BiswanathIt is believed that cores of neutron stars provide a natural laboratory where exotic high baryon density phases of quantum chromo dynamics (QCD) may exist. In fact, the theoretically well-established neutron superfluid phase is also believed to be found only inside neutron stars. Focus on neutrons stars has tremendously intensified in recent years with the direct detection of gravitational waves (GWs) by LIGO/Virgo from binary neutron star (BNS) merger events which has allowed the possibility of directly probing the properties of the interior of a neutron star. A truly remarkable phenomenon manifested by rapidly rotating neutron stars is in their avatar as Pulsars. The accuracy of pulsar timing can reach the level of one part in 1015, comparable to that of atomic clocks. Indeed, it was such a great accuracy which had allowed the first indirect detection of GWs from a BNS system. Such an incredible accuracy of pulse timings points to a very interesting possibility. Any deformation of the pulsar, even if it is extremely tiny, has the potential of leaving its imprints on the pulses through introduction of tiny perturbations in the entire moment of inertia (MI) tensor. While, the diagonal components of perturbed MI tensor affect the pulse timings, the off-diagonal components lead to wobbling of pulsar, directly affecting the pulse profile. This opens up a new window of opportunity for exploring various phase transitions occurring inside a pulsar core, through induced density fluctuations, which may be observable as perturbations in the pulse timing as well as its profile. Such perturbations also naturally induce a rapidly changing quadrupole moment of the star, thereby providing a new source of GW emission. Another remarkable possibility arises when we consider the effect of an external GW on neutron star. With the possibility of detecting any minute changes in its configuration through pulse observations, the neutron star has the potential of performing as a Weber detector of GW. This brief review will focus on these specific aspects of a pulsar. Specifically, the focus will be on the type of physics which can be probed by utilizing the effect of changes in the MI tensor of the pulsar on pulse properties.Item Gravitational-wave detection using redshifted 21-cm observations(APS, 2009-06) Sarkar, Tapomoy GuhaA gravitational-wave traversing the line of sight to a distant source produces a frequency shift which contributes to redshift space distortion. As a consequence, gravitational waves are imprinted as density fluctuations in redshift space. The gravitational-wave contribution to the redshift space power spectrum has a different μ dependence as compared to the dominant contribution from peculiar velocities. This, in principle, allows the two signals to be separated. The prospect of a detection is most favorable at the highest observable redshift z. Observations of redshifted 21-cm radiation from neutral hydrogen hold the possibility of probing very high redshifts. We consider the possibility of detecting primordial gravitational waves using the redshift space neutral hydrogen power spectrum. However, we find that the gravitational-wave signal, though present, will not be detectable on superhorizon scales because of cosmic variance and on subhorizon scales where the signal is highly suppressed.Item Probing dynamics of phase transitions occurring inside a pulsar(Springer, 2015-12) Layek, Biswanath; Das, ArpanDuring the evolution of a pulsar, various phase transitions may occur in its dense interior, such as superfluid transition, as well as transition to various exotic phases of quantum chromodynamics (QCD). We propose a technique which allows to probe these phases and associated transitions by detecting changes in rotation of the star arising from density changes and fluctuations during the transition affecting stars moment of inertia (MI). Our results suggest that these changes may be observable, and may possibly account for glitches and anti-glitches. Accurate measurements of pulsar timing and intensity modulations (from wobbling) may be used to pin down the particular phase transition occurring inside the pulsar core. We also discuss the possibility of observing gravitational waves from the changes in the quadrupole moment (QM) arising from these rapidly evolving density fluctuations.Item Bursts of gravitational waves due to crustquake from pulsars(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.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.