Manjari Bagchi

Rotational parameters of strange stars in comparison with neutron stars

Abstract I study stellar structures, i.e. the mass, the radius, the moment of inertia and the oblateness parameter at different spin frequencies for strange stars and neutron stars in a comparative manner. I also calculate the values of the radii of the marginally stable orbits and Keplerian orbital frequencies. By equating kHz QPO frequencies to Keplerian orbital frequencies, I find corresponding orbital radii. Knowledge about these parameters might be useful in further modeling of the observed features from LMXBs with advanced and improved future techniques for observations and data analysis.

Ruling out Kozai resonance in highly eccentric galactic binary millisecond pulsar PSR J1903+0327

We investigate the observational signatures associated with one of the proposed formation scenario for the recently discovered highly eccentric binary millisecond pulsar (MSP) PSR J1903+0327 in the galactic plane. The scenario requires that the MSP to be part of a hierarchical triple (HT), consisting of inner and outer binaries, experiencing the Kozai resonance. Numerical modelling of a bound point-mass HT, while incorporating the effects due to the quadrupolar interactions between the binary orbits and dominant contributions to the general relativistic periastron precession in the inner binary, reveals that, at the present epoch, the orbital eccentricity of the binary MSP should decrease for reasonable ranges in the HT parameters. The estimated decrements in the orbital eccentricity of the inner binary are few parts in 10 5 , substantially higher than the reported accuracies in the estimation of the orbital eccentricity of the binary MSP, while employing various general relativistic timing models for isolated binary pulsars. For wide ranges in the allowed orbital parameters, the estimated rate of change in the eccentricity of the inner binary is orders of magnitude higher than the value recently measured by the pulsar timing analysis. Therefore, we rule out the scenario that the MSP is part of an HT undergoing the Kozai oscillations. The origin of this system in a typical globular cluster is also shown to be less likely than inferred in the discovery paper.

ORBITAL ECCENTRICITY OF BINARY RADIO PULSARS IN GLOBULAR CLUSTERS AND THE INTERACTION BETWEEN STARS

We analyze the observed distribution of the orbital eccentricity and period of binary radio pulsars in globular clusters using computational tools to simulate binary-single-star interactions. Globular clusters have different groups of pulsars arising from separate interaction scenarios. Intermediate eccentricities of cluster pulsars can mostly be accounted for by fly-bys although locally lower stellar densities at pulsar positions may alter the situation. Very high eccentricities are likely to be the result of exchanges and/or mergers of single stars with the binary companion of the pulsar.

In what sense a neutron star-black hole binary is the holy grail for testing gravity?

Pulsars in binary systems have been very successful to test the validity of general relativity in the strong field regime [1-4]. So far, such binaries include neutron star-white dwarf (NS-WD) and neutron star-neutron star (NS-NS) systems. It is commonly believed that a neutron star-black hole (NS-BH) binary will be much superior for this purpose. But in what sense is this true? Does it apply to all possible deviations?

RADIO PULSAR BINARIES IN GLOBULAR CLUSTERS: THEIR ORBITAL ECCENTRICITIES AND STELLAR INTERACTIONS

High sensitivity searches of globular clusters (GCs) for radio pulsars by improved pulsar search algorithms and sustained pulsar timing observations have so far yielded some 140 pulsars in more than two dozen GCs. The observed distribution of orbital eccentricity and period of binary radio pulsars in GCs have imprints of the past interaction between single pulsars and binary systems or of binary pulsars and single passing noncompact stars. It is seen that GCs have different groups of pulsars. These may have arisen out of exchange or merger of a component of the binary with the incoming star or a fly-by in which the original binary remains intact but undergoes a change of eccentricity and orbital period. We consider the genesis of the distribution of pulsars using analytical and computational tools such as STARLAB, which performs numerical scattering experiments with direct N-body integration. Cluster pulsars with intermediate eccentricities can mostly be accounted for by fly-bys, whereas those with high eccentricities are likely to be the result of exchanges and/or mergers of single stars with the binary companion of the pulsar, although there are a few objects which do not easily fit into this description. The corresponding distribution for galactic field pulsarsmorexa0» shows notable differences from the GC pulsar orbital period and eccentricity distribution. The long orbital period pulsars in the galactic field with frozen out low eccentricities are largely missing from the GCs, and we show that the ionization of these systems in GCs cannot alone account for the peculiarities.«xa0less

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.

Bound for entropy and viscosity ratio of strange quark matter

Abstract High energy density (ϵ) and temperature (T) links general relativity and hydrodynamics leading to a lower bound for the ratio of shear viscosity ( η ) and entropy density ( s ). We get the interesting result that the bound is saturated in the simple model for quark matter that we use for strange stars at the surface for T ∼ 80 MeV . At this T we have the possibility of cosmic separation of phases. At the surface of the star where the pressure is zero—the density ϵ has a fixed value for all stars of various masses with correspondingly varying central energy density ϵ c . Inside the star where this density is higher, the ratio of η / s is larger and are like the known results found for perturbative QCD. This serves as a check of our calculation. The deconfined quarks at the surface of the strange star at T = 80 MeV seem to constitute the most perfect interacting fluid permitted by nature.

A Unified Model for Repeating and Non-repeating Fast Radio Bursts

The model that fast radio bursts (FRBs) are caused by plunges of asteroids onto neutron stars can explain both repeating and non-repeating bursts. If a neutron star passes through an asteroid belt around another star, there would be a series of bursts caused by a series of asteroid impacts. Moreover, the neutron star would cross the same belt repetitively if it were in a binary with the star hosting the asteroid belt, leading to a repeated series of bursts. I explore the properties of neutron star binaries that could lead to the only known repeating FRB so far (FRB121102). In this model, the next two epochs of bursts are expected around 2017 February 27 and 2017 December 18. On the other hand, if the asteroid belt is located around the neutron star itself, then a chance fall of an asteroid from that belt onto the neutron star would lead to a non-repeating burst. Even a neutron star grazing an asteroid belt can lead to a non-repeating burst caused by just one asteroid plunge during the grazing. This is possible even when the neutron star is in a binary with the asteroid-hosting star, if the belt and the neutron star orbit are non-coplanar.

High-Density Skyrmion Matter and Neutron Stars

We examine neutron star properties based on a model of dense matter composed of B = 1 skyrmions immersed in a mesonic mean field background. The model realizes spontaneous chiral symmetry breaking nonlinearly and incorporates scale breaking of QCD through a dilaton vacuum expectation value that also affects the mean fields. Quartic self-interactions among the vector mesons are introduced on grounds of naturalness in the corresponding effective field theory. Within a plausible range of the quartic couplings, the model generates neutron star masses and radii that are consistent with a preponderance of observational constraints, including recent ones that point to the existence of relatively massive neutron stars M ~ 1.7 M☉ and radii R ~ 12–14 km. If the existence of neutron stars with such dimensions is confirmed, matter at supranuclear density is stiffer than extrapolations of most microscopic models suggest.