IUCAA Preprints

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    Temperature profiles of accretion discs around rapidly rotating strange stars in general relativity : a comparison with neutron stars
    (2001-07-05) Bhattacharyya, Sudip; Thampan, A. V.; Bombaci, Ignazio
    We compute the temperature pro les of accretion discs around rapidly rotating strange stars, using constant gravitational mass equilibrium sequences of these objects, considering the full e ect of general relativity. Beyond a certain critical value of stellar angular momentum (J), we observe the radius (rorb) of the innermost stable circular orbit (ISCO) to increase with J (a property seen neither in rotating black holes nor in rotating neutron stars). The reason for this is traced to the crucial dependence of drorb=dJ on the rate of change of the radial gradient of the Keplerian angular velocity at rorb with respect to J. The structure parameters and temperature pro les obtained are compared with those of neutron stars, as an attempt to provide signatures for distinguishing between the two. We show that when the full gamut of strange star equation of state models, with varying degrees of sti ness are considered, there exists a substantial overlap in properties of both neutron stars and strange stars. However, applying accretion disc model constraints to rule out sti strange star equation of state models, we notice that neutron stars and strange stars exclusively occupy certain parameter spaces. This result implies the possibility of distinguishing these objects from each other by sensitive observations through future X{ray detectors.
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    Torque decay in the pulsar (p,p) diagrom effects of crustal ohmic dissipation and alignment
    (2001-07-05) Tauris, T. M.; Konar, Sushan
    We investigate the evolution of pulsars in the (P, ˙ P) diagram. We first present analytical formulae to follow the evolution of a pulsar using simple exponential models for magnetic field decay and alignment. We then compare these evolutionary tracks with detailed model calculations using ohmic decay of crustal neutron star magnetic fields. We find that, after an initial phase with a small braking index, n, pulsars evolve with enhanced torque decay (n ≫ 3) for about 1 Myr. The long term evolution depends on the impurity parameter of the crust. If impurities are negligible in older isolated pulsars we expect their true age to be approximately equal to their observed characteristic age, τ = P/(2 ˙ P). It is not possible from data to constrain model parameters of the neutron star crust.
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    Rapidly rotating strange stars for a new equation of state of strange quark matter
    (2000-12-18) Bombaci, Ignazio; Thampan, A. V.; Datta, Bhaskar
    For a new equation of state of strange quark matter, we construct equilibrium sequences of rapidly rotating strange stars in general relativity. The sequences are the normal and supramassive evolutionary sequences of constant rest mass. We also calculate equilibrium sequences for a constant value of Ω corresponding to the most rapidly rotating pulsar PSR 1937 +21. In addition to this, we calculate the radius of the marginally stable orbit and its dependence on Ω, relevant for modeling of kilo–Hertz quasi–periodic oscillations in X–ray binaries.
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    Magnetic field evolution of accreting neutron stars - IV effect of the curvature of space-time
    (2000-09-26) Konar, Sushan
    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.
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    Unification to the Pseudo-General-Relativistic Analysis of Accretion Disks around rotating Black Holes and Neutron Stars
    (2011-07-05) Mukhopadhyay, B.
    I analyse the relativistic accretion phenomena around rotating black holes and neutron stars and show both the kinds of disk can be treated in an unified manner. The corresponding accretion disks are described by pseudo-Newtonian approach. For this purpose, number of pseudo-Newtonian potentials are in literature, applicable to describe the relativistic properties of accretion disk. While, Kerr metric is used to describe the pseudo-Newtonian potential for accretion disk around black hole, the Hartle-Thorne metric is considered to describe disk around neutron star as the metric can describe continuously the space-time, inside the star as well as out-side of it. Two other potentials were proposed to describe the temporal effects of the accretion disk. All the potentials reproduce the marginally stable and bound orbits approximately or exactly as that of general relativity. These also reproduce the specific mechanical energy approximately. Using these potentials, I study the global parameter space of the accretion disk around black holes and neutron stars. I study, how the fluid properties get affected for different angular momentum of the compact object. I show that, for different angular momenta of the compact object, the valid disk parameter region dramatically changes and disk may become unstable in certain situations. Also I discuss about the possibility of shock in accretion disk around rotating black holes and neutron stars. When the angular momentum of compact object is chosen to be varied, the sonic locations of the accretion disk get shifted or disappear, making the disk unstable. To bring it in the stable situation, the angular momentum of the accreting matter has to be reduced/enhanced (for co/counter-rotating disk) by means of some physical process. I also study, how the fluid properties get changed with different rotations of the black holes, neutron stars and other gravitating central stars. Moreover, I show the effect of viscosity to the fluid properties of the disk. Thus, I find out the unified physical parameter regime, for which the stable accretion disk can be formed. Subsequently, a theoretical prediction of kHz QPO is given, for a fast rotating compact object as 4U 1636-53.
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    Self lensing effects for compact stars and their mass-radius relation
    (2011-07-06) Prasanna, A. R.; Ray, Subharthi
    During the last couple of years astronomers and astrophysicists have been debating on the fact whether the so called ‘strange stars’ - stars made up of strange quark matter, have been discovered with the candidates like SAX J1808.4−3658, 4U 1728−34, RX J1856.5−3754, etc. The main contention has been the estimation of radius of the star for an assumed mass of ∼ 1.4M⊙ and to see whether the point overlaps with the graphs for the neutron star equation of state or whether it goes to the region of stars made of strange matter equation of state. Using the well established formulae from general relativity for the gravitational redshift and the ‘lensing effect’ due to bending of photon trajectories, we, in this letter, relate the parameters M and R with the observable parameters, the redshift z and the radiation radius R∞, thus constraining both M and R for specific ranges, without any other arbitrariness. With the required inputs from observations, one ought to incorporate the effects of self lensing of the compact stars which has been otherwise ignored in all of the estimations done so far. Nonetheless, these effect of self lensing makes a marked difference and constraints on the M-R relation.