Browsing by Author "Ray, Subharthi"

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Astrophysical consequences of diquark formation on the surface of strange star
(2001-05-12) Dey, Mira; Sinha, Monika; Ray, Subharthi
Compactness of some stars is explained if they are strange stars (SS) as shown by Dey et al.(1998) (D98) and Li et al. (1999a,b). In D98, a realistic potential was used in a Fock mean eld approximation within Dirac phe- nomenology, thereby simplifying the spinorial structure of the quarks. Using the plane wave states for the beta equi- librated u, d and s quark Fermi seas from D98 and the conventional spin dependent force between these quarks, we now nd energy lowering in some speci c spin-aligned states. This is a short range pairing between quarks of dif- ferent ﬂavours and may lower the energy by several MeV for each diquark pair. The accretion driven X-ray and γ ray bursts in com- pact objects are believed to be due to thermonuclear or some other explosions which are short lived. These may in fact destroy the spin alignment of the quarks if these stars are SS. The subsequent alignment and production of copious diquark pairing may produce su cient energy to produce recurrent very long bursts seen in some such stars. The long time involved in these processes, we claim, is due to the weak interactions which are responsible for the equilibration of strange quark matter. In such a the case the elusive properties of some of the most massive objects in nature, the compact stars, can be accounted for by spin alignment of pairs of the smallest components of matter, - namely the quarks.
CMB polarization TE power spectrum estimation with non-circular beam
(IUCAA, 2015-02) Ramamonjisoa, Fidy A.; Ray, Subharthi; Mitra, Sanjit et.all
Density dependent strong coupling constant of QCD derived from compact star data
(2000-05-16) Ray, Subharthi; Dey, Jishnu; Dey, Mira
The present work is an endeavour to connect the properties of tiny nearly massless objects with those of some of the most massive ones, the compact stars. Since 1996 there is major inﬂux of X-ray and γ ray data from binary stars, one or both of which are compact objects that are diﬃcult to explain as neutron stars since they contain a mass M in too small a radius R . The suggestion has been put forward that these are strange quark stars (SS) explainable in a simple model with chiral symmetry restoration (CSR) for the quarks and the M, R and other properties like QPOs (quasi periodic oscillations) in their X-ray power spectrum. It would be nice if this astrophysical data could shed some light on fundamental properties of quarks obeying QCD. One can relate the strong coupling constant of QCD, αs to the quark mass through the Dyson-Schwinger gap equation using the real time formalism of Dolan and Jackiw. This enables us to obtain the density dependence of αs from the simple CSR referred to above. This way fundamental physics, diﬃcult to extract from other models like for example lattice QCD, can be constrained from present - day compact star data and may be put back to modelling the dense quark phase of early universe.
General Relativistic Effects of Strong Magnetic Fields on the gravitational force: A driving engine for Gamma-$ SGRM?
(2012-07-06) Malheiro, Manuel; Ray, Subharthi; Mosquera Cuesta, Herman J.; et al.
Abstract. In general relativity all forms of energy contribute to gravity and not only just ordinary matter as in Newtonian Physics. This fact can be seen in the modiﬁed hydrostatic equilibrium equation for relativistic stars pervaded by magnetic (B) ﬁelds. It has an additional term coupled to the matter part as well as an anisotropic term which is purely of magnetic origin. That additional term coming from the pressure changed by the radial component of the diagonal electromagnetic ﬁeld tensor, weakens the gravitational force when B is strong enough and can even produce an unexpected change in the attractive nature of the force by reversing its sign. In an extreme case, this new general relativistic (GR) eﬀect can even trigger an instability in the star as a consequence of the sudden reversal of the hydrostatic pressure gradient. We suggest here that this GR eﬀect may be the possible central engine driving the transient giant outbursts observed in Soft Gamma-ray Repeaters (SGRs). In small regions of the neutron star (NS), strong magnetic condensation can take place. Beyond a critical limit, these highly magnetised bubbles may explode releasing the trapped energy as a burst of γ-rays of ∼ 1036−40 erg.
Origin and interpretation of kilohertz QPOs from strange stars in X-ray Binary system: Theoretical hydrodynamical description
(2011-07-05) Mukhopadhyay, B.; Ray, Subharthi; Dey, Jishnu; et al.
We model and interpret the Kilohertz QPOs from the hydrodynamical description of accretion disk around a rapidly rotating compact strange star. The higher QPO frequency is described by the viscous eﬀects of accretion disk leading to shocks, while the lower one is taken to be the Keplerian motion of the accreting matter. Comparing our results with the observations for two of the fastest rotating compact stellar candidates namely, 4U 1636−53 and KS 1731−260, we ﬁnd that they match to a very good approximation, thus interpreting them as strange stars.
Physical properties of Tolman-Bayin solutions: Some cases of static charged fluid spheres in general relativity
(2005-05-01) Ray, Saibal; Rahaman, Farook; Ray, Subharthi; et al.
In this article, Einstein-Maxwell space-time has been considered in connection to some of the astrophysical solutions as previously obtained by Tolman (1939) and Bayin (1978). The eﬀect of inclusion of charge into these solutions has been investigated thoroughly and also the nature of ﬂuid pressure and mass density throughout the sphere have been discussed. Mass-radius and mass-charge relations have been derived for various cases of the charged matter distribution. Two cases are obtained where perfect ﬂuid with positive pressures give rise to electromagnetic mass models such that gravitational mass is of purely electromagnetic origin.
Possible evidence of surface vibration of realistic strange stars from stellar observations
(2011-07-05) Ray, Subharthi; Dey, Jishnu; Dey, Mira; et al.
Emission lines in the eV and keV range by certain stellar candidates from their recent analysis invoke the question of their possible origin. These stars under consideration, are the 4U 0614+091 (0.65, 0.86, and 1.31 keV ), 2S 0918−549 (0.8 keV with width 55 eV ), 4U 1543−624 (0.7 keV ), 4U 1850 −087 (0.7 keV ) and 4U 1820−30 (0.6 and 0.9 keV ) and also the 0.6 keV excess emission in RX J170930.2−263927. Recently, it has been suggested that the resonance absorption at ∼ in 0.7, 1.4, 2.1 and 2.8 keV 1E1207−5209 and 0.35, 0.7 and 1.4 keV RX J1856.5−3754 are due to harmonic surface vibrations in strange stars. We propose that these harmonic vibrations may also responsible for emission lines in the above mentioned compact stellar candidates.
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.