AR_final file_2018-19

Saibal Ray Anisotropic strange stars in Tolman-Kuchowicz spacetime We attempt to study a singularity-free model for the spherically symmetric anisotropic strange stars under Einstein’s general theory of relativity by ex- ploiting the Tolman-Kuchowicz metric. Further, We have assumed that the cosmological constant Λ is a scalar variable dependent on the spatial co- ordinate r . To describe the strange star candi- dates, we have considered that they are made of strange quark matter (SQM) distribution, which is assumed to be governed by the MIT Bag equation of state. To obtain unknown constants of the stellar system, we match the interior Tolman-Kuchowicz metric to the exterior modified Schwarzschild met- ric with the cosmological constant, at the surface of the system. We have predicted the exact values of the radii for different strange star candidates based on the observed values of the masses of the stellar objects and the chosen parametric values of the Λ as well as the Bag constant B . The set of solutions satisfies all the physical requirements to represent strange stars. Interestingly, the study reveals that as the values of the Λ and B increase the anisotropic system become gradually smaller in size turning the whole system into a more compact ultra-dense stellar object. This study has been done in collab- oration with M. K. Jasim, debabrata Deb, Y. K. Gupta, and Sourav Ray Chowdhary. Exploring physical features of anisotropic strange stars beyond standard maximum mass limit in f ( R, T ) gravity We have a specific model of anisotropic strange stars in the modified f ( R, T )-type gravity by deriving solutions to the modified Einstein field equations representing a spherically symmetric anisotropic stellar object. We take a standard as- sumption that f ( R, T ) = R +2 χ T , where R is Ricci scalar, T is the trace of the energy-momentum ten- sor of matter, and χ is a coupling constant. We suc- cessfully apply the ‘embedding class 1’ techniques, and also consider the case when the strange quark matter (SQM) distribution is governed by the sim- plified MIT Bag model equation of state given by, p r = 1 3 ( ρ − 4 B ), where B is the Bag constant. The radius of the strange star candidates by di- rectly solving the modified TOV equation with the observed values of the mass and some parametric values of B and χ has been obtained. The physi- cal acceptability of this solutions is verified by per- forming several physical tests. Interestingly, be- sides the SQM, another type of matter distribution originates due to the effect of coupling between the matter and curvature terms in the f ( R, T ) gravity theory. This study shows that with decreasing the value of χ , the stellar systems under investigations become gradually massive and larger in size, turn- ing them into less dense compact objects. It also reveals that for χ < 0, the f ( R, T ) gravity emerges as a suitable theory for explaining the observed massive stellar objects like massive pulsars, super- Chandrasekhar stars, and magnetars, etc., which remain obscure in the standard framework of gen- eral relativity (GR). This study has been done in collaboration with Debabrata Deb, Sergei V. Ke- tov, S. K. Maurya, and Maxim Khlopov, and P. H. R. S. Moraes. Biplab Raychaudhuri Study of the reflection spectrum of the bright atoll source GX 3+1 with NuSTAR We report on the NuSTAR observation of the atoll type neutron star (NS) low-mass X-ray bi- nary GX 3+1 performed on 17 October 2017. The source was found in a soft X-ray spectral state with 370keV luminosity of LX ∼ 3 × 10 37 ergss 1 ( ∼ 16% of the Eddington luminosity), assuming a distance of 6 kpc. A positive correlation between inten- sity and hardness ratio suggests that the source was in the banana branch during this observa- tion. The broadband 370 keV NuSTAR spec- tral data can be described by a two-component continuum model consisting of a disk blackbody ( kT disc ∼ 1 . 8 keV ) and a single temperature black- body model ( kT bb ∼ 2 . 7 keV ). The spectrum shows a clear and robust indication of relativis- tic reflection from the inner disc which is modelled with a self-consistent relativistic reflection model. The accretion disc is viewed at an inclination of i ≃ 22 0 26 0 and extended close to the NS, down to R in = (1 . 2 − 1 . 8) R ISCO ( ≃ 6 . 19 . 1 Rg or 1420 . 5 km), which allows an upper limit on the NS radius (6 13.5 km). Based on the measured flux and the mass accretion rate, the maximum radial extension for the boundary layer is estimated to be ∼ 6.3 Rg from the NS surface. However, if the disc is not truncated by the boundary layer but by the mag- netosphere, an estimated upper limit on the polar ( 210 )