AR_final file_2018-19

magnetic field would be of B ≤ 610 8 G . This obser- vation has been done in collaboration with Aditya S. Mondal, and Gulab C. Dewangan. NuSTAR view of the Z-type neutron star low-mass X-ray binary Cygnus X-2 We report on the NuSTAR observation of the Z- type neutron star low-mass X-ray binary Cygnus X-2 performed on 2015 January 7. During this ob- servation, the source exhibited a sudden decrease in count rate (dips) and stronger variability in 379 keV X-ray light curve. The hardnessintensity di- agram shows that the source remained in the so- called normal branch of the Z-track, although an extended flaring branch is observed during the dips. The source was in a soft spectral state with the 345 keV luminosity of L ≃ (0 . 51 . 1) × 10 38 ergs 1 , as- suming a distance of 8 kpc. Both the non-dip and dip X-ray spectra are well represented by models, in which the soft band is dominated by the emis- sion from the disc, while the hard X-ray band is dominated by the Comptonized emission from the boundary layer/corona and its reflected emission from the disc. The X-ray spectrum also revealed a broad Fe K α emission line which is nearly sym- metric at the higher flux and asymmetric when the flux is reduced by a factor of ∼ 2. The relativistic reflection model predicts the inner radius of the ac- cretion disc as R in 2 . 56 . 0 R ISCO ( ≃ 3073 km ) for the non-dip state and R in 2 . 02 . 6 R ISCO ( ≃ 2432 km )for the dip state. If the inner disc is truncated due to the pressure arising from a magnetic field, this im- plies an upper limit of the magnetic field strength of ≤ 7 . 6 × 10 9 G at the magnetic poles, which is con- sistent with other estimates. This observation has been done in collaboration with Aditya S. Mondal, and Gulab C. dewangan, and Mayukh Pahari. Prabir Rudra Generalised teleparallel quintom dark energy non- minimally coupled with the scalar torsion and a boundary term We propose a new generalized quintom dark en- ergy model in the teleparallel alternative of gen- eral relativity theory, by considering a non-minimal coupling between the scalar fields of a quintom model with the scalar torsion component T and the boundary term B . In the teleparallel alter- native of general relativity theory, the boundary term represents the divergence of the torsion vec- tor, B = 2 ∇ µ T µ , and is related to the Ricci scalar R and the torsion scalar T , by the fundamental re- lation: R = − T + B . We have investigated the dynamical properties of the present quintom sce- nario in the teleparallel alternative of general rel- ativity theory by performing a dynamical system analysis in the case of decomposable exponential potentials. The study analyzed the structure of the phase space, revealing the fundamental dynamical effects of the scalar torsion and boundary couplings in the case of a more general quintom scenario. Ad- ditionally, a numerical approach to the model is presented to analyze the cosmological evolution of the system. This observation has been done in col- laboration with Sebasbian Bahamonda, and Mihai Marciu. Time dependent geometry in massive gravity We analyze a time dependent geometry in a mas- sive theory of gravity. This is done by analyz- ing Vaidya spacetime in such a massive theory of gravity. The Vainshtein and dRGT mechanisms are used to obtain a ghost free massive gravity, and to construct such time dependent solutions. Singularities formed, The nature and strength of singularies formed are studied in detail. We also study the thermodynamical aspects of such a geom- etry by calculating the important thermodynamical quantities for such a system, and analyze the ther- modynamical behaviour of such quantities. This observation has been done in collaboration with Yaghoub Heydarzade, Behnam Pourhassan, Mir Faizal, Ahmed Farag Ali, and Farhad Darabi. Anirban Saha Footprint of spatial non commutativity in resonant detectors of gravitational wave The present day gravitational wave (GW) detec- tors strive to detect the length variation δL = hL , which owing to the smallness of the metric per- turbation ∼ h , is an extremely small length O ∼ 10 − 18 − 10 − 21 meter. The recently proposed non- commutative structure of space has a characteris- tic length-scale √ θ , which has an estimated upper- bound in similar length-scale range. We, therefore, propose that GW data can be used as an effective probe of noncommutative structure of space. In this work we demonstrate how spatial non commu- tativity modifies the responding frequency of the ( 211 )