AR-2019-2020

telescope and Ultraviolet/Optical Telescope aboard Swift, taken during the 4.5 months duration of the 2017 outburst. This is consistent with what was previously inferred for this source. We found a cor- relation between the simultaneous X-ray and ul- traviolet/optical data and our study suggests that most of the reprocessed flux must be coming out in the ultraviolet. This work has been done in collabo- ration with Bailey E. Tetarenko, Arash Bahramian, Diego Altamirano, Poshak Gandhi, et al. A broadband look of the accreting millisecond X- ray pulsar SAX J1748.9-2021 using AstroSat and XMM-Newton X-ray pulsars exhibit strict periodic variations in X-ray intensity. In some neutron stars (NS), X-ray pulsations of the order of millisecond have been detected (see e.g., Chakrabarty & Morgan 1998). They belong to peculiar type of X-ray pulsars called accretion powered millisecond X-ray pulsars (AMXPs). The short spin periods are caused by long-lasting mass transfer from a low-mass com- panion star through an accretion disc onto a ro- tating neutron star. AMXPs are perceived by the scientific community as astrophysical labora- tories that could be essential to our understand- ing of thermonuclear burst processes. So far, only 22 such AMXPs have been detected. We have analyzed and studied one of these sources SAX J1748.9-2021 using 200 kiloseconds (ks) long ob- servation made with Indias first multi-wavelength satellite AstroSat, and 60 ks long observation made with XMM-Newton launched by Europeon Space Agency (ESA). SAX J1748.9-2021 is a tran- sient AMXP, located in the globular cluster NGC 6440. We report on the spectral and timing anal- ysis of SAX J1748.9-2021 performed on AstroSat data taken during its faint and short outburst of 2017. We derived the best-fitting orbital solution for the 2017 outburst and obtained an average lo- cal spin frequency of 442.361098(3) Hz. The pulse profile obtained from 3-7 keV and 7-20 keV energy bands suggest constant fractional amplitude 0.5% for fundamental component, contrary to previously observed energy pulse profile dependence. The As- troSat observations revealed the source to be in a hard spectral state. The 1-50 keV spectrum from SXT and LAXPC on-board AstroSat can be well described with a single temperature blackbody and thermal Comptonization. Moreover, we found that the combined spectra from XMM-Newton (EPIC- PN) and AstroSat (SXT+LAXPC) indicated the presence of reflection features in the form of iron (Fe K α ) line that we modelled with the reflection model xillvercp. One of the two X-ray bursts ob- served during the AstroSat/LAXPC observation showed hard X-ray emission ( > 30 keV) due to Compton up-scattering of thermal photons by the hot corona. Time resolved analysis performed on the bursts revealed complex evolution in emission radius of blackbody for second burst suggestive of mild photospheric radius expansion. This work has been done in collaboration with Rahul Sharma, An- drea Sanna, and Anjan Dutta. Piyali Bhar Compact star in Tolman-Kuchowicz spacetime in the background of Einstein-Gauss-Bonnet gravity The present work is devoted to the study of anisotropic compact matter distributions within the framework of five-dimensional Einstein-Gauss- Bonnet gravity. To solve the field equations, we have considered that the inner geometry is de- scribed by Tolman-Kuchowicz spacetime. The Gauss-Bonnet Lagrangian, L GB is coupled to the Einstein-Hilbert action through a coupling con- stant, namely α . When this coupling tends to zero, general relativity results are recovered. We analyze the effect of this parameter on the prin- cipal salient features of the model, such as en- ergy density, radial and tangential pressure and anisotropy factor. These effects are contrasted with the corresponding general relativity results. Be- sides, we have checked the incidence on an im- portant mechanism: equilibrium by means of a generalized Tolman-Oppenheimer-Volkoff equation and stability through relativistic adiabatic index and Abreus criterion. Additionally, the behaviour of the subliminal sound speeds of the pressure waves in the principal directions of the configu- ration and the conduct of the energy-momentum tensor throughout the star are analyzed employ- ing the causality condition and energy conditions, respectively. All these subjects are illuminated by means of physical, mathematical and graphical sur- veys. The M-I and the M-R graphs imply that the stiffness of the equation of state increases with α ; however, it is less stiff than GR.This work has been done in collaboration with Ksh. Newton Singh, and Francisco Tello-Ortiz.