AR-2019-2020

lar mometum j s , and hence, the stellar disc size given by the exponential stellar disc scale length R D , of three superthins and seven LSBs lie above the 95.4 % confidence band of the j s - V rot regres- sion line for ordinary bulgeless disc galaxies, V rot being the asymptotic rotational velocity. Further, we find that superthins and LSBs have higher j s and R D values for a given value of stellar mass M s at high values of statistical significance, compared to ordinary disc galaxies. Therefore, we conclude, a superthin may be distinguished by a characteris- tically larger disc size, which could possibly explain the origin of its large planar-to-vertical axes ratio. Interestingly, we find that the median spin parame- ter is 0.13 ± 0.01 for superthin galaxies, which is an order of magnitude higher than those of LSBs and ordinary disc galaxies, which may have important implications for the existence of superthin stellar discs in these low surface brightness galaxies. This work has been done in collaboration with Vikas Jadhav Y. Sarmistha Banik Properties of massive rotating protoneutron stars with hyperons: Evolution and universality In this work, we study the properties and struc- ture of a massive and rapidly rotating protoneu- tron star (PNS) with hyperon content. We follow several stages of quasi-stationary evolution in an approximate way at four discrete steps. We use a density-dependent (DD) relativistic mean field the- ory (RMF) model and calculate different quantities such as mass, equatorial radius, moment of inertia, and quadrupole moment to get different rotating configurations upto the mass-shedding limit. We study the effect of the appearance of Λ, the lightest of all hyperons, on each of the evolutionary stages of the PNS. We also check its sensitivity to the in- clusion of φ vector meson as a mediator of Λ − Λ interaction in detail. Finally, we investigate the universal relations between moment of inertia and compactness in the context of a hot and young com- pact object. This work has been done in collabo- ration with Smruti S. Lenka, and Prasanta Char. Signatures of strangeness in neutron star merger remnants Neutron star (NS) mergers provide us with infor- mation rich in physics using multi-messenger as- trophysical observations. One of the probable rem- nants of such a merger is a differentially rotating hot hypermassive neutron star. The stability of the merger remnant depends crucially on the under- lying Equation of State (EoS) and thus, provides a method to probe the nature of dense matter in NSs. In this work, we search for possible signatures of strangeness containing matter in the NS inte- rior on the secular stability of the merger remnant. We also use recently proposed methods to make a rough estimate the collapse time of the merger remnant and the threshold mass above which the merger promptly collapses to a black hole. This work has been done in collaboration with Krishna Prakash Nunna, and Debarati Chatterjee. Aru Beri The black hole X-ray transient Swift J1357.2-0933 as seen with Swift and NuSTAR during its 2017 outburst One of our recent works involves a multi- wavelength approach to study an enigmatic X-ray binary, Swift J1357.2-0933. This work was also part of the SmartNet, which is the Simultane- ous Multiwavelength Astronomy Research in Tran- sients Network, which involves group of researchers all accross the globe. Multiwavelength observations are vital to understand the comprehensive picture of an X-ray binary. We report on observations of black hole Swift J1357.2-0933, a member of the modest population of very faint X-ray transients. This source has previously shown intense dips in the optical lightcurve, a phenomena that has been linked to the existence of aunique toroidal structure in the inner region of the disc, seen at a high incli- nation. Our observations, carried out by the Neil Gehrels Swift and NuSTAR X-ray observatories, do not show the presence of intense dips in the optical light curves. We find that the X-ray light curves do not show any features that would straightfor- wardly support an edge-on configuration or high inclination configuration of the orbit. This is sim- ilar to what was seen in the X-ray observations of the source during its 2011 outburst. Moreover, the broadband spectra were well described with an ab- sorbed power-law model without any signatures of cut-off at energies above 10 keV, or any reflection from the disc or the putative torus. Thus, the X- ray data do not support the unique obscuring torus scenario proposed for J1357. We also performed a multi-wavelength study using the data of X-ray