35th Annual Report (2022-2023) - ENGLISH

165 scalaron masses are narrowed down relative to the ones obtained from pericentre shift studies. In the field of observational astronomy, X- ray spectrumvariability of Mrk 478 has been studied using the observations from XMM- Newton, AstroSat and Swift. It has been found that for long – term (~ years) and intermediate – term (~ days to months) variability, the reflection fraction is anticorrelated with the flux and spectral index, which implies that the variability is due to the hard X-ray producing corona moving closer to and further from the black hole. (1) Effect of dark matter distribution on scalaron gravity near the Galactic Center black hole and its prospects; P C Lalremruati [thesis work] and Sanjeev nd Kalita (pub. date 22 December 2022), ApJ, 941, 183, 2022. PublicationsbyusingICARDfacilities: (2) Constraining spacetime metrics within and outside general relativity through the Galactic Center black hole (Sgr A*) shadow; Sanjeev Kalita and P. Bhattacharjee [research internship] th (pub. date 5 February 2023), Eur. Phys. J. C 83, 120, 2023. (3) Unscreening of f(R) gravity near the galactic center black hole: Testability through pericenter shift below S0-2's orbit; D. Paul [thesis work], Sanjeev Kalita and A. Talukdar [thesis work] th (pub. date 16 February, 2023), IJMPD, 32, 2350021-91, 2023. (4) Correlated variability of the reflection fraction with the X-ray flux spectral index for Mrk 478; S. Barua [thesis work], V. Jithesh, R. Misra, Biman J. rd Medhi and O. Adegoke (pub. date 23 September 2022), MNRAS, 517, 801 (2022). Publicoutreachevents The following sky viewing events were organised at Gauhati University Observatory. (1) Planet observation program for college students; visit from Pandu College, rd Guwahati (3 November, 2022). Research scholars of the department interacted with the students by illustrating on the naked eye planets. (2) Public outreach event – “The wanderers in the Gauhati University sky” was organised to view the planets th (11 January 2023). (3) Planet observation and interaction program for college students; visit th from Guwahati College, Guwahati (16 February 2023). Research scholars and faculty members of the department interacted with the students by illustrating on some bright stars and the planets. AreaofResearch: GRand Alternative Theory of Gravity BlackHole Physics Dark Energy andDarkMatter Gravitational Lensing andShadow Non-Linear Dynamics andChaos (a) We have investigated the orbits of spinning test particles around a Schwarzschild black hole under the influence of a quintessence matter field (SQBH). We begin with the dynamics of the spinning test particles around SQBH which is governed by the Mathisson–Papapetrou–Dixon equations under the pole–dipole approximation, where the gravitational field and the higher multipoles of the particle are neglected. Depending on the types of saddle points, the effective potential are classified and the possibility of chaotic orbits is discussed. The innermost stable circular orbits (ISCOs) of the spinning particle around SQBH are addressed, as are the effects of the parameters S (particles' spin) and ϵ (equation of state pa r ame t e r ) . La t e r , Pe r i as t r on precession is investigated up to the first-order spin correction for a spinning particle moving in nearly circular orbits around SQBH. It is noted that the addition of particle's spin revamps the results obtained for the non-spinning particles and also articulates some interesting observational properties of the SQBH. A d d i t i o n a l l y , we d i s c u s s t h e ramifications of employing first-order spin corrections for analysing ISCOs, as well as compare our results to the Schwarzschild BH to ensure that they are consistent in the limit when the equation of state parameter ϵ = −1/3 and normalization factor α → 0. (b) We have studied the stability of circular orbits in the background of a traversable wormhole (TWH) spacetime obtained as a solution of Einstein's field equations coupled conformally to a massless scalar field. The Lyapunov stability approach is employed to determine the stability of circular orbits (timelike and null) of non-spinning test particles around a TWH spacetime. In the case of timelike geodesics, the particle is confined to move in four different types of effective potentials depending on various values of the angular momentum ~ L ~ with both centrifugal and gravitational part. Coordinator: Dr. Hemwati Nandan ICARD Gurukula Kangri (Deemed to be University), Haridwar: Activities from 01 April 2022 till 31 March 2023