AR-2019-2020

energetic transients by observing them in optical, UV, X-ray, and radio wavebands using ground and space-based telescopes to study their properties, and also uGMRT to perform low-luminous radio observations of jetted TDEs. Recently, they have also detected Bowen Fluorescence effect in TDE. Modelling the energy dependent temporal behaviour of transient black hole binaries using AstroSat Accreting black hole X-ray binaries (BHXBs) in outburst exhibit random short-term variability in their flux, which may arise due to the perturbations occurring at different radii of the accretion disk propagating inwards. These perturbations cause variations to mass accretion rate at the inner regions of accretion disk on different time-scales. The X-ray variability in BHXBs are well represented by their power density spectra (PDS), which exhibit systematic changes throughout the course of an outburst with remarkable similarities among themselves, thereby suggesting a common under lying physical phenomenon. The PDS of most BHXBs are characterized by broadband continuum noise like features and sometimes narrow peak features called quasi-periodic oscillations (QPOs). The exact mechanism of origin of QPOs is still an open question, but the origin of broadband noise could be due to the inward propagation and coupling of perturbations occurring throughout the accretion disk resulting in flux variations. This scenario best explains the observed linearity of the rms-flux relationship in galactic black holes, and other type of X-ray sources shows that the rms-flux relationship of broadband noise is an universal feature of all accreting BHXBs, independent of their spectral state. Sneha Prakash, Bari Maqbool Bhat , Ranjeev Misra and others presented the results of the first broadband simultaneous spectral and temporal studies of this newly detected black hole binary MAXI J1820+070 as seen by SXT and LAXPC on-board AstroSat. MAXI J1820+070, brightest X-ray source till date, was first detected in the optical on 6th March 2018, by the ASSAS-SN project and later in X-rays on 11th March 2018, by MAXI. Subsequent multi-wavelength observations of the source categorized it as a galactic black hole transient. The soft lags between the energy bands 0.11 keV and 110 keV was seen for the first time in a BHXB. The combined spectra from NuSTAR and SWIFT/XRT revealed the presence of a dominant thermal Comptonization and weak disk emission along with relativistic reflection fraction. Moreover, the spectral studies estimated the source inclination angle to be ∼ 30 o and the inner disk radius to be ∼ 4.2 times the radius of the innermost stable circular orbit. The observed combined spectra in the energy range 0.7-80 keV were well modelled using disk blackbody emission, thermal Comptonization, and a reflection component. The spectral analysis revealed that the source was in its hard spectral state (Γ = 1.61) with a cool disk (kT in = 0.22 keV). They reported the energy dependent time-lag and root mean squared (rms) variability at different frequencies in the energy range 3-80 keV using LAXPC data. They also modelled the flux variability using a single zone stochastic propagation model to quan- tify the observed energy dependence of time-lag and fractional rms variability and then compared the results with that of Cygnus X-1. Additionally, They also confirmed the detection of a QPO with the centroid frequency at 47.7 mHz. (See Fig. 14). Relativistic reflection fraction and photon index in AGN The primary X-ray emission from Active Galactic Nuclei (AGN) is believed to be produced by the Comptonisation of the optical/UV photons from the accretion disc by the hot electrons in the corona. This primary power-law continuum irradiates the accretion disc and the circumnuclear material producing reflection features in the X-ray spectrum. Thus, the observed X-ray spectrum of AGN is a combination of the reflection spectrum and the primary power law. Since the reflection features are dependent on the structure, temperature, chemical composition and ionisation state of the gas, the reflection spectrum in AGN can give direct information about the physical conditions in the reflecting medium. Also, the area of the reflector and the location of the X-ray emitting region can affect the amount of reflection. Furthermore, the reflection features arising from the inner regions of the disc could be significantly modified by the relativistic effects near the black