AR-2019-2020

−3 Ó −4 Ó −3 Ó −3 )UHTXHQF\ [ 3RZHU PDS of MAXIJ1820+070 and Cygnus X−1 (January and June 2016) −4 −2 χ )UHTXHQF\ +] -20 0 20 40 60 4 8 16 32 Time-lag (ms) Energy (keV) Time-lag (ms) at 1.0 Hz Figure 14: Comparative PDS of MAXI J1820+070 (Black colour) and Cygnus X-1 January (Green colour) and June (Red colour) 2016 in the 380 keV energy band. PDS of the source is fitted with five Lorentzian components of which the two narrow peaks represent a QPO at 47.7 mHz and a weak oscillation at 109.4 mHz respectively. PDS of Cygnus X-1 is fitted with 2 and 3 Lorentzian components for January and June respectively (Left Panel). The right panel shows the time-lag as a function of energy at 1 Hz for MAXI J1820+070. The time-lags are increasing with the increase in energy and have been fitted using the fluctuation propagation model. hole. Hence, a study on the relativistic reflection fraction R f , defined as the ratio of the coronal intensity that illuminates the accretion disc to the coronal intensity observed directly, can provide important information regarding the geometry of the accretion disc and the corona. Here, Savithri Ezhikode , Gulab C. Dewangan , Ranjeev Misra , and Ninan Sajeeth Philip investigate the relationship between the relativistic reflection fraction R f and the hard X-ray photon index Γ of a NuSTAR sample of Seyfert 1 galaxies. The X-ray spectra are modelled using relxill code which helps to directly obtain the reflection fraction of a relativistically smeared reflection component. The parameter R f depends on the amount of Comptonised X-ray emission intercepted by the inner accretion disc. The parameter is found to range from ∼ 0.12(0.13) to ∼ 3.75(4.85) for different spin parameters ( a = 0(0 . 998)). They found a strong positive correlation between Γ and R f in our sample (See Fig. 15). As the slope of the X-ray power law is related to the rate of cooling of the plasma, steeper X-ray spectra indicate stronger cooling by the seed photons. The larger the area covered by the accretion disc as seen from the corona, the higher will be the seed photons entering the corona resulting in the steepening of the X-ray spectrum. Since the same accretion disc is responsible for the reflected emission, the larger area covered by the medium consequently enhances the reflection fraction. Thus, the observed R f − Γ correlation is most likely related to the variations in the disc-corona geometry of AGN. Spectral and timing properties of the galactic X-ray transient Swift J1658.2-4242 using AstroSat observations Black hole transients spend most of their lives in quiescence and are primarily discovered when they enter into outbursts characterized by abrupt changes in their X-ray luminosity by several orders of magnitudes. Swift J1658.2–4242 is a new Galactic X-ray transient source discovered by the BAT instrument onboard Swift on 2018 February 16 and extensively studied using different instruments in optical, radio and X-ray wavelength. Vadakkumthani Jithesh , Bari Maqbool Bhat and Ranjeev Misra studied the detailed X-ray timing and spectral analysis of Swift J1658.2– 4242 using the LAXPC and SXT instruments onboard AstroSat . They detected prominent C-type quasi-periodic oscillations (QPOs) of frequencies varying from ∼ 1 . 5 Hz to ∼ 6 . 6 Hz along with distinct 2nd harmonics and sub-harmonics. The QPO detected at ∼ 1 . 56 Hz drifts to a higher