We present a comprehensive spectral and temporal study of the black hole X-ray transient MAXI J1820+070 during its outbursts in 2018 using Swift/XRT, NICER, NuSTAR and AstroSat observations. The Swift/XRT and NICER spectral study shows a plateau in the light curve with spectral softening (hardness changes from ∼ 2.5 to 2) followed by a gradual decline without spectral softening during the first outburst. Also, spectral modelling suggests that the first outburst is in the low/hard state throughout with a truncated disk whereas the thermal disk emission dominates during the second outburst. During the entire outburst, strong reflection signature (reflection fraction varies between ∼ 0.38 − 3.8) is observed in the simultaneous wideband (NICER-NuSTAR, XRT-NuSTAR, AstroSat) data due to the presence of a dynamically evolving corona. The NICER timing analysis shows Quasi-periodic Oscillation (QPO) signatures and the characteristic frequency increases (decreases) in the plateau (decline) phase with time during the first outburst. We understand that the reduction of the electron cooling timescale in the corona due to spectral softening and the resonance oscillation with the local dynamical timescale may explain the above behaviour of the source during the outburst. Also, we propose a possible scenario of outburst triggering and the associated accretion geometry of the source.
A comprehensive wideband spectral analysis of the brightest black hole X-ray binary 4U 1543 − 47 during its 2021 outburst is carried out for the first time using NICER, NuSTAR, and AstroSat observations by phenomenological and reflection modelling. The source attains a super-Eddington peak luminosity and remains in the soft state, with a small fraction ($< 3\%$) of the inverse-Comptonized photons. The spectral modelling reveals a steep photon index (Γ ∼ 2 − 2.6) and relatively high inner disk temperature (Tin ∼ 0.9 − 1.27 keV). The line-of-sight column density varies between (0.45 − 0.54) × 1022 cm−2. Reflection modelling using the RELXILL model suggests that 4U 1543 − 47 is a low-inclination system (θ ∼ 32○ − 40○). The accretion disk is highly ionized (log ξ > 3) and has super solar abundance (3.6−10 AFe, ⊙) over the entire period of study. We detected a prominent dynamic absorption feature between ∼8 − 11 keV in the spectra throughout the outburst. This detection is the first of its kind for X-ray binaries. We infer that the absorption of the primary X-ray photons by the highly ionized, fast-moving disk-winds can produce the observed absorption feature. The phenomenological spectral modelling also shows the presence of a neutral absorption feature ∼7.1 − 7.4 keV, and both ionized and neutral absorption components follow each other with a delay of a typical viscous timescale of 10 − 15 days.
The objective of this paper is to study the thermodynamics and thermodynamic geometry of charged de-Sitter and charged anti de-Sitter black hole solutions in massive gravity. We study the effect of curvature parameter as well as the mass of graviton in the thermodynamics of the black hole system. We further extend our studies to different topology of the space time and its effects on phase transition and thermodynamics. In addition, the phase transition structure of the black hole and its interactions are reproduced using geometrothermodynamics.
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