We present detailed timing and spectral studies of the black hole candidate MAXI J0637–430 during its 2019-2020 outburst using observations with the Neutron Star Interior Composition Explorer (NICER) and the Neil Gehrels Swift Observatory. We find that the source evolves through the soft-intermediate, high-soft, hard-intermediate and low-hard states during the outburst. No evidence of quasi-periodic oscillations is found in the power density spectra of the source. Weak variability with fractional rms amplitude $<5{{\ \rm per\ cent}}$ is found in the softer spectral states. In the hard-intermediate and hard states, high variability with the fractional rms amplitude of $>20{{\ \rm per\ cent}}$ is observed. The 0.7 − 10 keV spectra with NICER are studied with a combined disk-blackbody and nthcomp model along with the interstellar absorption. The temperature of the disc is estimated to be 0.6 keV in the rising phase and decreased slowly to 0.1 keV in the declining phase. The disc component was not detectable or absent during the low hard state. From the state-transition luminosity and the inner edge of the accretion flow, we estimate the mass of the black hole to be in the range of 5–12 M⊙, assuming the source distance of d < 10 kpc.

We present the results obtained from broadband X-ray timing and spectral analysis of black hole candidate MAXI J1803–298 using an AstroSat observation on May 11–12, 2021. Four periodic absorption dips with a periodicity of 7.02 ± 0.18 hours are detected in the light curve. AstroSat observe the source when it was undergoing a transition from hard-intermediate state to soft-intermediate state. Our timing analysis reveals the presence of a sharp type-C quasi periodic oscillation (QPO) in the power density spectra (PDS) with an evolving QPO frequency ranging from 5.31 ± 0.02 Hz to 7.61 ± 0.09 Hz. We investigate the energy dependence of the QPO and do not find this feature in the PDS above 30 keV. The combined 0.7–80 keV SXT and LAXPC spectra are fitted with a model consisting of thermal multi-colour blackbody emission and Comptonized emission components. We perform time-resolved spectroscopy by extracting spectra during the dip and non-dip phases of the observation. A neutral absorber is detected during the dip and non-dip phases though a signature of an ionized absorber is also present in the dip phases. The spectral and temporal parameters are found to evolve during our observation. We estimate the mass function of the system as f(M) = 2.1–7.2 M⊙ and the mass of the black hole candidate in the range of MBH ∼ 3.5–12.5 M⊙.

We present the results obtained from analysis of two AstroSat observations of the high mass Xray binary pulsar OAO 1657-415. The observations covered 0.681-0.818 and 0.808-0.968 phases of the $ 10.4 day orbital period of the system, in March and July 2019, respectively. Despite being outside the eclipsing regime, the power density spectrum from the first observation lacks any signature of pulsation or quasi-periodic oscillations. However, during July observation, X-ray pulsations at a period of 37.0375 s were clearly detected in the light curves. The pulse profiles from the second observation consist of a broad single peak with a dip-like structure in the middle across the observed energy range. We explored evolution of the pulse profile in narrow time and energy segments. We detected pulsations in the light curves obtained from 0.808-0.92 orbital phase range, which is absent in the remaining part of the observation. The spectrum of OAO 1657-415 can be described by an absorbed power-law model along with an iron fluorescent emission line and a blackbody component for out-of-eclipse phase of the observation. Our findings are discussed in the frame of stellar wind accretion and accretion wake at late orbital phases of the binary.