The observed lognormal flux distributions in the high-energy emission from blazars have been interpreted as being due to variability stemming from non-linear multiplicative processes generated dynamically from the accretion disc. On the other hand, rapid minute scale variations in the flux point to a compact emitting region inside the jet, probably disconnected from the disc. In this work, we show that linear Gaussian variations of the intrinsic particle acceleration or escape time-scales can produce distinct non-Gaussian flux distributions, including lognormal ones. Moreover, the spectral index distributions can provide confirming evidence for the origin of the variability. Thus, modelling of the flux and index distributions can lead to quantitative identification of the micro-physical origin of the variability in these sources. As an example, we model the X-ray flux and index distribution of Mkn 421 obtained from ∼9 yr of MAXI observations and show that the variability in the X-ray emission is driven by Gaussian fluctuations of the particle acceleration process rather than that of the escape rate.

We report here results from detailed timing and spectral studies of the high mass X-ray binary pulsar 4U 1538-52 over several binary periods using observations made with the Rossi X-ray Timing Explorer (RXTE) and BeppoSAX satellites. Pulse timing analysis with the 2003 RXTE data over two binary orbits confirms an eccentric orbit of the system. Combining the orbitial parameters determined from this observation with the earlier measurements we did not find any evidence of orbital decay in this X-ray binary. We have carried out orbital phase resolved spectroscopy to measure changes in the spectral parameters with orbital phase, particularly the absorption column density and the iron line flux. The RXTE-PCA spectra in the 3–20 keV energy range were fitted ∼6.4 keV, whereas the BeppoSAX spectra needed only a power law and Gaussian emission line at ∼6.4 keV in the restricted energy range of 0.3–10.0 keV. An absorption along the line of sight was included for both the RXTE and BeppoSAX data. The variation of the free spectral parameters over the binary orbit was investigated and we found that the variation of the column density of absorbing material in the line of sight with orbital phase is in reasonable agreement with a simple model of a spherically symmetric stellar wind from the companion star.

Black holes are characterized by their mass and spin. X-ray observations can be used to estimate these two quantities in black hole systems such as black hole X-ray binaries (BHXRB). Here we analysed near-simultaneous data of BHXRB MAXI J1348–630 from Neutron Star Interior Composition Explorer (NICER) (0.6–10.0 keV) and Nuclear Spectroscopic Telescope Array (NuSTAR) (3.0–79 keV) during the soft and hard states. Considering the distance of the source to be 2.2 kpc, the spin and mass of the black hole were estimated from the modeling of the soft state spectrum of the source. The spin and mass of the black hole were found to be |$0.80_{-0.02}^{+0.02}$| and |$8.7_{-0.3}^{+0.3}\, \mathrm{M}_\odot$|⁠, respectively. The inclination of the source was determined to be |$36.5^{\circ}\, \pm \, 1.0^{\circ}$|⁠. Using these quantities in the hard state spectral model, the inner radius of the accretion disc was constrained for all three hard states of the source.

We analysed the archival XMM–Newton data of the radio-loud narrow-line Seyfert 1 galaxy PMN J0948+0022 in the energy range 0.3–10.0 keV. The X-ray data reveal that the spectrum in the 0.3–10.0 keV energy band is not a simple power law as previously described in the literature. Instead it consists of a power law with soft excess below 2.5 keV. The X-ray spectrum was fitted with four different models and it was shown the soft excess component of the spectrum in the 0.3–2.5 keV energy range could be described reasonably well within the framework of the thermal Comptonization model as well as relativistically blurred reflection model. The power-law component required to fit the spectrum beyond 2.5 keV was found to be rather hard compared to the ones observed in other unobscured active galactic nuclei. It is also shown that the Swift/X-Ray Telescope spectrum from the source could not reveal the soft excess component due to poor statistics. The fractional variability estimated from XMM–Newton data indicates the presence of independently varying components in the spectrum above and below 1 keV.