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We study the Fourier time lags due to the Comptonization of disc-emitted photons in a spherical, uniform, and stationary X-ray corona, which located on the rotational axis of the black hole. We use monk , a general relativistic Monte Carlo radiative transfer code, to calculate Compton scattering of photons emitted by a thin disc with a Novikov–Thorne temperature profile. We find that the model time lags due to Comptonization remain constant up to a characteristic frequency and then rapidly decrease to zero at higher frequencies. We provide equations that can be used to determine the time lags and cross-spectra for a wide range of values for the corona radius, temperature, optical depth, height, and for various accretion rates and black hole masses. We also provide an equation for the X-ray luminosity of a single corona, as a function of its characteristics and location above the disc. Remarkably, the observed X-ray time lags of nearby, bright active galaxies can be successfully reproduced by inverse Comptonization process of multiple dynamic coronae. [ABSTRACT FROM AUTHOR]

Relativistic Fe K emission lines from accretion discs and from disc winds encode key information about black holes, and their accretion and feedback mechanisms. We show that these two processes can in principle produce indistinguishable line profiles, such that they cannot be disentangled spectrally. We argue that it is likely that in many cases both processes contribute to the net line profile, and their relative contributions cannot be constrained purely by Fe K spectroscopy. In almost all studies of Fe K emission to date, a single process (either disc reflection or wind Compton scattering) is assumed to dominate the total line profile. We demonstrate that fitting a single process emission model (pure reflection or pure wind) to a hybrid line profile results in large systematic biases in the estimates of key parameters, such as mass outflow rate and spin. We discuss various strategies to mitigate this effect, such as including high-energy data covering the Compton hump, and the implications for future X-ray missions. [ABSTRACT FROM AUTHOR]

We analytically study the dynamical and thermal properties of the optically thin gases at the parsec-scale when they are spherically accreted on to low luminous active galactic nuclei. The falling gases are irradiated by the central X-ray radiation with the Compton temperature of 5–15 × 107 K. The radiative heating/cooling and the bulge stellar potential in galaxies are taken into account. We analyse the effect of accretion rate, luminosity, gas temperature, and Compton temperature on steady solutions of dynamical and thermal properties. The steady solutions are obviously different from Bondi solution. Compared to our models, the Bondi model underestimates the accretion rate. We give the boundary between thermal stability and instability. The boundary is significantly affected by Compton temperature. When Compton temperature is higher, the falling gases tend to become thermally unstable. When thermal instability takes place in the irradiated gases, the gases become two phases (i.e. hot gases and cool gases) and the hot gases may become outflows. This effect may reduce the accretion rates. [ABSTRACT FROM AUTHOR]

We present X-ray spectral and long-term variability analyses of an unbiased sample of 361 optically selected radio-loud quasars (RLQs) utilizing sensitive serendipitous X-ray data from the Chandra and XMM-Newton archives. The spectral and temporal properties of RLQs are compared with those of radio-quiet quasars (RQQs) matched in L 2500Å and z. The median power-law photon index (Γ) of RLQs is |$1.84_{-0.01}^{+0.01}$|⁠ , which is close to that of matched RQQs (⁠|$1.90_{-0.01}^{+0.02}$|⁠). No significant correlations between Γ and radio-loudness, L x/ L x, rqq (the X-ray luminosity over that expected from the L x– L uv relation for RQQs), redshift, or Eddington ratio are found for our RLQs. The stacked X-ray spectra of our RLQs show strong iron-line emission and a possible Compton-reflection hump. The intrinsic X-ray variability amplitude is ≈40 per cent for RLQs on time-scales of months-to-years in the rest frame, which is somewhat smaller than for the matched RQQs (≈60 per cent) on similar time-scales, perhaps due to the larger black hole masses and lower Eddington ratios in our RLQ sample. The X-ray spectral and variability results for our RLQs generally support the idea that the X-ray emission of typical RLQs is dominated by the disc/corona, as is also indicated by a recent luminosity correlation study. [ABSTRACT FROM AUTHOR]