The supermassive black holes in most galaxies in the universe are powered by hot accretion flows. Both theoretical analysis and numerical simulations have indicated that, depending on the degree of magnetization, black hole hot accretion flow is divided into two modes, namely SANE (standard and normal evolution) and MAD (magnetically arrested disk). It has been an important question which mode the hot accretion flows in individual sources should belong to in reality, SANE or MAD. This issue has been investigated in some previous works but they all suffer from various uncertainties. By using the measured rotation measure (RM) values in the prototype low-luminosity active galactic nuclei in M87 at 2, 5, and 8 GHz along the jet at various distances from the black hole, combined with three-dimensional general relativity magnetohydrodynamical numerical simulations of SANE and MAD, we show in this paper that the RM values predicted by MAD are well consistent with observations, while the SANE model overestimates the RM by over two orders of magnitude and thus is ruled out.

Relativistic reflection features in the X-ray spectra of black hole binaries and active galactic nuclei are thought to be produced through illumination of a cold accretion disk by a hot corona. In this work, we assume that the corona has the shape of an infinitesimally thin disk with its central axis the same as the rotational axis of the black hole. The corona can either be static or corotate with the accretion disk. We calculate the disk’s emissivity profiles and iron line shapes for a set of coronal radii and heights. We incorporate these emissivity profiles into relxill_nk and we simulate some observations of a black hole binary with the Nuclear Spectroscopic Telescope Array to study the impact of a disk-like coronal geometry on the measurement of the properties of the system, and in particular, on the possibility of testing the Kerr nature of the source. We find that, in general, the astrophysical properties of the accretion disk are recovered well even if we fit the data with a model employing a broken power law or a lamppost emissivity profile, while it is more challenging to constrain the geometric properties of the black hole spacetime.

Hydrodynamical interactions between binaries and circumbinary disks (CBDs) play an important role in a variety of astrophysical systems, from young stellar binaries to supermassive black hole binaries. Previous simulations of CBDs have mostly employed locally isothermal equations of state. We carry out 2D viscous hydrodynamic simulations of CBDs around equal-mass, circular binaries, treating the gas thermodynamics by thermal relaxation toward equilibrium temperature (the constant-β cooling ansatz, where β is the cooling time in units of the local Keplerian time). As an initial study, we use the grid-based code Athena++ on a polar grid, covering an extended disk outside the binary co-orbital region. We find that with a longer cooling time, the accretion variability is gradually suppressed, and the morphology of the CBD becomes more symmetric. The disk also shows evidence of hysteresis behavior depending on the initial conditions. Gas cooling also affects the rate of angular momentum transfer between the binary and the CBD, where given our adopted disk thickness and viscosity (H/r ∼ 0.1 and α ∼ 0.1), the binary orbit expands while undergoing accretion for most β values between 0 and 4.0 except over a narrow range of intermediate β values. The validity of using a polar grid excising the central domain is also discussed.

X-ray reflection spectroscopy is potentially a powerful tool to probe the spacetime geometry around astrophysical black holes and test general relativity in the strong field regime. However, precision tests of general relativity are only possible if we employ the correct astrophysical model and if we can limit the systematic uncertainties. It is thus crucial to select the sources and the observations most suitable for these tests. In this work, we analyze simultaneous observations of XMM-Newton and NuSTAR of the supermassive black hole in Fairall 9. This source has a number of properties that make it a promising candidate for tests of general relativity using X-ray reflection spectroscopy. Nevertheless, we find that with the available data there is not a unique interpretation of the spectrum of Fairall 9, which prevents, for the moment, the use of this source for robust tests of general relativity. This issue may be solved by future X-ray missions with a higher energy resolution near the iron line.