Observational effects of strong gravity

Abstract

It is now a century since Einstein tore down the edifice of classical physics, ultimately replacing it with his crowning achievement, the General Theory of Relativity, the most remarkable prediction of which is the black hole. There are many astrophysical examples of black holes, understanding which has long been a goal of high-energy astronomy. We review how these observations can be explained in terms of a two-phase accretion flow. Hard X-ray photons produced in an optically thin gas are reflected from a cool accretion disk, resulting in a complex reflection spectra, which are dominated by a narrow Iron Кα fluorescence feature at 6.4 keV (dependent on the ionisation state of the cool disk). The photons that form this spectral feature originate in rapidly moving material, close to the black event horizon. They are therefore subject to the combined dynamical effects of the accretion disk and those of General Relativity, resulting in a highly broadened line Profile. The observed form of the line can then, in principle, be used as a test of the strong gravitational field of the black hole. We have developed a new, extremely fast strong gravity code that accurately calculates the effect of strong gravity on photons originating close to the black hole event horizon, including the ability to calculate the trajectories of photons that perform multiple orbits of the black hole. We compare results from the code to the standard models describing relativistic smeared lines available to the community, finding that they match to within ~ 5%. We apply this code to the observed shape of the Iron Κα line and show that the (poorly understood) vertical structure of the accretion disk strongly affects the derived radial emissivity profile, which has important consequences for the interpretation of observational data. Following on from this, we consider the spectral and imaging properties of thin Keplerian accretion disks, fully including the effects of photons that perform multiple orbits of the black hole. Viewed at high inclinations, these photons can carry as much as ~ 60% of the total luminosity of the system, which returns to the disk at a range of radii. At low inclinations, the multiple orbit photons re-intercept the disk plane close to the black hole. For a Schwarzschild black hole, this lies within the plunging region and so the photons need not be absorbed by the disk. The resultant ring is bright it may well be possible to use these as a future test of strong gravity via X-ray interferometrie images of accreting black holes. Finally, we examine the observational properties of accretion flows where angular momentum transport is provided by the Magneto-Rotational Instability. It is shown that the dissipation profile derived from the magnetic 4-current density in these simulations provides a remarkably close match to that derived from the standard relativistic disk model at large radii. At small radii however, the descriptions of dissipation in the two models are rather different, which has important observational consequences. With this model of dissipation, we examine the observed properties of optically thin accretion flows, discussing the implications of these calculations for the low I hard state of Galactic Black Holes, Additionally, we describe a simple reflection geometry for Iron Kα fluorescence, assuming that this MHD flow is optically thick in the equatorial plane. The resultant line shapes are markedly different to those predicted in the standard relativistic disk model, showing that the (currently unknown) flow dynamics are also important in shaping the line.