General relativistic radiation hydrodynamics of accretion flows - I. Bondi-Hoyle accretion

Abstract

We present a new code for performing general‐relativistic radiation‐hydrodynamic simulations of accretion flows on to black holes. The radiation field is treated in the optically thick approximation, with the opacity contributed by the Thomson scattering and thermal bremsstrahlung. Our analysis concentrates on a detailed numerical investigation of hot (T∼ 1010 K) two‐dimensional, Bondi–Hoyle accretion flows with various Mach numbers. The asymptotic velocity is in the range v∞∼ (0.08–0.18)c, while the initial rest‐mass density is of the order of a few ρ∼ 10−12 g cm−3. We find significant differences with respect to purely hydrodynamical evolutions. In particular, once the system relaxes to a radiation‐pressure‐dominated regime, the accretion rates become about two orders of magnitude smaller than in the purely hydrodynamical case, remaining however super‐Eddington as well as the luminosities. Furthermore, when increasing the Mach number of the inflowing gas, the accretion rates become smaller because of the smaller cross‐section of the black hole, but the luminosities increase as a result of a stronger emission in the shocked regions. Overall, our approach provides the first self‐consistent calculation of the Bondi–Hoyle luminosity, most of which is emitted within r∼ 100M from the black hole, with typical values L/LEdd≃ 1–7, and corresponding energy efficiencies . The possibility of computing luminosities self‐consistently has also allowed us to compare with the bremsstrahlung luminosity often used in modelling the electromagnetic counterparts to supermassive black hole binaries, to find that in the optically thick regime these more crude estimates are about 20 times larger than our radiation‐hydrodynamic results.

Really focused stellar winds in X-ray binaries

“…In this asymmetric case, the classical approximation of the BHL accretion rate is less accurate (Walder 1997) and the accretion is unstable (Ruffert 1997(Ruffert , 1999. There are sophisticated general-relativistic radiation-or magneto-hydrodynamic models (cf., e.g., Zanotti et al 2011, and references therein) of the BHL-accretion. These models are needed to treat in detail the accretion in the close vicinity of the compact component and the generation of the X-ray radiation.…”

“…According to von Zeipel's theorem (von Zeipel 1924), the distribution of the radiative flux F and the effective temperature T eff across the tidally or rotationally distorted surface of a star should be given by the local gravity acceleration g as…”

Current Status of Simulations

“…The other big thing to happen (mostly) within the past year is that a number of groups have now tackled, for the first time, the challenge of developing codes for relativistic radiation MHD in black hole environments (Farris et al 2008;Zanotti et al 2011;Roedig et al 2012;Sadowski et al 2013;Takahashi et al 2013). So far none of these groups have gotten to the point of simulating accretion disks in the way Ohsuga and Mineshige (2011) did (they are still mostly at the stage of code tests and simple one-and two-dimensional problems), but with so many groups joining the chase, one can surely expect rapid progress in the near future.…”

“…So far none of these groups have gotten to the point of simulating accretion disks in the way Ohsuga and Mineshige (2011) did (they are still mostly at the stage of code tests and simple one-and two-dimensional problems), but with so many groups joining the chase, one can surely expect rapid progress in the near future. One result of some astrophysical interest is the study of Bondi-Hoyle (wind) accretion onto a black hole, including the effects of radiation (Zanotti et al 2011;Roedig et al 2012) (see Figure 14).…”

General relativistic radiation-hydrodynamics of accretion flows

“…Based on this, a number of specific physical systems in the optically thick regime have been investigated. [2][3][4] In addition, the presence of stiff source terms in the radiation hydrodynamic equations has been successfully treated via an IMEX RungeKutta scheme 5 and the transition between the optically thick and the optically thin regime, sometimes referred to as the grey regime, has also been accounted for by means of the so-called "M1 closure" [6][7][8][9] . In this work, we study the grey regime of radiation hydrodynamics by combining a stiff source solver with the truncated moment formalism of Thorne, which has been recently formulated in the full general relativistic framework 10 .…”