Recent developments in compact object astrophysics, especially the discovery of merging neutron stars by LIGO, the imaging of the black hole in M87 by the Event Horizon Telescope, and high-precision astrometry of the Galactic Center at close to the event horizon scale by the GRAVITY experiment motivate the development of numerical source models that solve the equations of general relativistic magnetohydrodynamics (GRMHD). Here we compare GRMHD solutions for the evolution of a magnetized accretion flow where turbulence is promoted by the magnetorotational instability from a set of nine GRMHD codes: Athena++, BHAC, Cosmos++, ECHO, H-AMR, iharm3D, HARM-Noble, IllinoisGRMHD, and KORAL. Agreement among the codes improves as resolution increases, as measured by a consistently applied, specially developed set of code performance metrics. We conclude that the community of GRMHD codes is mature, capable, and consistent on these test problems.

We use global magnetohydrodynamic simulations to study the influence of net vertical magnetic fields on the structure of geometrically thin (H/r ≈ 0.05) accretion disks in the Newtonian limit. We consider initial mid-plane gas to magnetic pressure ratios β 0 = 1000, 300 and 100, spanning the transition between weakly and strongly magnetized accretion regimes. We find that magnetic pressure is important for the disks' vertical structure in all three cases, with accretion occurring at z/R ≈ 0.2 in the two most strongly magnetized models. The disk midplane shows outflow rather than accretion. Accretion through the surface layers is driven mainly by stress due to coherent large scale magnetic field rather than by turbulent stress. Equivalent viscosity parameters measured from our simulations show similar dependencies on initial β 0 to those seen in shearing box simulations, though the disk midplane is not magnetic pressure dominated even for the strongest magnetic field case. Winds are present but are not the dominant driver of disk evolution. Over the (limited) duration of our simulations, we find evidence that the net flux attains a quasi-steady state at levels that can stably maintain a strongly magnetized disk. We suggest that geometrically thin accretion disks in observed systems may commonly exist in a magnetically "elevated" state, characterized by non-zero but modest vertical magnetic fluxes, with potentially important implications for disk phenomenology in X-ray binaries (XRBs) and active galactic nuclei (AGN).

We present results from general relativistic calculations of the tidal disruption of white dwarf stars from near encounters with intermediate mass black holes. We follow the evolution of 0.2 and 0.6M ⊙ stars on parabolic trajectories that approach 10 3 -10 4 M ⊙ black holes as close as a few Schwarzschild radii at periapsis, paying particular attention to the effect tidal disruption has on thermonuclear reactions and the synthesis of intermediate to heavy ion elements. These encounters create diverse thermonuclear environments characteristic of Type I supernovae and capable of producing both intermediate and heavy mass elements in arbitrary ratios, depending on the strength (or proximity) of the interaction. Nuclear ignition is triggered in all of our calculations, even at weak tidal strengths β ∼ 2.6 and large periapsis radius R P ∼ 28 Schwarzschild radii. A strong inverse correlation exists between the mass ratio of calcium to iron group elements and tidal strength, with β 5 producing predominately calcium-rich debris. At these moderate to weak interactions, nucleosynthesis is not especially efficient, limiting the total mass and outflows of calcium group elements to < 15% of available nuclear fuel. Iron group elements however continue to be produced in greater quantity and ratio with increasing tidal strength, peaking at ∼ 60% mass conversion efficiency in our closest encounter cases. These events generate short bursts of gravitational waves with characteristic frequencies 0.1-0.7 Hz and strain amplitudes 0.5 × 10 −22 -3.5 × 10 −22 at 10 Mpc source distance.

We present results from two-dimensional, general relativistic, viscous, radiation hydrodynamic numerical simulations of Shakura-Sunyaev thin disks accreting onto stellar mass Schwarzschild black holes. We consider cases on both the gas-and radiation-pressure-dominated branches of the thermal equilibrium curve, with mass accretion rates spanning the range fromṀ = 0.01L Edd /c 2 to 10L Edd /c 2 . The simulations directly test the stability of this standard disk model on the different branches. We find clear evidence of thermal instability for all radiation-pressure-dominated disks, resulting universally in the vertical collapse of the disks, which in some cases then settle onto the stable, gas-pressuredominated branch. Although these results are consistent with decades-old theoretical predictions, they appear to be in conflict with available observational data from black hole X-ray binaries. We also find evidence for a radiation-pressure-driven instability that breaks the unstable disks up into alternating rings of high and low surface density on a timescale comparable to the thermal collapse. Since radiation is included self-consistently in the simulations, we are able to calculate lightcurves and power density spectra (PDS). For the most part, we measure radiative efficiencies (ratio of luminosity to mass accretion rate) close to 6%, as expected for a non-rotating black hole. The PDS appear as broken power laws, with a break typically around 100 Hz. There is no evidence of significant excess power at any frequencies, i.e. no quasi-periodic oscillations are observed.