grmonty is a radiative transport code that uses the Monte Carlo technique to compute orientation-dependent broadband spectra from general relativistic magnetohydrodynamical simulations of black hole accretion. It is fully relativistic, includes angle and energy dependent synchrotron emission/absorption, and treats Compton scattering off of relativistic thermal electrons without approximation. The published version uses the “fast-light” approximation, but the code has since been extended to properly treat light travel time effects in time-dependent flows. The code, sans light travel time effects, is available freely here.

Our very own supermassive black hole, Sgr A*

The black hole at the center of the Milky Way is about four million times as massive as the Sun and about 27,000 lightyears away. Gas falling into the hole emits radiation across the electromagnetic spectrum from radio to X-rays, and loads of multiwavelength data have been and are still being recorded. With the Event Horizon Telescope, astronomers are even beginning to directly probe the structure of the gas on scales comparable with the black hole's event horizon. By building numerical models of this infalling gas and performing mock observations, we can try to get a handle on the physics of the accretion flow and the properties of the black hole spacetime.

Equatorial slice of the density from a 3D GRMHD simulation.

GRMHD simulation of black hole accretion This image shows a volume rendering from a 3D GRMHD simulation of an accretion flow around a black hole, highlighting the main body of the disk and the nearly evacuated funnel. The simulation was carried out with Scott Noble's 3D version of the HARM code.


Simulated image of Sgr A* black hole Simulated image of Sgr A* black hole Simulated image of Sgr A* black hole Simulated image of Sgr A* black hole Simulated image of Sgr A* black hole Simulated image of Sgr A* black hole


230 GHz (1.3 mm) images (logarithmically stretched) computed by time-dependent relativistic ray-tracing through a 4D GRMHD data hypercube. Inclinations shown are relative to the black hole spin axis (which points along the funnel in the volume rendered image above).

Quasi-periodic Oscillations

QPOs are observed in numerous galactic black hole binaries and at least one active galactic nucleus. Their origin is still unknown. When analyzing the multiwavelength light curves produced by grmonty using 3D GRMHD simulation data, we found that our light curves showed QPOs, the first unambiguosly seen QPOs in simulated observations of MHD/GRMHD models...

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