Tilted black hole accretion disc models of Sagittarius A*: time-variable millimetre to near-infrared emission

Abstract

High-resolution, multi-wavelength, and time-domain observations of the Galactic centre black hole candidate, Sgr A*, allow for a direct test of contemporary accretion theory. To date, all models have assumed alignment between the accretion disc and black hole angular momentum axes, but this is unjustified for geometrically thick accretion flows like that onto Sgr A*. Instead, we calculate images and spectra from a set of simulations of accretion flows misaligned ('tilted') by 15 degrees from the black hole spin axis and compare them with millimetre (mm) to near-infrared (NIR) observations. Non-axisymmetric standing shocks from eccentric fluid orbits dominate the emission, leading to a wide range of possible image morphologies. These effects invalidate previous parameter estimates from model fitting, including estimates of the dimensionless black hole spin, except possibly at low values of spin or tilt. At 1.3mm, the images have crescent morphologies, and the black hole shadow may still be accessible to future mm-VLBI observations. Shock heating leads to high energy electrons (T > 10^12 K), which can naturally produce the observed NIR flux, spectral index, and rapid variability ('flaring'). This NIR emission is uncorrelated with that in the mm, which also agrees with observations. These are the first models to self-consistently explain the time-variable mm to NIR emission of Sgr A*. Predictions of the model include significant structural changes observable with mm-VLBI on both the dynamical (hour) and Lense-Thirring precession (day-year) timescales; and ~30-50 microarcsecond changes in centroid position from extreme gravitational lensing events during NIR flares, detectable with the future VLT instrument GRAVITY. If the observed NIR emission is caused by shock heating in a tilted accretion disc, then the Galactic centre black hole has a positive, non-zero spin parameter (a > 0).

17 pages, 18 figures, submitted to MNRAS; for movies and version with high-res figures see http://astro.berkeley.edu/~jdexter/tiltedsgra

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• Testing Generalized Einstein-Cartan-Kibble-Sciama Gravity using Weak Deflection Angle and Shadow Cast
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• An 8 hour characteristic time-scale in submillimetre light curves of Sagittarius A*
• Persistent Asymmetric Structure of Sagittarius A* on Event Horizon Scales
• Detection of intrinsic source structure at ~3 Schwarzschild radii with Millimeter-VLBI observations of SAGITTARIUS A*
• Probing the magnetic field structure in Sgr A* on Black Hole Horizon Scales with Polarized Radiative Transfer Simulations
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• Simulations of imaging the event horizon of Sagittarius A* from space
• Fast Variability and mm/IR flares in GRMHD Models of Sgr A* from Strong-Field Gravitational Lensing
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• A geometric crescent model for black hole images
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• Black Hole Images as Tests of General Relativity: Effects of Plasma Physics
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• A Detection of Sgr A* in the far infrared
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• Physical Properties of the Inner Shocks in Hot, Tilted Black Hole Accretion Flows
• Far Infrared Variability of Sagittarius A*: 25.5 Hours of Monitoring with $Herschel$
• Simultaneous Monitoring of X-ray and Radio Variability in Sagittarius A*
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• Aberrational Effects for Shadows of Black Holes
• Probing spacetime around Sagittarius A* using modeled VLBI closure phases
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• Images and Spectra of Time Dependent Two Component Advective Flow in Presence of Outflows
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• Electron Heating in the Trans-Relativistic Perpendicular Shocks of Tilted Accretion Flows
• Classification of a black hole spin out of its shadow using support vector machines
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