Search

Sagittarius A* (Sgr A*), the supermassive black hole at the centre of the Milky Way, undergoes large-amplitude near-infrared (NIR) flares that can coincide with the continuous rotation of the NIR emission region. One promising explanation for this observed NIR behaviour is a magnetic flux eruption, which occurs in three-dimensional General Relativistic Magneto-Hydrodynamic (3D GRMHD) simulations of magnetically arrested accretion flows. After running two-temperature 3D GRMHD simulations, where the electron temperature is evolved self-consistently along with the gas temperature, it is possible to calculate ray-traced images of the synchotron emission from thermal electrons in the accretion flow. Changes in the gas-dominated (σ = b 2/2ρ < 1) regions of the accretion flow during a magnetic flux eruption reproduce the NIR flaring and NIR emission region rotation of Sgr A* with durations consistent with observation. In this paper, we demonstrate that these models also predict that large (1.5x – 2x) size increases of the sub-millimeter (sub-mm) and millimeter (mm) emission region follow most NIR flares by 20–50 min. These size increases occur across a wide parameter space of black hole spin (a  = 0.3, 0.5, −0.5, and 0.9375) and initial tilt angle between the accretion flow and black hole spin axes θ0 (θ0 = 0°, 16°, and 30°). We also calculate the sub-mm polarization angle rotation and the shift of the sub-mm spectral index from zero to –0.8 during a prominent NIR flare in our high spin (a  = 0.9375) simulation. We show that, during a magnetic flux eruption, a large (∼10 r g), magnetically dominated (σ > 1), low-density, and high-temperature 'bubble' forms in the accretion flow. The drop in density inside the bubble and additional electron heating in accretion flow between 15 r g and 25 r g leads to a sub-mm size increase in corresponding images. [ABSTRACT FROM AUTHOR]

Using general relativistic magnetohydrodynamic simulations of accreting black holes, we show that a suitable subtraction of the linear polarization per pixel from total intensity images can enhance the photon ring feature. We find that the photon ring is typically a factor of ≃2 less polarized than the rest of the image. This is due to a combination of plasma and general relativistic effects, as well as magnetic turbulence. When there are no other persistently depolarized image features, adding the subtracted residuals over time results in a sharp image of the photon ring. We show that the method works well for sample, viable GRMHD models of Sgr A* and M87*, where measurements of the photon ring properties would provide new measurements of black hole mass and spin, and potentially allow for tests of the 'no-hair' theorem of general relativity. [ABSTRACT FROM AUTHOR]

The Galactic centre black hole candidate Sgr A* is the best target for studies of low-luminosity accretion physics, including with near-infrared (NIR) and submillimetre wavelength long baseline interferometry experiments. Here, we compare images and spectra generated from a parameter survey of general relativistic MHD simulations to a set of radio to NIR observations of Sgr A*. Our models span the limits of weak and strong magnetization and use a range of sub-grid prescriptions for electron heating. We find two classes of scenarios can explain the broad shape of the submillimetre spectral peak and the highly variable NIR flaring emission. Weakly magnetized 'disc-jet' models where most of the emission is produced near the jet wall, consistent with past work, as well as strongly magnetized (magnetically arrested disc) models where hot electrons are present everywhere. Disc-jet models are strongly depolarized at submillimetre wavelengths as a result of strong Faraday rotation, inconsistent with observations of Sgr A*. We instead favour the strongly magnetized models, which provide a good description of the median and highly variable linear polarization signal. The same models can also explain the observed mean Faraday rotation measure and potentially the polarization signals seen recently in Sgr A* NIR flares. [ABSTRACT FROM AUTHOR]

Non-very long baseline interferometry (VLBI) measurements of Faraday rotation at millimetre wavelengths have been used to constrain mass accretion rates (...) on to supermassive black holes in the centre of the Milky Way and in the centre of M87. We constructed general relativistic magnetohydrodynamics models for these sources that qualitatively well describe their spectra and radio/mm images invoking a coupled jet–disc system. Using general relativistic polarized radiative transfer, we now also model the observed mm rotation measure (RM) of M87. The models are tied to the observed radio flux; however, the electron temperature and accretion rate are degenerate parameters and are allowed to vary. For the inferred low viewing angles of the M87 jet, the RM is low even as the black hole ... increases by a factor of ≃100. In jet-dominated models, the observed linear polarization is produced in the forward jet, while the dense accretion disc depolarizes the bulk of the near-horizon scale emission that originates in the counter jet. In the jet-dominated models, with increasing ... and increasing Faraday optical depth, one is progressively sensitive only to polarized emission in the forward jet, keeping the measured RM relatively constant. The jet-dominated model reproduces a low net-polarization of ≃1 per cent and RMs in agreement with observed values due to Faraday depolarization, however, with ... much larger than the previously inferred limit of 9 x 10-4M⊙ yr-1. All jet-dominated models produce much higher RMs for inclination angles i ≳ 30°, where the line of sight passes through the accretion flow, thereby providing independent constraints on the viewing geometry of the M87 jet. [ABSTRACT FROM AUTHOR]

We apply image moment invariant analysis to total intensity and polarimetric images calculated from general relativistic magnetohydrodynamic simulations of accreting black holes. We characterize different properties of the models in our library by their invariant distributions and their evolution in time. We show that they are highly sensitive to different physical effects present in the system which allow for model discrimination. We propose a new model scoring method based on image moment invariants that is uniformly applicable to total intensity and polarimetric images simultaneously. The method does not depend on the type of images considered and its application to other non-ring like images (e.g. jets) is straight forward. [ABSTRACT FROM AUTHOR]

We present the first spectral energy distributions produced self-consistently by 2.5D general relativistic magnetohydrodynamical (GRMHD) numerical simulations, where radiative cooling is included in the dynamical calculation. As a case study, we focus on the accretion flow around the supermassive black hole in the Galactic Centre, Sagittarius A* (Sgr A*), which has the best constrained physical parameters. We compare the simulated spectra to the observational data of Sgr A* and explore the parameter space of our model to determine the effect of changing the initial magnetic field configuration, the ion-to-electron temperature ratio Ti/Te and the target accretion rate. We find the best description of the data for a mass accretion rate of ∼10−9 M⊙ yr−1, and rapid spin (0.7 < a* < 0.9). The submillimetre peak flux seems largely independent of initial conditions, while the higher energies can be very sensitive to the initial magnetic field configuration. Finally, we also discuss flaring features observed in some simulations, which may be due to artefacts of the 2D configuration. [ABSTRACT FROM AUTHOR]

ABSTRACT We present general relativistic magnetohydrodynamic numerical simulations of the accretion flow around the supermassive black hole in the Galactic Centre, Sagittarius A* (Sgr A*). The simulations include for the first time radiative cooling processes (synchrotron, bremsstrahlung and inverse Compton) self-consistently in the dynamics, allowing us to test the common simplification of ignoring all cooling losses in the modelling of Sgr A*. We confirm that for Sgr A*, neglecting the cooling losses is a reasonable approximation if the Galactic Centre is accreting below ∼10−8 M⊙ yr−1, i.e. [ABSTRACT FROM AUTHOR]