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Register a new userGeneral relativistic magnetohydrodynamical $kappa$-jet models for Sgr A*
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The observed spectral energy distribution of an accreting supermassive black hole typically forms a power-law spectrum in the Near Infrared (NIR) and optical wavelengths, that may be interpreted as a signature of accelerated electrons along the jet. However, the details of acceleration remain uncertain. In this paper, we study the radiative properties of jets produced in axisymmetric GRMHD simulations of hot accretion flows onto underluminous supermassive black holes both numerically and semi-analytically, with the aim of investigating the differences between models with and without accelerated electrons inside the jet. We assume that electrons are accelerated in the jet regions of our GRMHD simulation. To model them, we modify the electrons distribution function in the jet regions from a purely relativistic thermal distribution to a combination of a relativistic thermal distribution and the $kappa$-distribution function. Inside the disk, we assume a thermal distribution for the electrons. We calculate jet spectra and synchrotron maps by using the ray tracing code {tt RAPTOR}, and compare the synthetic observations to observations of Sgr~A*. Finally, we compare numerical models of jets to semi-analytical ones. We find that in the $kappa$-jet models, the radio-emitting region size, radio flux, and spectral index in NIR/optical bands increase for decreasing values of the $kappa$ parameter, which corresponds to a larger amount of accelerated electrons. The model with $kappa = 3.5$, $eta_{rm acc}=5-10%$ (the percentage of electrons that are accelerated), and observing angle $i = 30^{rm o}$ fits the observed Sgr~A* emission in the flaring state from the radio to the NIR/optical regimes, while $kappa = 3.5$, $eta_{rm acc}< 1%$, and observing angle $i = 30^{rm o}$ fit the upper limits in quiescence.
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General relativistic numerical simulations of magnetized accretion flows around black holes show a disordered electromagnetic structure in the disk and corona and a highly relativistic, Poynting-dominated funnel jet in the polar regions. The polar jet is nearly consistent with the stationary paraboloidal Blandford-Znajek model of an organized field threading the polar regions of a rotating black hole. How can a disordered accretion disk and corona lead to an ordered jet? We show that the polar jet is associated with a strikingly simple angular-integrated toroidal current distribution $dI_phi/dr propto r^{-5/4}$, where $I_phi(r)$ is the toroidal current enclosed inside radius $r$. We demonstrate that the poloidal magnetic field in the simulated jet agrees well with the force-free field solution for a non-rotating thin disk with an $r^{-5/4}$ toroidal current, suggesting rotation leads to negligible self-collimation. We find that the polar field is confined/collimated by the corona. The electromagnetic field in the disk also scales as $r^{-5/4}$, which is consistent with some Newtonian accretion models that assume rough equipartition between magnetic and gas pressure. However, the agreement is accidental since toward the black hole the magnetic pressure increases faster than the gas pressure. This field dominance near the black hole is associated with magnetic stresses that imply a large effective viscosity parameter $alphasim 1$, whereas the typically assumed value of $alphasim 0.1$ holds far from the black hole.[abridged]
The generation of turbulence at magnetized shocks and its subsequent interaction with the latter is a key question of plasma- and high-energy astrophysics. This paper presents two-dimensional magnetohydrodynamic simulations of a fast shock front interacting with incoming upstream perturbations, described as harmonic entropy or fast magnetosonic waves, both in the relativistic and the sub-relativistic regimes. We discuss how the disturbances are transmitted into downstream turbulence and we compare the observed response for small amplitude waves to a recent linear calculation. In particular, we demonstrate the existence of a resonant response of the corrugation amplitude when the group velocity of the outgoing downstream fast mode matches the velocity of the shock front. We also present simulations of large amplitude waves to probe the non-linear regime.
The half opening angle of a Kerr black-hole shadow is always equal to (5+-0.2)GM/Dc^2, where M is the mass of the black hole and D is its distance from the Earth. Therefore, measuring the size of a shadow and verifying whether it is within this 4% range constitutes a null hypothesis test of General Relativity. We show that the black hole in the center of the Milky Way, Sgr A*, is the optimal target for performing this test with upcoming observations using the Event Horizon Telescope. We use the results of optical/IR monitoring of stellar orbits to show that the mass-to-distance ratio for Sgr A* is already known to an accuracy of +-4%. We investigate our prior knowledge of the properties of the scattering screen between Sgr A and the Earth, the effects of which will need to be corrected for in order for the black-hole shadow to appear sharp against the background emission. Finally, we explore an edge detection scheme for interferometric data and a pattern matching algorithm based on the Hough/Radon transform and demonstrate that the shadow of the black hole at 1.3 mm can be localized, in principle, to within ~9%. All these results suggest that our prior knowledge of the properties of the black hole, of scattering broadening, and of the accretion flow can only limit this General Relativistic null hypothesis test with Event Horizon Telescope observations of Sgr A* to 10%.
The compact radio source Sgr A* is coincident with a 4 million solar mass black hole at the dynamical center of the Galaxy and is surrounded by dense orbiting ionized and molecular gas. We present high resolution radio continuum images of the central 3 and report a faint continuous linear structure centered on Sgr A* with a PA~60 degrees. The extension of this feature appears to be terminated symmetrically by two linearly polarized structures at 8.4 GHz, ~75 from Sgr A*. A number of weak blobs of radio emission with X-ray counterparts are detected along the axis of the linear structure. The linear structure is best characterized by a mildly relativistic jet from Sgr A* with an outflow rate 10^-6 solar mass per year. The near and far-sides of the jet are interacting with orbiting ionized and molecular gas over the last 1-3 hundred years and are responsible for a 2 hole, the minicavity, characterized by disturbed kinematics, enhanced FeII/III line emission, and diffuse X-ray gas. The estimated kinetic luminosity of the outflow is ~1.2x10^{41} erg/s, so the interaction with the bar may be responsible for the Galactic center X-ray flash inferred to be responsible for much of the fluorescent Fe Kalpha line emission from the inner 100pc of the Galaxy.
We study the environment of Sgr A* using spectral and continuum observations with the ALMA and VLA. Our analysis of sub-arcsecond H30alpha, H39alpha, H52alpha and H56alpha line emission towards Sgr A* confirm the recently published broad peak ~500 km/s~spectrum toward Sgr~A*. We also detect emission at more extreme radial velocities peaking near -2500 and 4000 km/s, within 0.2. We then present broad band radio continuum images at multiple frequencies on scales from arcseconds to arcminutes. A number of elongated continuum structures lie parallel to the Galactic plane, extending from ~0.4 to 10. We note a nonthermal elongated structure on an arcminute scale emanating from Sgr A* at low frequencies between 1 and 1.4 GHz where thermal emission from the mini-spiral is depressed by optical depth effects. The position angle of this elongated structure and the sense of motion of ionized features with respect to Sgr A* suggest a symmetric, collimated jet emerging from Sgr A* with an opening angle of ~30deg and a position angle of ~60deg punching through the medium before accelerating a significant fraction of the orbiting ionized gas to high velocities. The jet with estimated mass flow rate ~1.4x10^{-5} solar mass/yr emerges perpendicular to the equatorial plane of the accretion flow near the event horizon of Sgr A* and runs along the Galactic plane. To explain a number of east-west features near Sgr A*, we also consider the possibility of an outflow component with a wider-angle launched from the accretion flow at larger radii.
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