scholarly journals On the comparison of AGN with GRMHD simulations: I. Sgr A*

2020 ◽  
Vol 493 (1) ◽  
pp. 1404-1418 ◽  
Author(s):  
Richard Anantua ◽  
Sean Ressler ◽  
Eliot Quataert
Keyword(s):  
Low Frequency ◽  
Current Data ◽  
Magnetic Pressure ◽  
Galactic Centre ◽  
Spectral Slope ◽  
Thermal Pressure ◽  
Sagittarius A ◽  
General Relativistic ◽  
Sgr A ◽  
Two Temperature

ABSTRACT We present models of Galactic Centre emission in the vicinity of Sagittarius A* that use parametrizations of the electron temperature or energy density. These models include those inspired by two-temperature general relativistic magnetohydrodynamic (GRMHD) simulations as well as jet-motivated prescriptions generalizing equipartition of particle and magnetic energies. From these models, we calculate spectra and images and classify them according to their distinct observational features. Some models produce morphological and spectral features, e.g. image sizes, the sub-mm bump, and low-frequency spectral slope compatible with observations. Models with spectra consistent with observations produce the most compact images, with the most prominent, asymmetric photon rings. Limb-brightened outflows are also visible in many models. Of all the models we consider, that which represents the current data the best is one in which electrons are relativistically hot when magnetic pressure is larger than the thermal pressure, but cold (i.e. negligibly contributing to the emission) otherwise. This work is part of a series also applying the ‘observing’ simulations methodology to near-horizon regions of supermassive black holes in M87 and 3C 279.

scholarly journals Micro-arcsecond structure of Sagittarius A∗ revealed by high-sensitivity 86 GHz VLBI observations

2019 ◽  
Vol 621 ◽  
pp. A119 ◽  
Author(s):  
Christiaan D. Brinkerink ◽  
Cornelia Müller ◽  
Heino D. Falcke ◽  
Sara Issaoun ◽  
Kazunori Akiyama ◽  
...  
Keyword(s):  
High Sensitivity ◽  
Galactic Centre ◽  
Model Parameters ◽  
Sagittarius A ◽  
Long Baseline ◽  
Vlbi Observations ◽  
Size Frequency ◽  
Frequency Relation ◽  
Sgr A ◽  
Source Geometry

Context. The compact radio source Sagittarius A∗ (Sgr A∗) in the Galactic centre is the primary supermassive black hole candidate. General relativistic magnetohydrodynamical (GRMHD) simulations of the accretion flow around Sgr A∗ predict the presence of sub-structure at observing wavelengths of ∼3 mm and below (frequencies of 86 GHz and above). For very long baseline interferometry (VLBI) observations of Sgr A∗ at this frequency the blurring effect of interstellar scattering becomes sub-dominant, and arrays such as the high sensitivity array (HSA) and the global mm-VLBI array (GMVA) are now capable of resolving potential sub-structure in the source. Such investigations help to improve our understanding of the emission geometry of the mm-wave emission of Sgr A∗, which is crucial for constraining theoretical models and for providing a background to interpret 1 mm VLBI data from the Event Horizon Telescope (EHT). Aims. Following the closure phase analysis in our first paper, which indicates asymmetry in the 3 mm emission of Sgr A∗, here we have used the full visibility information to check for possible sub-structure. We extracted source size information from closure amplitude analysis, and investigate how this constrains a combined fit of the size-frequency relation and the scattering law for Sgr A∗. Methods. We performed high-sensitivity VLBI observations of Sgr A∗ at 3 mm using the Very Long Baseline Array (VLBA) and the Large Millimeter Telescope (LMT) in Mexico on two consecutive days in May 2015, with the second epoch including the Greenbank Telescope (GBT). Results. We confirm the asymmetry for the experiment including GBT. Modelling the emission with an elliptical Gaussian results in significant residual flux of ∼10 mJy in south-eastern direction. The analysis of closure amplitudes allows us to precisely constrain the major and minor axis size of the main emission component. We discuss systematic effects which need to be taken into account. We consider our results in the context of the existing body of size measurements over a range of observing frequencies and investigate how well-constrained the size-frequency relation is by performing a simultaneous fit to the scattering law and the size-frequency relation. Conclusions. We find an overall source geometry that matches previous findings very closely, showing a deviation in fitted model parameters less than 3% over a time scale of weeks and suggesting a highly stable global source geometry over time. The reported sub-structure in the 3 mm emission of Sgr A∗ is consistent with theoretical expectations of refractive noise on long baselines. However, comparing our findings with recent results from 1 mm and 7 mm VLBI observations, which also show evidence for east-west asymmetry, we cannot exclude an intrinsic origin. Confirmation of persistent intrinsic substructure will require further VLBI observations spread out over multiple epochs.

scholarly journals Spectral and imaging properties of Sgr A* from high-resolution 3D GRMHD simulations with radiative cooling

2020 ◽  
Vol 499 (3) ◽  
pp. 3178-3192
Author(s):  
D Yoon ◽  
K Chatterjee ◽  
S B Markoff ◽  
D van Eijnatten ◽  
Z Younsi ◽  
...  
Keyword(s):  
Accretion Rate ◽  
Dynamical Evolution ◽  
Radiative Properties ◽  
Spectral Energy ◽  
Radiative Cooling ◽  
Galactic Centre ◽  
Accretion Flow ◽  
Sagittarius A ◽  
Sgr A ◽  
The Impact

ABSTRACT The candidate supermassive black hole in the Galactic Centre, Sagittarius A* (Sgr A*), is known to be fed by a radiatively inefficient accretion flow (RIAF), inferred by its low accretion rate. Consequently, radiative cooling has in general been overlooked in the study of Sgr A*. However, the radiative properties of the plasma in RIAFs are poorly understood. In this work, using full 3D general–relativistic magnetohydrodynamical simulations, we study the impact of radiative cooling on the dynamical evolution of the accreting plasma, presenting spectral energy distributions and synthetic sub-millimetre images generated from the accretion flow around Sgr A*. These simulations solve the approximated equations for radiative cooling processes self-consistently, including synchrotron, bremsstrahlung, and inverse Compton processes. We find that radiative cooling plays an increasingly important role in the dynamics of the accretion flow as the accretion rate increases: the mid-plane density grows and the infalling gas is less turbulent as cooling becomes stronger. The changes in the dynamical evolution become important when the accretion rate is larger than $10^{-8}\, M_{\odot }~{\rm yr}^{-1}$ ($\gtrsim 10^{-7} \dot{M}_{\rm Edd}$, where $\dot{M}_{\rm Edd}$ is the Eddington accretion rate). The resulting spectra in the cooled models also differ from those in the non-cooled models: the overall flux, including the peak values at the sub-mm and the far-UV, is slightly lower as a consequence of a decrease in the electron temperature. Our results suggest that radiative cooling should be carefully taken into account in modelling Sgr A* and other low-luminosity active galactic nuclei that have a mass accretion rate of $\dot{M} \gt 10^{-7}\, \dot{M}_{\rm Edd}$.

Sagittarius A

1967 ◽  
Vol 31 ◽  
pp. 393-404 ◽  
Author(s):  
J. Lequeux
Keyword(s):  
Short Wave ◽  
Galactic Centre ◽  
High Angular Resolution ◽  
Thermal Source ◽  
Thermal Index ◽  
Sagittarius A ◽  
The Galaxy ◽  
External Galaxies ◽  
Sgr A ◽  
Thermal Sources

The available high-resolution observations of the complex of radio sources in the region of the galactic centre are reviewed and analysed. As noted by Downes and Maxwell, the spectrum of the strong central source, Sagittarius A, is markedly non-thermal (index 0·7) at high frequencies; below 3000 MHz the spectrum may be flat, but flux values obtained at high angular resolution are badly needed.Various arguments indicate that the whole source complex is located near the nucleus of the Galaxy. From the short-wave observations collected by Downes and Maxwell dimensions, densities and masses are estimated for the thermal sources. The total mass of ionized hydrogen in these sources is about 106M⊙; the mechanism of ionization is uncertain.The non-thermal source Sgr A may be similar to the optical nuclei of external galaxies. Its synchrotron emission, as well as the outward motions and tilted features observed in the 21-cm line and continuum, are signs of activity in the galactic nucleus; but the time-scales of the various phenomena appear to disagree.

scholarly journals Kozai resonance model for Sagittarius A* stellar orbits

2006 ◽  
Vol 2 (S238) ◽  
pp. 201-206 ◽  
Author(s):  
Ladislav Šubr ◽  
Vladimír Karas ◽  
Jaroslav Haas
Keyword(s):  
Galactic Centre ◽  
Tidal Disruption ◽  
Resonance Mechanism ◽  
Resonance Model ◽  
Stellar Orbits ◽  
Stellar Cluster ◽  
Sagittarius A ◽  
Sgr A ◽  
Different Sources ◽  
Kozai Resonance

AbstractWe study a possibility of tidal disruptions of stars orbiting a supermassive black hole due to eccentricity oscillations driven by Kozai's mechanism. We apply the model to conditions relevant for the Galactic Centre where we consider two different sources of the perturbation to the central potential, which trigger the resonance mechanism. Firstly, it is a disc of young massive stars orbiting Sgr A* atr≳ 0.08 pc, and, secondly, a molecular circumnuclear disc. Each of the two possibilities appears to be capable of exciting eccentricities to values sufficient for the tidal disruption of ∼100 stars from the nuclear stellar cluster on a time-scale of 0.1–10 Myrs. Tidally disrupted stars may cause periods of increased accretion activity of Sgr A*.

scholarly journals On the prospects of imaging Sagittarius A* from space

2018 ◽  
Vol 14 (S342) ◽  
pp. 24-28
Author(s):  
Freek Roelofs ◽  
Heino Falcke ◽  
Christiaan Brinkerink ◽  
Monika Moscibrodzka ◽  
Leonid I. Gurvits ◽  
...  
Keyword(s):  
General Relativity ◽  
Galactic Center ◽  
Tests Of General Relativity ◽  
Sagittarius A ◽  
Long Baseline ◽  
General Relativistic ◽  
New Space ◽  
Sgr A ◽  
Relativistic Magnetohydrodynamics ◽  
Earth Orbits

AbstractVery Long Baseline Interferometry (VLBI) at sub-millimeter waves has the potential to image the shadow of the black hole in the Galactic Center, Sagittarius A* (Sgr A*), and thereby test basic predictions of the theory of general relativity. We investigate the imaging prospects of a new Space VLBI mission concept. The setup consists of two satellites in polar or equatorial circular Medium-Earth Orbits with slightly different radii, resulting in a dense spiral-shaped uv-coverage with long baselines, allowing for extremely high-resolution and high-fidelity imaging of radio sources. We simulate observations of a general relativistic magnetohydrodynamics model of Sgr A* for this configuration with noise calculated from model system parameters. After gridding the uv-plane and averaging visibilities accumulated over multiple months of integration, images of Sgr A* with a resolution of up to 4 μ as could be reconstructed, allowing for stronger tests of general relativity and accretion models than with ground-based VLBI.

scholarly journals Flares in the Galactic Centre – I. Orbiting flux tubes in magnetically arrested black hole accretion discs

2021 ◽  
Vol 502 (2) ◽  
pp. 2023-2032
Author(s):  
O Porth ◽  
Y Mizuno ◽  
Z Younsi ◽  
C M Fromm
Keyword(s):  
Black Hole ◽  
Magnetic Energy ◽  
Gravity Data ◽  
Accretion Discs ◽  
Galactic Centre ◽  
Flux Tubes ◽  
Black Hole Accretion ◽  
General Relativistic ◽  
Sgr A ◽  
The Magnetic Field

ABSTRACT Recent observations of Sgr A* by the GRAVITY instrument have astrometrically tracked infrared (IR) flares at distances of ∼10 gravitational radii (rg). In this paper, we study a model for the flares based on 3D general relativistic magnetohydrodynamic (GRMHD) simulations of magnetically arrested accretion discs (MADs) that exhibit violent episodes of flux escape from the black hole magnetosphere. These events are attractive for flare modelling for several reasons: (i) the magnetically dominant regions can resist being disrupted via magnetorotational turbulence and shear; (ii) the orientation of the magnetic field is predominantly vertical as suggested by the GRAVITY data; and (iii) the magnetic reconnection associated with the flux eruptions could yield a self-consistent means of particle heating/acceleration during the flare events. In this analysis, we track erupted flux bundles and provide distributions of sizes, energies, and plasma parameter. In our simulations, the orbits tend to circularize at a range of radii from ${\sim} 5\hbox{ to }40\, r_{\rm g}$. The magnetic energy contained within the flux bundles ranges up to ${\sim} 10^{40}\,\rm erg$, enough to power IR and X-ray flares. We find that the motion within the magnetically supported flow is substantially sub-Keplerian, in tension with the inferred period–radius relation of the three GRAVITY flares.

scholarly journals Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole

2018 ◽  
Vol 615 ◽  
pp. L15 ◽  
Author(s):  
◽  
R. Abuter ◽  
A. Amorim ◽  
N. Anugu ◽  
M. Bauböck ◽  
...  
Keyword(s):  
Black Hole ◽  
Adaptive Optics ◽  
Gravitational Redshift ◽  
Galactic Centre ◽  
Order Effects ◽  
Massive Black Hole ◽  
Black Hole Candidate ◽  
Very Large Telescope ◽  
General Relativistic ◽  
Sgr A

The highly elliptical, 16-year-period orbit of the star S2 around the massive black hole candidate Sgr A✻ is a sensitive probe of the gravitational field in the Galactic centre. Near pericentre at 120 AU ≈ 1400 Schwarzschild radii, the star has an orbital speed of ≈7650 km s−1, such that the first-order effects of Special and General Relativity have now become detectable with current capabilities. Over the past 26 years, we have monitored the radial velocity and motion on the sky of S2, mainly with the SINFONI and NACO adaptive optics instruments on the ESO Very Large Telescope, and since 2016 and leading up to the pericentre approach in May 2018, with the four-telescope interferometric beam-combiner instrument GRAVITY. From data up to and including pericentre, we robustly detect the combined gravitational redshift and relativistic transverse Doppler effect for S2 of z = Δλ / λ ≈ 200 km s−1/c with different statistical analysis methods. When parameterising the post-Newtonian contribution from these effects by a factor f , with f = 0 and f = 1 corresponding to the Newtonian and general relativistic limits, respectively, we find from posterior fitting with different weighting schemes f = 0.90 ± 0.09|stat ± 0.15|sys. The S2 data are inconsistent with pure Newtonian dynamics.

scholarly journals 2D kinematics of massive stars near the Galactic Centre

2020 ◽  
Vol 500 (3) ◽  
pp. 3213-3239
Author(s):  
Mattia Libralato ◽  
Daniel J Lennon ◽  
Andrea Bellini ◽  
Roeland van der Marel ◽  
Simon J Clark ◽  
...  
Keyword(s):  
Moving Objects ◽  
Hubble Space Telescope ◽  
Massive Stars ◽  
Line Of Sight ◽  
Galactic Centre ◽  
Harsh Environment ◽  
Proper Motions ◽  
Sgr A ◽  
Better Than ◽  
Rule Out

ABSTRACT The presence of massive stars (MSs) in the region close to the Galactic Centre (GC) poses several questions about their origin. The harsh environment of the GC favours specific formation scenarios, each of which should imprint characteristic kinematic features on the MSs. We present a 2D kinematic analysis of MSs in a GC region surrounding Sgr A* based on high-precision proper motions obtained with the Hubble Space Telescope. Thanks to a careful data reduction, well-measured bright stars in our proper-motion catalogues have errors better than 0.5 mas yr−1. We discuss the absolute motion of the MSs in the field and their motion relative to Sgr A*, the Arches, and the Quintuplet. For the majority of the MSs, we rule out any distance further than 3–4 kpc from Sgr A* using only kinematic arguments. If their membership to the GC is confirmed, most of the isolated MSs are likely not associated with either the Arches or Quintuplet clusters or Sgr A*. Only a few MSs have proper motions, suggesting that they are likely members of the Arches cluster, in agreement with previous spectroscopic results. Line-of-sight radial velocities and distances are required to shed further light on the origin of most of these massive objects. We also present an analysis of other fast-moving objects in the GC region, finding no clear excess of high-velocity escaping stars. We make our astro-photometric catalogues publicly available.

scholarly journals Formation of transient high-β plasmas in a magnetized, weakly collisional regime

2021 ◽  
Vol 87 (1) ◽  
Author(s):  
T. Byvank ◽  
D. A. Endrizzi ◽  
C. B. Forest ◽  
S. J. Langendorf ◽  
K. J. McCollam ◽  
...  
Keyword(s):  
Experimental Data ◽  
Magnetic Flux ◽  
Plasma Physics ◽  
System Size ◽  
Magnetic Pressure ◽  
Magnetized Plasma ◽  
Inertial Fusion ◽  
Thermal Pressure ◽  
Astrophysical Objects ◽  
Collisional Regime

We present experimental data providing evidence for the formation of transient ( ${\sim }20\ \mathrm {\mu }\textrm {s}$ ) plasmas that are simultaneously weakly magnetized (i.e. Hall magnetization parameter $\omega \tau > 1$ ) and dominated by thermal pressure (i.e. ratio of thermal-to-magnetic pressure $\beta > 1$ ). Particle collisional mean free paths are an appreciable fraction of the overall system size. These plasmas are formed via the head-on merging of two plasmas launched by magnetized coaxial guns. The ratio $\lambda _{\textrm {gun}}=\mu _0 I_{\textrm {gun}}/\psi _{\textrm {gun}}$ of gun current $I_{\textrm {gun}}$ to applied magnetic flux $\psi _{\textrm {gun}}$ is an experimental knob for exploring the parameter space of $\beta$ and $\omega \tau$ . These experiments were conducted on the Big Red Ball at the Wisconsin Plasma Physics Laboratory. The transient formation of such plasmas can potentially open up new regimes for the laboratory study of weakly collisional, magnetized, high- $\beta$ plasma physics; processes relevant to astrophysical objects and phenomena; and novel magnetized plasma targets for magneto-inertial fusion.

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