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 A parameter survey of Sgr A* radiative models from GRMHD simulations with self-consistent electron heating

2020 ◽  
Vol 494 (3) ◽  
pp. 4168-4186 ◽  
Author(s):  
J Dexter ◽  
A Jiménez-Rosales ◽  
S M Ressler ◽  
A Tchekhovskoy ◽  
M Bauböck ◽  
...  
Keyword(s):  
Faraday Rotation ◽  
Near Infrared ◽  
Good Description ◽  
Galactic Centre ◽  
Electron Heating ◽  
Black Hole Candidate ◽  
Long Baseline ◽  
Rotation Measure ◽  
Accretion Physics ◽  
Sgr A

ABSTRACT 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.

scholarly journals Quantum gravity effects around Sagittarius A*

2016 ◽  
Vol 25 (12) ◽  
pp. 1644021 ◽  
Author(s):  
Hal M. Haggard ◽  
Carlo Rovelli
Keyword(s):  
Black Hole ◽  
Quantum Gravity ◽  
Event Horizon ◽  
Sagittarius A ◽  
Vlbi Observations ◽  
Gravity Effects ◽  
Sgr A ◽  
Schwarzschild Radius ◽  
Number Of Authors

Recent VLBI observations have resolved Sagittarius A* at horizon scales. The event horizon telescope is expected to provide increasingly good images of the region around the Schwarzschild radius [Formula: see text] of Sgr A* soon. A number of authors have recently pointed out the possibility that nonperturbative quantum gravitational phenomena could affect the space surrounding a black hole. Here, we point out that the existence of a region around [Formula: see text], where these effects should be maximal.

scholarly journals Simulations of M87 and Sgr A* imaging with the Millimetron Space Observatory on near-Earth orbits

2020 ◽  
Vol 500 (4) ◽  
pp. 4866-4877
Author(s):  
A S Andrianov ◽  
A M Baryshev ◽  
H Falcke ◽  
I A Girin ◽  
T de Graauw ◽  
...  
Keyword(s):  
Black Hole ◽  
Space Mission ◽  
Dynamic Imaging ◽  
Image Resolution ◽  
Fast Time ◽  
Supermassive Black Hole ◽  
Long Baseline ◽  
Vlbi Observations ◽  
Sgr A ◽  
Earth Orbits

ABSTRACT High-resolution imaging of supermassive black hole shadows is a direct way to verify the theory of general relativity under extreme gravity conditions. Very Long Baseline Interferometry (VLBI) observations at millimetre/submillimetre wavelengths can provide such angular resolution for the supermassive black holes located in Sgr A* and M87. Recent VLBI observations of M87 with the Event Horizon Telescope (EHT) have shown such capabilities. The maximum obtainable spatial resolution of the EHT is limited by the Earth's diameter and atmospheric phase variations. In order to improve the image resolution, longer baselines are required. The Radioastron space mission successfully demonstrated the capabilities of space–Earth VLBI with baselines much longer than the Earth's diameter. Millimetron is the next space mission of the Russian Space Agency and will operate at millimetre wavelengths. The nominal orbit of the observatory will be located around the Lagrangian L2 point of the Sun–Earth system. In order to optimize the VLBI mode, we consider a possible second stage of the mission that could use a near-Earth high elliptical orbit (HEO). In this paper, a set of near-Earth orbits is used for synthetic space–Earth VLBI observations of Sgr A* and M87 in a joint Millimetron and EHT configuration. General relativistic magnetohydrodynamic models for the supermassive black hole environments of Sgr A* and M87 are used for static and dynamic imaging simulations at 230 GHz. A comparison preformed between ground and space–Earth baselines demonstrates that joint observations with Millimetron and EHT significantly improve the image resolution and allow the EHT + Millimetron to obtain snapshot images of Sgr A*, probing the dynamics at fast time-scales.

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 $\dot{M} \gt 10^{-7}\, \dot{M}_{\rm Edd}$.

scholarly journals First Space-VLBI Observations of Sagittarius A*

2021 ◽  
Vol 922 (2) ◽  
pp. L28
Author(s):  
Michael D. Johnson ◽  
Yuri Y. Kovalev ◽  
Mikhail M. Lisakov ◽  
Petr A. Voitsik ◽  
Carl R. Gwinn ◽  
...  
Keyword(s):  
Angular Resolution ◽  
Galactic Center ◽  
Global Network ◽  
Sagittarius A ◽  
Vlbi Observations ◽  
Submillimeter Wavelengths ◽  
Short Baselines ◽  
Sgr A ◽  
Strong Scattering ◽  
Long Baselines

Abstract We report results from the first Earth-space VLBI observations of the Galactic Center supermassive black hole, Sgr A*. These observations used the space telescope Spektr-R of the RadioAstron project together with a global network of 20 ground telescopes, observing at a wavelength of 1.35 cm. Spektr-R provided baselines up to 3.9 times the diameter of the Earth, corresponding to an angular resolution of approximately 55 μas and a spatial resolution of 5.5R Sch at the source, where R Sch ≡ 2GM/c 2 is the Schwarzschild radius of Sgr A*. Our short ground baseline measurements ( ≲ 80 Mλ) are consistent with an anisotropic Gaussian image, while our intermediate ground baseline measurements (100–250 Mλ) confirm the presence of persistent image substructure in Sgr A*. Both features are consistent with theoretical expectations for strong scattering in the ionized interstellar medium, which produces Gaussian scatter-broadening on short baselines and refractive substructure on long baselines. We do not detect interferometric fringes on any of the longer ground baselines or on any ground–space baselines. While space-VLBI offers a promising pathway to sharper angular resolution and the measurement of key gravitational signatures in black holes, such as their photon rings, our results demonstrate that space-VLBI studies of Sgr A* will require sensitive observations at submillimeter wavelengths.

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 Event horizon scale emission models for Sagittarius A*

2013 ◽  
Vol 9 (S303) ◽  
pp. 298-302
Author(s):  
J. Dexter
Keyword(s):  
Black Holes ◽  
Black Hole ◽  
Event Horizon ◽  
Near Infrared ◽  
Sagittarius A ◽  
Long Baseline ◽  
Black Hole Accretion ◽  
Emission Models ◽  
Infrared Interferometry ◽  
Sgr A

AbstractVery long baseline interferometry observations at millimeter wavelengths have detected source structure in Sgr A* on event horizon scales. Near-infrared interferometry will achieve similar resolution in the next few years. These experiments provide an unprecedented opportunity to explore strong gravity around black holes, but interpreting the data requires physical modeling. I discuss the calculation of images, spectra, and light curves from relativistic MHD simulations of black hole accretion. The models provide an excellent description of current observations, and predict that we may be on the verge of detecting a black hole shadow, which would constitute the first direct evidence for the existence of black holes.

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 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.

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