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 Self-consistent spectra from GRMHD simulations with radiative cooling A link to reality for Sgr A*

2010 ◽  
Vol 6 (S275) ◽  
pp. 104-105
Author(s):  
Samia Drappeau ◽  
Salomé Dibi ◽  
Sera Markoff ◽  
Chris Fragile
Keyword(s):  
Energy Density ◽  
Total Energy ◽  
Accretion Rate ◽  
Radiative Cooling ◽  
Accretion Flow ◽  
Self Consistent ◽  
Sgr A ◽  
Cooling Function ◽  
Simulation Parameters ◽  
Total Energy Conservation

AbstractCosmos++ (Anninos et al. 2005) is one of the first fully relativistic magneto-hydro-dynamical (MHD) codes that can self-consistently account for radiative cooling, in the optically thin regime. As the code combines a total energy conservation formulation with a radiative cooling function, we have now the possibility to produce spectra energy density from these simulations and compare them to data. In this paper, we present preliminary results of spectra calculated using the same cooling functions from 2D Cosmos++ simulations of the accretion flow around Sgr A*. The simulation parameters were designed to roughly reproduce Sgr A*'s behavior at very low (10−8–10−7 M⊙/yr) accretion rate, but only via spectra can we test that this has been achieved.

scholarly journals The surprisingly small impact of magnetic fields on the inner accretion flow of Sagittarius A* fueled by stellar winds

2019 ◽  
Vol 492 (3) ◽  
pp. 3272-3293 ◽  
Author(s):  
S M Ressler ◽  
E Quataert ◽  
J M Stone
Keyword(s):  
Angular Momentum ◽  
Magnetic Fields ◽  
Event Horizon ◽  
Initial Conditions ◽  
Rotational Instability ◽  
Stellar Winds ◽  
Accretion Flow ◽  
Sagittarius A ◽  
Mhd Simulations ◽  
Sgr A

ABSTRACT We study the flow structure in 3D magnetohydrodynamic (MHD) simulations of accretion on to Sagittarius A* via the magnetized winds of the orbiting Wolf–Rayet stars. These simulations cover over 3 orders of magnitude in radius to reach ≈300 gravitational radii, with only one poorly constrained parameter (the magnetic field in the stellar winds). Even for winds with relatively weak magnetic fields (e.g. plasma β ∼ 106), flux freezing/compression in the inflowing gas amplifies the field to β ∼ few well before it reaches the event horizon. Overall, the dynamics, accretion rate, and spherically averaged flow profiles (e.g. density, velocity) in our MHD simulations are remarkably similar to analogous hydrodynamic simulations. We attribute this to the broad distribution of angular momentum provided by the stellar winds, which sources accretion even absent much angular momentum transport. We find that the magneto-rotational instability is not important because of (i) strong magnetic fields that are amplified by flux freezing/compression, and (ii) the rapid inflow/outflow times of the gas and inefficient radiative cooling preclude circularization. The primary effect of magnetic fields is that they drive a polar outflow that is absent in hydrodynamics. The dynamical state of the accretion flow found in our simulations is unlike the rotationally supported tori used as initial conditions in horizon scale simulations, which could have implications for models being used to interpret Event Horizon Telescope and GRAVITY observations of Sgr A*.

scholarly journals Nature of the Dusty S-cluster Object (DSO/G2): Pre-main-sequence star with non-spherical dusty envelope

2016 ◽  
Vol 11 (S322) ◽  
pp. 231-232
Author(s):  
M. Zajaček ◽  
M. Valencia-S. ◽  
B. Shahzamanian ◽  
F. Peissker ◽  
A. Eckart ◽  
...  
Keyword(s):  
Energy Distribution ◽  
Main Sequence ◽  
Near Infrared ◽  
Spectral Energy Distribution ◽  
Main Sequence Star ◽  
Continuum Emission ◽  
Spectral Energy ◽  
Galactic Centre ◽  
Infrared Excess ◽  
Sgr A

AbstractNear-infrared observations reveal several infrared-excess sources near the Galactic Centre with emission lines present in their spectra. One of these objects, DSO/G2, which moves around the supermassive black hole (Sgr A*) on a highly eccentric orbit, passed the pericentre at approximately 160 AU in 2014. It remained compact, which implies that at least in this case it is a star embedded in a dusty envelope. The spectral energy distribution and the detection of polarized continuum emission indicate that it is probably a pre-main-sequence star surrounded by a dense envelope with bipolar cavities. In addition, the star associated with DSO/G2 plausibly develops a bow shock due to its supersonic motion. The model of the star surrounded by the non-spherical dusty envelope can reproduce the main characteristics of the DSO/G2 source: 1. spectral energy distribution in near-infrared bands; 2. linear polarization in Ks band; and 3. the overall compact behaviour.

scholarly journals Positron Effects on Polarized Images and Spectra from Jet and Accretion Flow Models of M87* and Sgr A*

2021 ◽  
Vol 923 (2) ◽  
pp. 272
Author(s):  
Razieh Emami ◽  
Richard Anantua ◽  
Andrew A. Chael ◽  
Abraham Loeb
Keyword(s):  
Power Law ◽  
Pressure Ratio ◽  
Spectral Energy ◽  
Emission Region ◽  
Flow Models ◽  
Accretion Flow ◽  
Positron Fraction ◽  
General Relativistic ◽  
Sgr A ◽  
Polarization Fraction

Abstract We study the effects of including a nonzero positron-to-electron fraction in emitting plasma on the polarized spectral energy distributions and submillimeter images of jet and accretion flow models for near-horizon emission from M87* and Sgr A*. For M87*, we consider a semi-analytic fit to the force-free plasma regions of a general relativistic magnetohydrodynamic jet simulation, which we populate with power-law leptons with a constant electron-to-magnetic pressure ratio. For Sgr A*, we consider a standard self-similar radiatively inefficient accretion flow where the emission is predominantly from thermal leptons with a small fraction in a power-law tail. In both models, we fix the positron-to-electron ratio throughout the emission region. We generate polarized images and spectra from our models using the general relativistic ray tracing and radiative transfer from GRTRANS. We find that a substantial positron fraction reduces the circular polarization fraction at IR and higher frequencies. However, in submillimeter images, higher positron fractions increase polarization fractions due to strong effects of Faraday conversion. We find an M87* jet model that best matches the available broadband total intensity, and 230 GHz polarization data is a sub-equipartition, with positron fraction of ≃10%. We show that jet models with significant positron fractions do not satisfy the polarimetric constraints at 230 GHz from the Event Horizon Telescope (EHT). Sgr A* models show similar trends in their polarization fractions with increasing pair fraction. Both models suggest that resolved, polarized EHT images are useful to constrain the presence of pairs at 230 GHz emitting regions of M87* and Sgr A*.

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.

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 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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