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 Simulating the pericentre passage of the Galactic centre star S2

2018 ◽  
Vol 616 ◽  
pp. L8 ◽  
Author(s):  
M. Schartmann ◽  
A. Burkert ◽  
A. Ballone
Keyword(s):  
Power Law ◽  
Stellar Wind ◽  
Adaptive Mesh Refinement ◽  
Mass Loss Rate ◽  
Galactic Centre ◽  
Massive Black Hole ◽  
X Ray ◽  
Flow Models ◽  
Accretion Flow ◽  
Sgr A

Context. Our knowledge of the density distribution of the accretion flow around Sgr A* – the massive black hole (BH) at our Galactic centre (GC) – relies on two measurements only: one at a distance of a few Schwarzschild radii (Rs) and one at roughly 105 Rs, which are usually bridged by a power law, which is backed by magnetohydrodynamical simulations. The so-called S2 star reached its closest approach to the massive BH at around 1500 Rs in May 2018. It has been proposed that the interaction of its stellar wind with the high-density accretion flow at this distance from Sgr A* will lead to a detectable, month-long X-ray flare. Aims. Our goal is to verify whether or not the S2 star wind can be used as a diagnostic tool to infer the properties of the accretion flow towards Sgr A* at its pericentre (an unprobed distance regime), putting important constraints on BH accretion flow models. Methods. We run a series of three-dimensional adaptive mesh refinement simulations with the help of the RAMSES code which include the realistic treatment of the interaction of S2’s stellar wind with the accretion flow along its orbit and – apart from hydrodynamical and thermodynamical effects – include the tidal interaction with the massive BH. These are post-processed to derive the X-ray emission in the observable 2–10 keV window. Results. No significant excess of X-ray emission from Sgr A* is found for typical accretion flow models. A measurable excess is produced for a significantly increased density of the accretion flow. This can, however, be ruled out for standard power-law accretion flow models as in this case the thermal X-ray emission without the S2 wind interaction would already exceed the observed quiescent luminosity. Only a significant change of the wind parameters (increased mass loss rate and decreased wind velocity) might lead to an (marginally) observable X-ray flaring event. Conclusion. Even the detection of an (month-long) X-ray flare during the pericentre passage of the S2 star would not allow for strict constraints to be put on the accretion flow around Sgr A* due to the degeneracy caused by the dependence on multiple parameters (of the accretion flow model as well as the stellar wind).

scholarly journals General relativistic magnetohydrodynamical κ-jet models for Sagittarius A*

2018 ◽  
Vol 612 ◽  
pp. A34 ◽  
Author(s):  
J. Davelaar ◽  
M. Mościbrodzka ◽  
T. Bronzwaer ◽  
H. Falcke
Keyword(s):  
Distribution Function ◽  
Power Law ◽  
Numerical Models ◽  
Radiative Properties ◽  
Emission Region ◽  
Accelerated Electrons ◽  
Thermal Distribution ◽  
Upper Limits ◽  
General Relativistic ◽  
Sgr A

Context. 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. Aim. In this paper, we study the radiative properties of jets produced in axisymmetric general relativistic magnetohydrodynamics (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. Methods. 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 κ-distribution function (the κ-distribution function is itself a combination of a relativistic thermal and a non-thermal power-law distribution, and thus it describes accelerated electrons). Inside the disk, we assume a thermal distribution for the electrons. In order to resolve the particle acceleration regions in the GRMHD simulations, we use a coordinate grid that is optimized for modeling jets. We calculate jet spectra and synchrotron maps by using the ray tracing code RAPTOR, and compare the synthetic observations to observations of Sgr A*. Finally, we compare numerical models of jets to semi-analytical ones. Results. We find that in the κ-jet models, the radio-emitting region size, radio flux, and spectral index in NIR/optical bands increase for decreasing values of the κ parameter, which corresponds to a larger amount of accelerated electrons. This is in agreement with analytical predictions. In our models, the size of the emission region depends roughly linearly on the observed wavelength λ, independently of the assumed distribution function. The model with κ = 3.5, ηacc = 5–10% (the percentage of electrons that are accelerated), and observing angle i = 30° fits the observed Sgr A* emission in the flaring state from the radio to the NIR/optical regimes, while κ = 3.5, ηacc < 1%, and observing angle i = 30° fit the upper limits in quiescence. At this point, our models (including the purely thermal ones) cannot reproduce the observed source sizes accurately, which is probably due to the assumption of axisymmetry in our GRMHD simulations. The κ-jet models naturally recover the observed nearly-flat radio spectrum of Sgr A* without invoking the somewhat artificial isothermal jet model that was suggested earlier. Conclusions. From our model fits we conclude that between 5% and 10% of the electrons inside the jet of Sgr A* are accelerated into a κ distribution function when Sgr A* is flaring. In quiescence, we match the NIR upper limits when this percentage is <1%.

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 How to tell an accreting boson star from a black hole

2020 ◽  
Vol 497 (1) ◽  
pp. 521-535 ◽  
Author(s):  
Hector Olivares ◽  
Ziri Younsi ◽  
Christian M Fromm ◽  
Mariafelicia De Laurentis ◽  
Oliver Porth ◽  
...  
Keyword(s):  
Black Hole ◽  
Radiative Transfer ◽  
Event Horizon ◽  
Polar Regions ◽  
Radio Observations ◽  
Accretion Flow ◽  
Supermassive Black Hole ◽  
General Relativistic ◽  
Sgr A ◽  
Magnetohydrodynamic Simulations

ABSTRACT The capability of the Event Horizon Telescope (EHT) to image the nearest supermassive black hole candidates at horizon-scale resolutions offers a novel means to study gravity in its strongest regimes and to test different models for these objects. Here, we study the observational appearance at 230 GHz of a surfaceless black hole mimicker, namely a non-rotating boson star, in a scenario consistent with the properties of the accretion flow on to Sgr A*. To this end, we perform general relativistic magnetohydrodynamic simulations followed by general relativistic radiative transfer calculations in the boson star space–time. Synthetic reconstructed images considering realistic astronomical observing conditions show that, despite qualitative similarities, the differences in the appearance of a black hole – either rotating or not – and a boson star of the type considered here are large enough to be detectable. These differences arise from dynamical effects directly related to the absence of an event horizon, in particular, the accumulation of matter in the form of a small torus or a spheroidal cloud in the interior of the boson star, and the absence of an evacuated high-magnetization funnel in the polar regions. The mechanism behind these effects is general enough to apply to other horizonless and surfaceless black hole mimickers, strengthening confidence in the ability of the EHT to identify such objects via radio observations.

scholarly journals Sgr A* near-infrared flares from reconnection events in a magnetically arrested disc

2020 ◽  
Vol 497 (4) ◽  
pp. 4999-5007 ◽  
Author(s):  
J Dexter ◽  
A Tchekhovskoy ◽  
A Jiménez-Rosales ◽  
S M Ressler ◽  
M Bauböck ◽  
...  
Keyword(s):  
Black Hole ◽  
Magnetic Flux ◽  
Linear Polarization ◽  
Near Infrared ◽  
Broad Class ◽  
Emission Region ◽  
Polarization Angle ◽  
Continuous Rotation ◽  
General Relativistic ◽  
Sgr A

ABSTRACT Large-amplitude Sgr A* near-infrared (NIR) flares result from energy injection into electrons near the black hole event horizon. Astrometry data show continuous rotation of the emission region during bright flares, and corresponding rotation of the linear polarization angle. One broad class of physical flare models invokes magnetic reconnection. Here, we show that such a scenario can arise in a general relativistic magnetohydrodynamic simulation of a magnetically arrested disc. Saturation of magnetic flux triggers eruption events, where magnetically dominated plasma is expelled from near the horizon and forms a rotating, spiral structure. Dissipation occurs via reconnection at the interface of the magnetically dominated plasma and surrounding fluid. This dissipation is associated with large increases in NIR emission in models of Sgr A*, with durations and amplitudes consistent with the observed flares. Such events occur at roughly the time-scale to re-accumulate the magnetic flux from the inner accretion disc, ≃10 h for Sgr A*. We study NIR observables from one sample event to show that the emission morphology tracks the boundary of the magnetically dominated region. As the region rotates around the black hole, the NIR centroid and linear polarization angle both undergo continuous rotation, similar to the behaviour seen in Sgr A* flares.

scholarly journals Radiatively Inefficient Accretion Flow Models of Sgr A*

2003 ◽  
Vol 324 (S1) ◽  
pp. 435-443 ◽  
Author(s):  
Eliot Quataert
Keyword(s):  
Flow Models ◽  
Accretion Flow ◽  
Sgr A

scholarly journals Constraining the Accretion Flow in Sgr A* by General Relativistic Dynamical and Polarized Radiative Modeling

2012 ◽  
Vol 8 (S290) ◽  
pp. 309-310 ◽  
Author(s):  
Roman V. Shcherbakov ◽  
Robert F. Penna ◽  
Jonathan C. McKinney
Keyword(s):  
Flow Model ◽  
Accretion Rate ◽  
Three Dimensional ◽  
Hydrodynamic Simulations ◽  
Accretion Flow ◽  
Dynamical Flow ◽  
General Relativistic ◽  
The Mean ◽  
Sgr A ◽  
Radiative Modeling

AbstractWe briefly summarize the method of simulating Sgr A* polarized sub-mm spectra from the accretion flow and fitting the observed spectrum. The dynamical flow model is based on three-dimensional general relativistic magneto hydrodynamic simulations. Fully self-consistent radiative transfer of polarized cyclo-synchrotron emission is performed. We compile a mean sub-mm spectrum of Sgr A* and fit it with the mean simulated spectra. We estimate the ranges of inclination angle θ=42°–75°, mass accretion rate Ṁ=(1.4-7.0)×10−8M⊙year−1, and electron temperature Te=(3–4)×1010K at 6M. We discuss multiple caveats in dynamical modeling, which must be resolved to make further progress.

scholarly journals The flux distribution of Sgr A*

2020 ◽  
Vol 638 ◽  
pp. A2 ◽  
Author(s):  
◽  
R. Abuter ◽  
A. Amorim ◽  
M. Bauböck ◽  
J. B. Berger ◽  
...  
Keyword(s):  
Flux Density ◽  
Power Law ◽  
Lognormal Distribution ◽  
Flux Distribution ◽  
Spectral Energy Distribution ◽  
Model Fitting ◽  
Light Curves ◽  
Spectral Energy ◽  
Noise Characteristics ◽  
Sgr A

The Galactic center black hole Sagittarius A* is a variable near-infrared (NIR) source that exhibits bright flux excursions called flares. When flux from Sgr A* is detected, the light curve has been shown to exhibit red noise characteristics and the distribution of flux densities is non-linear, non-Gaussian, and skewed to higher flux densities. However, the low-flux density turnover of the flux distribution is below the sensitivity of current single-aperture telescopes. For this reason, the median NIR flux has only been inferred indirectly from model fitting, but it has not been directly measured. In order to explore the lowest flux ranges, to measure the median flux density, and to test if the previously proposed flux distributions fit the data, we use the unprecedented resolution of the GRAVITY instrument at the VLTI. We obtain light curves using interferometric model fitting and coherent flux measurements. Our light curves are unconfused, overcoming the confusion limit of previous photometric studies. We analyze the light curves using standard statistical methods and obtain the flux distribution. We find that the flux distribution of Sgr A* turns over at a median flux density of (1.1 ± 0.3) mJy. We measure the percentiles of the flux distribution and use them to constrain the NIR K-band spectral energy distribution. Furthermore, we find that the flux distribution is intrinsically right-skewed to higher flux density in log space. Flux densities below 0.1 mJy are hardly ever observed. In consequence, a single powerlaw or lognormal distribution does not suffice to describe the observed flux distribution in its entirety. However, if one takes into account a power law component at high flux densities, a lognormal distribution can describe the lower end of the observed flux distribution. We confirm the rms–flux relation for Sgr A* and find it to be linear for all flux densities in our observation. We conclude that Sgr A* has two states: the bulk of the emission is generated in a lognormal process with a well-defined median flux density and this quiescent emission is supplemented by sporadic flares that create the observed power law extension of the flux distribution.

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