Decomposing the internal faraday rotation of black hole accretion flows

ABSTRACT Faraday rotation has been seen at millimeter wavelengths in several low-luminosity active galactic nuclei, including Event Horizon Telescope (EHT) targets M87* and Sgr A*. The observed rotation measure (RM) probes the density, magnetic field, and temperature of material integrated along the line of sight. To better understand how accretion disc conditions are reflected in the RM, we perform polarized radiative transfer calculations using a set of general relativistic magnetohydrodynamic (GRMHD) simulations appropriate for M87*. We find that in spatially resolved millimetre wavelength images on event horizon scales, the RM can vary by orders of magnitude and even flip sign. The observational consequences of this spatial structure include significant time-variability, sign-flips, and non-λ2 evolution of the polarization plane. For some models, we find that internal RM can cause significant bandwidth depolarization even across the relatively narrow fractional bandwidths observed by the EHT. We decompose the linearly polarized emission in these models based on their RM and find that emission in front of the mid-plane can exhibit orders of magnitude less Faraday rotation than emission originating from behind the mid-plane or within the photon ring. We confirm that the spatially unresolved (i.e. image integrated) RM is a poor predictor of the accretion rate, with substantial scatter stemming from time variability and inclination effects. Models can be constrained with repeated observations to characterize time variability and the degree of non-λ2 evolution of the polarization plane.

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

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa922 ◽

2020 ◽

Vol 494 (3) ◽

pp. 4168-4186 ◽

Cited By ~ 5

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.

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A GENERAL RELATIVISTIC NULL HYPOTHESIS TEST WITH EVENT HORIZON TELESCOPE OBSERVATIONS OF THE BLACK HOLE SHADOW IN Sgr A*

The Astrophysical Journal ◽

10.1088/0004-637x/814/2/115 ◽

2015 ◽

Vol 814 (2) ◽

pp. 115 ◽

Cited By ~ 66

Author(s):

Dimitrios Psaltis ◽

Feryal Özel ◽

Chi-Kwan Chan ◽

Daniel P. Marrone

Keyword(s):

Black Hole ◽

Event Horizon ◽

Null Hypothesis ◽

Hypothesis Test ◽

General Relativistic ◽

Sgr A

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How to tell an accreting boson star from a black hole

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa1878 ◽

2020 ◽

Vol 497 (1) ◽

pp. 521-535 ◽

Cited By ~ 1

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.

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General relativistic MHD simulations of non-thermal flaring in Sagittarius A*

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stab2466 ◽

2021 ◽

Author(s):

K Chatterjee ◽

S Markoff ◽

J Neilsen ◽

Z Younsi ◽

G Witzel ◽

...

Keyword(s):

Particle Acceleration ◽

Near Infrared ◽

Power Spectra ◽

X Rays ◽

Thermal Electron ◽

X Ray ◽

Particle Distributions ◽

Accretion Flows ◽

General Relativistic ◽

Sgr A

Abstract Sgr A* exhibits regular variability in its multiwavelength emission, including daily X-ray flares and roughly continuous near-infrared (NIR) flickering. The origin of this variability is still ambiguous since both inverse Compton and synchrotron emission are possible radiative mechanisms. The underlying particle distributions are also not well constrained, particularly the non-thermal contribution. In this work, we employ the GPU-accelerated general relativistic magnetohydrodynamics (GRMHD) code H-AMR to perform a study of flare flux distributions, including the effect of particle acceleration for the first time in high-resolution 3D simulations of Sgr A*. For the particle acceleration, we use the general relativistic ray-tracing (GRRT) code BHOSS to perform the radiative transfer, assuming a hybrid thermal+non-thermal electron energy distribution. We extract ∼60 hr lightcurves in the sub-millimetre, NIR and X-ray wavebands, and compare the power spectra and the cumulative flux distributions of the lightcurves to statistical descriptions for Sgr A* flares. Our results indicate that non-thermal populations of electrons arising from turbulence-driven reconnection in weakly magnetised accretion flows lead to moderate NIR and X-ray flares and reasonably describe the X-ray flux distribution while fulfilling multiwavelength flux constraints. These models exhibit high rms per cent amplitudes, $\gtrsim 150{{\ \rm per\ cent}}$ both in the NIR and the X-rays, with changes in the accretion rate driving the 230 GHz flux variability, in agreement with Sgr A* observations.

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CONFRONTING THE JET MODEL OF Sgr A* WITH THE FARADAY ROTATION MEASURE OBSERVATIONS

The Astrophysical Journal ◽

10.1088/0004-637x/798/1/22 ◽

2014 ◽

Vol 798 (1) ◽

pp. 22 ◽

Cited By ~ 7

Author(s):

Ya-Ping Li ◽

Feng Yuan ◽

Q. Daniel Wang

Keyword(s):

Faraday Rotation ◽

Rotation Measure ◽

Sgr A

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Strategy to Explore Magnetized Cosmic Web with Forthcoming Large Surveys of Rotation Measure

Galaxies ◽

10.3390/galaxies6040118 ◽

2018 ◽

Vol 6 (4) ◽

pp. 118 ◽

Cited By ~ 2

Author(s):

Takuya Akahori

Keyword(s):

Magnetic Field ◽

Faraday Rotation ◽

Line Of Sight ◽

Wide Field ◽

Sky Surveys ◽

Rotation Measure ◽

Polarized Emission ◽

Linearly Polarized ◽

Intergalactic Magnetic Field ◽

The Universe

The warm-hot intergalactic medium (WHIM) is a candidate for the missing baryons in the Universe. If the WHIM is permeated with the intergalactic magnetic field (IGMF), the Faraday rotation measure (RM) of the WHIM is imprinted in linearly-polarized emission from extragalactic objects. In this article, we discuss strategies to explore the WHIM’s RM from forthcoming radio broadband and wide-field polarization sky surveys. There will be two observational breakthroughs in the coming decades; the RM grid and Faraday tomography. They will allow us to find ideal RM sources for the study of the IGMF and give us unique information of the WHIM along the line of sight.

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Status of GRMHD simulations and radiative models of Sgr A*

Proceedings of the International Astronomical Union ◽

10.1017/s174392131601259x ◽

2016 ◽

Vol 11 (S322) ◽

pp. 43-49

Author(s):

Monika Mościbrodzka

Keyword(s):

Black Hole ◽

Galactic Center ◽

Kerr Black Hole ◽

Theoretical Models ◽

Radiative Properties ◽

X Ray ◽

Accretion Flows ◽

General Relativistic ◽

Sgr A ◽

Magnetohydrodynamic Simulations

AbstractThe Galactic center is a perfect laboratory for testing various theoretical models of accretion flows onto a supermassive black hole. Here, I review general relativistic magnetohydrodynamic simulations that were used to model emission from the central object - Sgr A*. These models predict dynamical and radiative properties of hot, magnetized, thick accretion disks with jets around a Kerr black hole. Models are compared to radio-VLBI, mm-VLBI, NIR, and X-ray observations of Sgr A*. I present the recent constrains on the free parameters of the model such as accretion rate onto the black hole, the black hole angular momentum, and orientation of the system with respect to our line of sight.

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Magnetic field at a jet base: extreme Faraday rotation in 3C 273 revealed by ALMA

Astronomy and Astrophysics ◽

10.1051/0004-6361/201832587 ◽

2019 ◽

Vol 623 ◽

pp. A111 ◽

Cited By ~ 6

Author(s):

T. Hovatta ◽

S. O’Sullivan ◽

I. Martí-Vidal ◽

T. Savolainen ◽

A. Tchekhovskoy

Keyword(s):

Magnetic Field ◽

Faraday Rotation ◽

Position Angle ◽

High Angular Resolution ◽

Systematic Uncertainties ◽

Rotation Measure ◽

Linearly Polarized ◽

Polarization Fraction ◽

The Magnetic Field ◽

Quasar 3C 273

Aims. We studied the polarization behavior of the quasar 3C 273 over the 1 mm wavelength band at ALMA with a total bandwidth of 7.5 GHz across 223–243 GHz at 0.8′′ resolution, corresponding to 2.1 kpc at the distance of 3C 273. With these observations we were able to probe the optically thin polarized emission close to the jet base, and constrain the magnetic field structure. Methods. We computed the Faraday rotation measure using simple linear fitting and Faraday rotation measure synthesis. In addition, we modeled the broadband behavior of the fractional Stokes Q and U parameters (qu-fitting). The systematic uncertainties in the polarization observations at ALMA were assessed through Monte Carlo simulations. Results. We find the unresolved core of 3C 273 to be 1.8% linearly polarized. We detect a very high rotation measure (RM) of (5.0 ± 0.3) × 105 rad m−2 over the 1 mm band when assuming a single polarized component and an external RM screen. This results in a rotation of >40° of the intrinsic electric vector position angle, which is significantly higher than typically assumed for millimeter wavelengths. The polarization fraction increases as a function of wavelength, which according to our qu-fitting could be due to multiple polarized components of different Faraday depth within our beam or to internal Faraday rotation. With our limited wavelength coverage we cannot distinguish between the cases, and additional multifrequency and high angular resolution observations are needed to determine the location and structure of the magnetic field of the Faraday active region. Comparing our RM estimate with values obtained at lower frequencies, the RM increases as a function of observing frequency, following a power law with an index of 2.0 ± 0.2, consistent with a sheath surrounding a conically expanding jet. We also detect ~0.2% circular polarization, although further observations are needed to confirm this result.

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