The Effects of Accretion Flow Dynamics on the Black Hole Shadow of Sagittarius A*

Pu, Hung-Yi; Akiyama, Kazunori; Asada, Keiichi

doi:10.3847/0004-637x/831/1/4

Cited by 38 publications

(39 citation statements)

References 53 publications

(81 reference statements)

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“…We adopted three representative images at 1.3 mm (230 GHz) for flow models with dynamics characterized by (i) a Keplerian shell that is rigidly rotating outside the innermost stable circular orbit (ISCO) and in-falling with a constant angular momentum inside the ISCO (henceforth Keplerian Model), (ii) Sub-Keplerian motion (henceforth Sub-Keplerian Model), and (iii) free-falling motion with zero angular momentum at infinity (henceforth Freefall Model). All three images, shown in Figure 1, are broadly consistent with visibility amplitude measurements of prior EHT observations in 2009 and/or 2013 (see Pu et al 2016, for details).…”

Section: Imaging Methods and The Total Square Variationsupporting

confidence: 83%

Superresolution Interferometric Imaging with Sparse Modeling Using Total Squared Variation: Application to Imaging the Black Hole Shadow

Kuramochi

Akiyama

Ikeda

et al. 2018

ApJ

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We propose a new superresolution imaging technique for interferometry using sparse modeling, utilizing two regularization terms: the 1 -norm and a new function named Total Squared Variation (TSV) of the brightness distribution. TSV is an edge-smoothing variant of Total Variation (TV), leading to reducing the sum of squared gradients. First, we demonstrate that our technique may achieve super-resolution of ∼ 30% compared to the traditional CLEAN beam size using synthetic observations of two point sources. Second, we present simulated observations of three physically motivated static models of Sgr A* with the Event Horizon Telescope (EHT) to show the performance of proposed techniques in greater detail. We find that 1 +TSV regularization outperforms 1 +TV regularization with the popular isotropic TV term and the Cotton-Schwab CLEAN algorithm, demonstrating that TSV is well-matched to the expected physical properties of the astronomical images, which are often nebulous. Remarkably, in both the image and gradient domains, the optimal beam size minimizing root-mean-squared errors is 10 % of the traditional CLEAN beam size for 1 +TSV regularization, and non-convolved reconstructed images have smaller errors than beam-convolved reconstructed images. This indicates that the traditional post-processing technique of Gaussian convolution in interferometric imaging may not be required for the 1 +TSV regularization. We also propose a feature extraction method to detect circular features from the image of a black hole shadow with the circle Hough transform (CHT) and use it to evaluate the performance of the image reconstruction. With our imaging technique and the CHT, the EHT can constrain the radius of the black hole shadow with an accuracy of ∼ 10 − 20 % in present simulations for Sgr A*, suggesting that the EHT would be able to provide useful independent measurements of the mass of the supermassive black holes in Sgr A* and also another primary target, M87.

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Section: Imaging Methods and The Total Square Variationsupporting

confidence: 83%

Superresolution Interferometric Imaging with Sparse Modeling Using Total Squared Variation: Application to Imaging the Black Hole Shadow

Kuramochi

Akiyama

Ikeda

et al. 2018

ApJ

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“…where α, κ ∈ [0, 1], are two free-parameters that control the rate of free-fall and the sub-Keplerian motion respectively. Note that our definition of α differs from that of Pu et al (2016). For the Keplerian component, outside the ISCO, u r K = 0.…”

Section: Density Profile Evolutionmentioning

confidence: 99%

Spacetime Tomography Using the Event Horizon Telescope

Broderick

Gold

et al. 2020

ApJ

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We have now entered the new era of high-resolution imaging astronomy with the beginning of the Event Horizon Telescope (EHT). The EHT can resolve the dynamics of matter in the immediate vicinity around black holes at and below the horizon scale. One of the candidate black holes, Sagittarius A*, flares 1-4 times a day depending on the wavelength. A possible interpretation of these flares could be hotspots generated through magnetic reconnection events in the accretion flow. In this paper, we construct a semi-analytical model for hotspots that include the effects of shearing as a spot moves along the accretion flow. We then explore the ability of the EHT to recover these hotspots. Even including significant systematic uncertainties, such as thermal noise, diffractive scattering, and background emission due to an accretion disk, we were able to recover the hotspots and spacetime structure to sub-percent precision. Moreover, by observing multiple flaring events we show how the EHT could be used to tomographically map spacetime. This provides new avenues for testing relativistic fluid dynamics and general relativity near the event horizon of supermassive black holes.

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“…ODYSSEY Written in CUDA (Compute Unified Device Architecture) C/C++, ODYSSEY 112 (Pu et al 2016b) is a GPU-based, public code for radiative transfer in curved spacetime, based on the ray-tracing algorithm in Fuerst & Wu (2004) and the radiative transfer formula in Younsi et al (2012). ODYSSEY performs radiative transfer for unpolarized thermal and nonthermal synchrotron emission (Pu et al 2016a). An extension of ODYSSEY with a polarized GRRT scheme for thermal synchrotron emission is described in Pu & Broderick (2018).…”

Section: Ipolementioning

confidence: 99%

Verification of Radiative Transfer Schemes for the EHT

Gold¹,

Broderick²,

Younsi³

et al. 2020

ApJ

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The Event Horizon Telescope (EHT) Collaboration has recently produced the first resolved images of the central supermassive black hole in the giant elliptical galaxy M87. Here we report on tests of the consistency and accuracy of the general relativistic radiative transfer codes used within the collaboration to model M87 * and Sgr A * . We compare and evaluate (1) deflection angles for equatorial null geodesics in a Kerr spacetime; (2) images calculated from a series of simple, parameterized matter distributions in the Kerr metric using simplified emissivities and absorptivities;(3) for a subset of codes, images calculated from general relativistic magnetohydrodynamics simulations using different realistic synchrotron emissivities and absorptivities; (4) observables for the 2017 configuration of EHT, including visibility amplitudes and closure phases. The error in total flux is of order 1% when the codes are run with production numerical parameters. The dominant source of discrepancies for small camera distances is the location and detailed setup of the software "camera" that each code uses to produce synthetic images. We find that when numerical parameters are suitably chosen and the camera is sufficiently far away the images converge and that for given transfer coefficients, numerical uncertainties are unlikely to limit parameter estimation for the current generation of EHT observations. The purpose of this paper is to describe a verification and comparison of EHT radiative transfer codes. It is not to verify EHT models more generally.

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The Effects of Accretion Flow Dynamics on the Black Hole Shadow of Sagittarius A*

Cited by 38 publications

References 53 publications

Superresolution Interferometric Imaging with Sparse Modeling Using Total Squared Variation: Application to Imaging the Black Hole Shadow

Superresolution Interferometric Imaging with Sparse Modeling Using Total Squared Variation: Application to Imaging the Black Hole Shadow

Spacetime Tomography Using the Event Horizon Telescope

Verification of Radiative Transfer Schemes for the EHT

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