Dynamical Imaging with Interferometry

Johnson, Michael D.; Bouman, Katherine L.; Blackburn, L.; Chael, Andrew; Rosen, Julian; Shiokawa, Hotaka; Roelofs, Freek; Akiyama, Kazunori; Fish, Vincent L.; Doeleman, Sheperd S.

doi:10.3847/1538-4357/aa97dd

Cited by 71 publications

(108 citation statements)

References 79 publications

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“…Since the dynamics and spectrum of the diffuse medium are usually expected to be gradual and without strong edges, TSV would be a reasonable choice to regularize the brightness distribution along the third dimension. Recently, Johnson et al (2017) presented movie reconstruction techniques, including a technique using MEM regularization for twodimensional image and TSV regularization for time direction, and demonstrated that TSV can be a good regularization function in the time direction. Such an extension to higher dimensions will enable our techniques to handle intra-day time-variable sources like Sgr A* and also high-fidelity multi-epoch imaging.…”

Section: Circular Features Extracted With the Chtmentioning

confidence: 99%

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: Circular Features Extracted With the Chtmentioning

confidence: 99%

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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“…The most recent application of closure measurements as a robust sky measurement independent of antenna‐based calibration involves interferometric measurements of the supermassive Black Hole at the Galactic center. These include millimeter and submillimeter VLBI measurements, in which time variations of the phase contributions of the antenna electronics, and/or the troposphere, preclude accurate antenna‐based calibration (Johnson et al, ; Roelofs et al, ).…”

Section: Closure Phase Reviewmentioning

confidence: 99%

“…As is well known, calculation of closure quantities from interferometric visibilities is independent of phase and amplitude terms introduced by antenna optics and electronics (e.g., standing waves), as well as to electrical path length and attenuation contributions due to the ionosphere and troposphere above the antenna. Hence, closure quantities represent a robust measurement of the sky signal, independent of antenna‐based calibration and calibration errors (Johnson et al, ; Roelofs et al, ).…”

Section: Introductionmentioning

confidence: 99%

H I 21‐cm Cosmology and the Bispectrum: Closure Diagnostics in Massively Redundant Interferometric Arrays

Carilli¹,

Nikolic²,

Thyagarajan³

et al. 2018

Radio Science

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New, massively redundant low‐frequency arrays allow for a novel investigation of closure relations in interferometry. We employ commissioning data from the Hydrogen Epoch of Reionization Array to investigate closure quantities in this densely packed grid array of 14‐m antennas operating at 100 to 200 MHz. We investigate techniques that utilize closure phase spectra for redundant triads to estimate departures from redundancy for redundant baseline visibilities. We find a median absolute deviation from redundancy in closure phase across the observed frequency range of about 4.5°. This value translates into a nonredundancy per visibility phase of about 2.6°, using prototype electronics. The median absolute deviations from redundancy decrease with longer baselines. We show that closure phase spectra can be used to identify ill‐behaved antennas in the array, independent of calibration. We investigate the temporal behavior of closure spectra. The Allan variance increases after a 1‐min stride time, due to passage of the sky through the primary beam of the transit telescope. However, the closure spectra repeat to well within the noise per measurement at corresponding local sidereal times from day to day. In future papers in this series we will develop the technique of using closure phase spectra in the search for the H I 21‐cm signal from cosmic reionization.

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“…The combination of the EHT array and VLBI imaging algorithms designed to address the EHT's particular challenges is expected to be capable of reconstructing static images of Sgr A* at this resolution, and has done so for M87 (see, e.g., Honma et al 2014;Bouman et al 2016;Chael et al 2016;Johnson et al 2017;Akiyama et al 2017a,b;Bouman et al 2018;Kuramochi et al 2018;Chael et al 2018a;Event Horizon Telescope Collaboration et al 2019b). However, imaging time-variable structure around supermassive black holes requires well-sampled spatial baseline coverage (conventionally described in the (u, v) plane) on timescales comparable to the innermost stable circular orbit (or ISCO).…”

Section: Introductionmentioning

confidence: 99%

Metrics and Motivations for Earth–Space VLBI: Time-resolving Sgr A* with the Event Horizon Telescope

Palumbo

Doeleman

Johnson

et al. 2019

ApJ

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Very-long-baseline interferometry (VLBI) at frequencies above 230 GHz with Earth-diameter baselines gives spatial resolution finer than the ∼50µas "shadow" of the supermassive black hole at the Galactic Center, Sagittarius A* (Sgr A*). Imaging static and dynamical structure near the "shadow" provides a test of general relativity and may allow measurement of black hole parameters. However, traditional Earth-rotation synthesis is inapplicable for sources (such as Sgr A*) with intra-day variability. Expansions of ground-based arrays to include space-VLBI stations may enable imaging capability on time scales comparable to the prograde innermost stable circular orbit (ISCO) of Sgr A*, which is predicted to be 4-30 minutes, depending on black hole spin. We examine the basic requirements for space-VLBI, and we develop tools for simulating observations with orbiting stations. We also develop a metric to quantify the imaging capabilities of an array irrespective of detailed image morphology or reconstruction method. We validate this metric on example reconstructions of simulations of Sgr A* at 230 and 345 GHz, and use these results to motivate expanding the Event Horizon Telescope (EHT) to include small dishes in Low Earth Orbit (LEO). We demonstrate that high-sensitivity sites such as the Atacama Large Millimeter/Submillimeter Array (ALMA) make it viable to add small orbiters to existing ground arrays, as space-ALMA baselines would have sensitivity comparable to ground-based non-ALMA baselines. We show that LEO-enhanced arrays sample half of the diffraction-limited Fourier plane of Sgr A* in less than 30 minutes, enabling reconstructions of near-horizon structure with normalized root-mean-square error 0.3 on sub-ISCO timescales.

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Dynamical Imaging with Interferometry

Cited by 71 publications

References 79 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

H I 21‐cm Cosmology and the Bispectrum: Closure Diagnostics in Massively Redundant Interferometric Arrays

Metrics and Motivations for Earth–Space VLBI: Time-resolving Sgr A* with the Event Horizon Telescope

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