Imaging the Schwarzschild-radius-scale Structure of M87 with the Event Horizon Telescope Using Sparse Modeling

Akiyama, Kazunori; Kuramochi, Kazuki; Ikeda, Shiro; Fish, Vincent L.; Tazaki, Fumie; Honma, Mareki; Doeleman, Sheperd S.; Broderick, Avery E.; Dexter, Jason; Mościbrodzka, Monika; Bouman, Katherine L.; Chael, Andrew; Zaizen, Masamichi

doi:10.3847/1538-4357/aa6305

Cited by 144 publications

(136 citation statements)

References 63 publications

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“…A natural extension of our previous work (Honma et al 2014;Akiyama et al 2016;Ikeda et al 2016) to full-polarimetric imaging is given by…”

Section: The Proposed Methodsmentioning

confidence: 99%

“…which is less affected by the number of visibilities N and the total flux density of the target source, which should be close to the maximum value of the visibility amplitudes at Stokes I (i.e., I max i i |˜|), following Akiyama et al (2016). The other additional term is TV regularization with a regularization parameter t L .…”

Section: The Proposed Methodsmentioning

confidence: 99%

“…This is a convex relaxation of Equation (1) and can be transformed into the Lagrange form: (Honma et al 2014;Akiyama et al 2016;Ikeda et al 2016) and found that LASSO can potentially reconstruct structure on scales ∼4 times finer than D max l (Honma et al 2014). Techniques of compressed sensing are beginning to be used in other fields of radio interferometry 15 after pioneering work by Wiaux et al (2009a) and Wiaux et al (2009b) (see Garsden et al 2015, and references therein).…”

Section: Introductionmentioning

confidence: 99%

“…We have taken another approach by adding total variation (TV) regularization (e.g., Wiaux et al 2010;McEwen & Wiaux 2011;Uemura et al 2015;Chael et al 2016), which produces an image that is sparse in its gradient domain. TV regularization has been shown to be effective for imaging with visibility amplitudes and closure phases (e.g., Akiyama et al 2016) in the superresolution regime finer than the diffraction limit.…”

Section: Introductionmentioning

confidence: 99%

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Superresolution Full-polarimetric Imaging for Radio Interferometry with Sparse Modeling

Akiyama

Ikeda

Pleau

et al. 2017

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We propose a new technique for radio interferometry to obtain superresolution full-polarization images in all four Stokes parameters using sparse modeling. The proposed technique reconstructs the image in each Stokes parameter from the corresponding full-complex Stokes visibilities by utilizing two regularization functions: the ℓ 1 norm and the total variation (TV) of the brightness distribution. As an application of this technique, we present simulated linear polarization observations of two physically motivated models of M87 with the Event Horizon Telescope. We confirm that ℓ 1 +TV regularization can achieve an optimal resolution of ∼25%-30% of the diffraction limit D max l , which is the nominal spatial resolution of a radio interferometer for both the total intensity (i.e., Stokes I) and linear polarizations (i.e., Stokes Q and U). This optimal resolution is better than that obtained from the widely used Cotton-Schwab CLEAN algorithm or from using ℓ 1 or TV regularizations alone. Furthermore, we find that ℓ 1 +TV regularization can achieve much better image fidelity in linear polarization than other techniques over a wide range of spatial scales, not only in the superresolution regime, but also on scales larger than the diffraction limit. Our results clearly demonstrate that sparse reconstruction is a useful choice for high-fidelity full-polarimetric interferometric imaging.

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“…A natural extension of our previous work (Honma et al 2014;Akiyama et al 2016;Ikeda et al 2016) to full-polarimetric imaging is given by…”

Section: The Proposed Methodsmentioning

confidence: 99%

Section: The Proposed Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Superresolution Full-polarimetric Imaging for Radio Interferometry with Sparse Modeling

Akiyama

Ikeda

Pleau

et al. 2017

Self Cite

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“…Given its proximity, M87 has been a prime target to probe scenarios for the formation of relativistic jets with high-resolution radio observations exploring scales down to some tens of r g and much effort has been recently dedicated into this direction (e.g., Acciari et al 2009;Doeleman et al 2012;Hada et al 2014Hada et al , 2016Kino et al 2015;Akiyama et al 2015Akiyama et al , 2017. At VHE energies, M87 has revealed at least three active γ-ray episodes during which day-scale flux variability (i.e., h = c∆τ obs ∼ r g ) has been observed (Aharonian et al 2006;Albert et al 2008;Acciari et al 2009;Abramowski et al 2012;Aliu et al 2012).…”

Section: M87mentioning

confidence: 99%

Magnetospheric Gamma-Ray Emission in Active Galactic Nuclei

Katsoulakos

Rieger

2018

ApJ

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The rapidly variable, very high-energy (VHE) gamma-ray emission from Active Galactic Nuclei (AGN) has been frequently associated with non-thermal processes occurring in the magnetospheres of their supermassive black holes. The present work aims to explore the adequacy of different gap-type (unscreened electric field) models to account for the observed characteristics. Based on a phenomenological description of the gap potential, we estimate the maximum extractable gap power L gap for different magnetospheric set-ups, and study its dependence on the accretion state of the source. L gap is found to be in general proportional to the Blandford-Znajek jet power L BZ and a sensitive function of gap sizeβ , where the power index β ≥ 1 is dependent on the respective gapsetup. The transparency of the black hole vicinity to VHE photons generally requires a radiatively inefficient accretion environment and thereby imposes constraints on possible accretion rates, and correspondingly on L BZ . Similarly, rapid variability, if observed, may allow to constrain the gap size h ∼ c∆t. Combining these constraints, we provide a general classification to assess the likelihood that the VHE gamma-ray emission observed from an AGN can be attributed to a magnetospheric origin. When applied to prominent candidate sources these considerations suggest that the variable (day-scale) VHE activity seen in the radio galaxy M87 could be compatible with a magnetospheric origin, while such an origin appears less likely for the (minute-scale) VHE activity in IC310.

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Imaging VGOS observations and investigating source structure effects

Mei-ling

Savolainen

Zubko

et al. 2020

Preprint

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Geodetic very long baseline interferometry (VLBI) is a space-geodetic technique that has regularly made global astrometric/geodetic observations since 1979, which are the basis for creating the International Celestial Reference Frame (ICRF2; Fey et al., 2015) and obtaining a full set of Earth Orientation Parameters. Together with other three space-geodetic techniques, that is, Global Navigation Satellite Systems (GNSS), Satellite Laser Ranging (SLR), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DO-RIS), VLBI plays an important role in establishing the International Terrestrial Reference Frame (ITRF; Altamimi et al., 2016). Requirements for the ITRF have increased dramatically since the 1980s, and the most stringent one from sea level studies is 1 mm position accuracy and 0.1 mm/yr velocity stability on global scales. The International VLBI Service for Geodesy and Astrometry (IVS; Nothnagel et al., 2017; Schuh & Behrend, 2012; please see https://ivscc.gsfc.nasa.gov/index.html) is developing the next-generation geodetic VLBI system, known as the VLBI Global Observing System (VGOS; Niell et al., 2007;Petrachenko et al., 2009) in order to achieve the goal of 1 mm accuracy. This is one order of magnitude beyond the capability of the traditional S/X VLBI system. Although the VGOS observations from the 500-km-long baseline, GGAO12M-WESTFORD, have obtained geodetic results of a weighted root-mean-square deviation of the baseline length residuals about the weighted mean of 1.6 mm (Niell et al., 2018), few studies have analyzed error sources in the VGOS observations, for instance source structure effects, to evaluate the capability of this new VLBI system. Source structure effects hardly have any influence on this short baseline but can cause significant errors in observations of longer baselines, as demonstrated by the residual delays from geodetic solutions of VGOS observations in Bolotin et al. (2019). Celestial radio sources observed by geodetic VLBI, mostly quasars, are treated as point-like sources in routine geodetic solutions, but in fact they are

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Imaging the Schwarzschild-radius-scale Structure of M87 with the Event Horizon Telescope Using Sparse Modeling

Cited by 144 publications

References 63 publications

Superresolution Full-polarimetric Imaging for Radio Interferometry with Sparse Modeling

Superresolution Full-polarimetric Imaging for Radio Interferometry with Sparse Modeling

Magnetospheric Gamma-Ray Emission in Active Galactic Nuclei

Imaging VGOS observations and investigating source structure effects

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