Publications

2017
F. Roelofs, M. Johnson, H. Shiokawa, S. Doeleman, and H. Falcke. 2017. “Quantifying Intrinsic Variability of Sagittarius A* Using Closure Phase Measurements of the Event Horizon Telescope.” ApJ, 847, 55.Abstract
General relativistic magnetohydrodynamic (GRMHD) simulations of accretion disks and jets associated with supermassive black holes show variability on a wide range of timescales. On timescales comparable to or longer than the gravitational timescale tG=GM/c3, variation may be dominated by orbital dynamics of the inhomogeneous accretion flow. Turbulent evolution within the accretion disk is expected on timescales comparable to the orbital period, typically an order of magnitude larger than tG. For Sgr A*, tG is much shorter than the typical duration of a VLBI experiment, enabling us to study this variability within a single observation. Closure phases, the sum of interferometric visibility phases on a triangle of baselines, are particularly useful for studying this variability. In addition to a changing source structure, variations in observed closure phase can also be due to interstellar scattering, thermal noise, and the changing geometry of projected baselines over time due to Earth rotation. We present a metric that is able to distinguish the latter two from intrinsic or scattering variability. This metric is validated using synthetic observations of GRMHD simulations of Sgr A*. When applied to existing multi-epoch EHT data of Sgr A*, this metric shows that the data are most consistent with source models containing intrinsic variability from source dynamics, interstellar scattering, or a combination of those. The effects of black hole inclination, orientation, spin, and morphology (disk or jet) on the expected closure phase variability are also discussed.
Sheperd Doeleman. 2017. “Seeing the Unseeable.” Nature Astronomy, 1, Pp. 646.Abstract
The Event Horizon Telescope, an Earth-sized interferometer, aims to capture an image of a black hole's event horizon to test the theory of general relativity and probe accretion processes.
K. Akiyama, S. Ikeda, M. Pleau, V. L. Fish, F. Tazaki, K. Kuramochi, A. Broderick, J. Dexter, M. Mościbrodzka, M. Gowanlock, M. Honma, and S. S. Doeleman. 2017. “Superresolution Full-polametric Imaging for Radio Interferometry with Sparse Modeling.” Astronomical Journal, 153, 159.Abstract
We propose a new technique for radio interferometry to obtain super-resolution 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 ℓ1-norm and 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 (EHT). We confirm that ℓ1+TV regularization can achieve an optimal resolution of ∼25−30\% of the diffraction limit λ/Dmax, 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 ℓ1 or TV regularizations alone. Furthermore, we find that ℓ1+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 super-resolution 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.
H. Shiokawa, C. Gammie, and S. Doeleman. 2017. “Time Domain Filtering of Resolved Images of SgrA*.” ApJ, 846, 29.Abstract
The goal of the Event Horizon Telescope (EHT) is to provide spatially resolved images of Sgr A*, the source associated with the Galactic Center black hole. Because Sgr A* varies on timescales short compared to an EHT observing campaign, it is interesting to ask whether variability contains information about the structure and dynamics of the accretion flow. In this paper, we introduce "time-domain filtering", a technique to filter time fluctuating images with specific temporal frequency ranges, and demonstrate the power and usage of the technique by applying it to mock millimeter wavelength images of Sgr A*. The mock image data is generated from General Relativistic Magnetohydrodynamic (GRMHD) simulation and general relativistic ray-tracing method. We show that the variability on each line of sight is tightly correlated with a typical radius of emission. This is because disk emissivity fluctuates on a timescale of order the local orbital period. Time-domain filtered images therefore reflect the model dependent emission radius distribution, which is not accessible in time-averaged images. We show that, in principle, filtered data have the power to distinguish between models with different black hole spins, different disk viewing angles, and different disk orientations in the sky.
2016
Christiaan D. Brinkerink, Cornelia Müller, Heino Falcke, Geoffrey C. Bower, Thomas P. Krichbaum, Edgar Castillo, Adam T. Deller, Sheperd S. Doeleman, Raquel Fraga-Encinas, Ciriaco Goddi, Antonio Hernández-Gómez, David H. Hughes, Michael Kramer, Jonathan Léon-Tavares, Laurent Loinard, Alfredo Montaña, Monika Mościbrodzka, Gisela N. Ortiz-León, David Sanchez-Arguelles, Remo P. J. Tilanus, Grant W. Wilson, and J. Anton Zensus. 2016. “Asymmetric structure in Sgr A* at 3 mm from closure phase measurements with VLBA, GBT and LMT.” Monthly Notices of the Royal Astronomical Society, 462, Pp. 1382-1392. Publisher's VersionAbstract
We present the results of a closure phase analysis of 3 mm very longbaseline interferometry measurements performed on Sagittarius A* (SgrA*). We have analysed observations made in 2015 May using the Very LongBaseline Array, the Robert C. Byrd Green Bank Telescope and the LargeMillimeter Telescope Alfonso Serrano and obtained non-zero closure phasemeasurements on several station triangles - indicative of anon-point-symmetric source structure. The data are fitted with anasymmetric source structure model in Sgr A*, represented by a simpletwo-component model, which favours a fainter component due east of themain source. This result is discussed in light of a scattering screenwith substructure or an intrinsically asymmetric source.
Geoffrey C. Bower, Adam Deller, Paul Demorest, Andreas Brunthaler, Heino Falcke, Monika Moscibrodzka, Ryan M. O’Leary, Ralph P. Eatough, Michael Kramer, K. J. Lee, Laura Spitler, Gregory Desvignes, Anthony P. Rushton, Sheperd Doeleman, and Mark J. Reid. 2016. “Erratum: “The Proper Motion of the Galactic Center Pulsar Relative to Sagittarius A*” (2015, ApJ, 798, 120).” The Astrophysical Journal, 821. Publisher's VersionAbstract
Not Available
A. A. Chael, M. D. Johnson, R. Narayan, S. S. Doeleman, J. F. C. Wardle, and K. L. Bouman. 2016. “High-resolution Linear Polarimetric Imaging for the Event Horizon Telescope.” \apj, 829, Pp. 11.
Andrew A. Chael, Michael D. Johnson, Ramesh Narayan, Sheperd S. Doeleman, John F. C. Wardle, and Katherine L. Bouman. 2016. “High-resolution Linear Polarimetric Imaging for the Event Horizon Telescope.” The Astrophysical Journal, 829. Publisher's VersionAbstract
Images of the linear polarizations of synchrotron radiation aroundactive galactic nuclei (AGNs) highlight their projected magnetic fieldlines and provide key data for understanding the physics of accretionand outflow from supermassive black holes. The highest-resolutionpolarimetric images of AGNs are produced with Very Long BaselineInterferometry (VLBI). Because VLBI incompletely samples the Fouriertransform of the source image, any image reconstruction that fills inunmeasured spatial frequencies will not be unique and reconstructionalgorithms are required. In this paper, we explore some extensions ofthe Maximum Entropy Method (MEM) to linear polarimetric VLBI imaging. Incontrast to previous work, our polarimetric MEM algorithm combines aStokes I imager that only uses bispectrum measurements that are immuneto atmospheric phase corruption, with a joint Stokes Q and U imager thatoperates on robust polarimetric ratios. We demonstrate the effectivenessof our technique on 7 and 3 mm wavelength quasar observations from theVLBA and simulated 1.3 mm Event Horizon Telescope observations of Sgr A*and M87. Consistent with past studies, we find that polarimetric MEM canproduce superior resolution compared to the standard CLEAN algorithm,when imaging smooth and compact source distributions. As an imagingframework, MEM is highly adaptable, allowing a range of constraints onpolarization structure. Polarimetric MEM is thus an attractive choicefor image reconstruction with the EHT.
Ru-Sen Lu, Freek Roelofs, Vincent L. Fish, Hotaka Shiokawa, Sheperd S. Doeleman, Charles F. Gammie, Heino Falcke, Thomas P. Krichbaum, and J. Anton Zensus. 2016. “Imaging an Event Horizon: Mitigation of Source Variability of Sagittarius A*.” The Astrophysical Journal, 817. Publisher's VersionAbstract
The black hole in the center of the Galaxy, associated with the compactsource Sagittarius A* (Sgr A*), is predicted to cast a shadow upon theemission of the surrounding plasma flow, which encodes the influence ofgeneral relativity (GR) in the strong-field regime. The Event HorizonTelescope (EHT) is a Very Long Baseline Interferometry (VLBI) networkwith a goal of imaging nearby supermassive black holes (in particularSgr A* and M87) with angular resolution sufficient to observe stronggravity effects near the event horizon. General relativisticmagnetohydrodynamic (GRMHD) simulations show that radio emission fromSgr A* exhibits variability on timescales of minutes, much shorter thanthe duration of a typical VLBI imaging experiment, which usually takesseveral hours. A changing source structure during the observations,however, violates one of the basic assumptions needed for aperturesynthesis in radio interferometry imaging to work. By simulatingrealistic EHT observations of a model movie of Sgr A*, we demonstratethat an image of the average quiescent emission, featuring thecharacteristic black hole shadow and photon ring predicted by GR, cannonetheless be obtained by observing over multiple days and subsequentprocessing of the visibilities (scaling, averaging, and smoothing)before imaging. Moreover, it is shown that this procedure can becombined with an existing method to mitigate the effects of interstellarscattering. Taken together, these techniques allow the black hole shadowin the Galactic center to be recovered on the reconstructed image.
Gisela N. Ortiz-León, Michael D. Johnson, Sheperd S. Doeleman, Lindy Blackburn, Vincent L. Fish, Laurent Loinard, Mark J. Reid, Edgar Castillo, Andrew A. Chael, Antonio Hernández-Gómez, David H. Hughes, Jonathan León-Tavares, Ru-Sen Lu, Alfredo Montaña, Gopal Narayanan, Katherine Rosenfeld, David Sánchez, F. Peter Schloerb, Zhi-qiang Shen, Hotaka Shiokawa, Jason SooHoo, and Laura Vertatschitsch. 2016. “The Intrinsic Shape of Sagittarius A* at 3.5 mm Wavelength.” The Astrophysical Journal, 824. Publisher's VersionAbstract
The radio emission from Sgr A{}\ast is thought to be poweredby accretion onto a supermassive black hole of ˜ 4×{10}6 {M}ȯ at the Galactic Center. Atmillimeter wavelengths, Very Long Baseline Interferometry (VLBI)observations can directly resolve the bright innermost accretion regionof Sgr A{}\ast . Motivated by the addition of many sensitivelong baselines in the north–south direction, we developed a fullVLBI capability at the Large Millimeter Telescope Alfonso Serrano (LMT).We successfully detected Sgr A{}\ast at 3.5 mm with an arrayconsisting of six Very Long Baseline Array telescopes and the LMT. Wemodel the source as an elliptical Gaussian brightness distribution andestimate the scattered size and orientation of the source from closureamplitude and self-calibration analysis, obtaining consistent resultsbetween methods and epochs. We then use the known scattering kernel todetermine the intrinsic two-dimensional source size at 3.5 mm: (147+/- 7μ {{as}})× (120+/- 12 μ {{as}}), at position angle 88^\circ+/- 7^\circ east of north. Finally, we detect non-zero closure phaseson some baseline triangles, but we show that these are consistent withbeing introduced by refractive scattering in the interstellar medium anddo not require intrinsic source asymmetry to explain.
Avery E. Broderick, Vincent L. Fish, Michael D. Johnson, Katherine Rosenfeld, Carlos Wang, Sheperd S. Doeleman, Kazunori Akiyama, Tim Johannsen, and Alan L. Roy. 2016. “Modeling Seven Years of Event Horizon Telescope Observations with Radiatively Inefficient Accretion Flow Models.” The Astrophysical Journal, 820. Publisher's VersionAbstract
An initial three-station version of the Event Horizon Telescope, amillimeter-wavelength very-long baseline interferometer, has observedSagittarius A* (Sgr A*) repeatedly from 2007 to 2013, resulting in themeasurement of a variety of interferometric quantities. Of particularimportance is that there is now a large set of closure phases measuredover a number of independent observing epochs. We analyze theseobservations within the context of a realization of semi-analyticradiatively inefficient disk models, implicated by the low luminosity ofSgr A*. We find a broad consistency among the various observing epochsand between different interferometric data types, with the latterproviding significant support for this class of model of Sgr A*. The newdata significantly tighten existing constraints on the spin magnitudeand its orientation within this model context, finding a spin magnitudeof a={0.10}-0.10-0.10+0.30+0.56, an inclinationwith respect to the line of sight of θ ={60^\circ}-{8^\circ -{13}^\circ}+{5^\circ +{10}^\circ }, and aposition angle of ξ ={156^\circ }-{17^\circ-{27}^\circ }+{10^\circ+{14}^\circ } east of north. These are in goodagreement with previous analyses. Notably, the previous 180°degeneracy in the position angle has now been conclusively broken by theinclusion of the closure-phase measurements. A reflection degeneracy inthe inclination remains, permitting two localizations of the spin vectororientation, one of which is in agreement with the orbital angularmomentum of the infrared gas cloud G2 and the clockwise disk of youngstars. This may support a relationship between Sgr A*'s accretion flowand these larger-scale features.
V. Fish, K. Akiyama, K. Bouman, A. Chael, M. Johnson, S. Doeleman, L. Blackburn, J. Wardle, and W. Freeman. 2016. “Observing\mdashand Imaging\mdashActive Galactic Nuclei with the Event Horizon Telescope.” Galaxies, 4, Pp. 54.
Vincent L. Fish, Michael D. Johnson, Sheperd S. Doeleman, Avery E. Broderick, Dimitrios Psaltis, Ru-Sen Lu, Kazunori Akiyama, Walter Alef, Juan Carlos Algaba, Keiichi Asada, Christopher Beaudoin, Alessandra Bertarini, Lindy Blackburn, Ray Blundell, Geoffrey C. Bower, Christiaan Brinkerink, Roger Cappallo, Andrew A. Chael, Richard Chamberlin, Chi-Kwan Chan, Geoffrey B. Crew, Jason Dexter, Matt Dexter, Sergio A. Dzib, Heino Falcke, Robert Freund, Per Friberg, Christopher H. Greer, Mark A. Gurwell, Paul T. P. Ho, Mareki Honma, Makoto Inoue, Tim Johannsen, Junhan Kim, Thomas P. Krichbaum, James Lamb, Jonathan León-Tavares, Abraham Loeb, Laurent Loinard, David MacMahon, Daniel P. Marrone, James M. Moran, Monika Mościbrodzka, Gisela N. Ortiz-León, Tomoaki Oyama, Feryal Özel, Richard L. Plambeck, Nicolas Pradel, Rurik A. Primiani, Alan E. E. Rogers, Katherine Rosenfeld, Helge Rottmann, Alan L. Roy, Chester Ruszczyk, Daniel L. Smythe, Jason SooHoo, Justin Spilker, Jordan Stone, Peter Strittmatter, Remo P. J. Tilanus, Michael Titus, Laura Vertatschitsch, Jan Wagner, John F. C. Wardle, Jonathan Weintroub, David Woody, Melvyn Wright, Paul Yamaguchi, André Young, Ken H. Young, J. Anton Zensus, and Lucy M. Ziurys. 2016. “Persistent Asymmetric Structure of Sagittarius A* on Event Horizon Scales.” The Astrophysical Journal, 820. Publisher's VersionAbstract
The Galactic Center black hole Sagittarius A* (Sgr A*) is a primeobserving target for the Event Horizon Telescope (EHT), which canresolve the 1.3 mm emission from this source on angular scalescomparable to that of the general relativistic shadow. Previous EHTobservations have used visibility amplitudes to infer the morphology ofthe millimeter-wavelength emission. Potentially much richer sourceinformation is contained in the phases. We report on 1.3 mm phaseinformation on Sgr A* obtained with the EHT on a total of 13 observingnights over four years. Closure phases, which are the sum of visibilityphases along a closed triangle of interferometer baselines, are usedbecause they are robust against phase corruptions introduced byinstrumentation and the rapidly variable atmosphere. The median closurephase on a triangle including telescopes in California, Hawaii, andArizona is nonzero. This result conclusively demonstrates that themillimeter emission is asymmetric on scales of a few Schwarzschild radiiand can be used to break 180° rotational ambiguities inherent fromamplitude data alone. The stability of the sign of the closure phaseover most observing nights indicates persistent asymmetry in the imageof Sgr A* that is not obscured by refraction due to interstellarelectrons along the line of sight.
T. Johannsen, C. Wang, A. E. Broderick, S. S. Doeleman, V. L. Fish, A. Loeb, and D. Psaltis. 2016. “Testing General Relativity with Accretion-Flow Imaging of Sgr A$^*$.” Physical Review Letters, 117, 9, Pp. 091101.
Tim Johannsen, Avery E. Broderick, Philipp M. Plewa, Sotiris Chatzopoulos, Sheperd S. Doeleman, Frank Eisenhauer, Vincent L. Fish, Reinhard Genzel, Ortwin Gerhard, and Michael D. Johnson. 2016. “Testing General Relativity with the Shadow Size of Sgr A*.” Physical Review Letters, 116. Publisher's VersionAbstract
In general relativity, the angular radius of the shadow of a black holeis primarily determined by its mass-to-distance ratio and depends onlyweakly on its spin and inclination. If general relativity is violated,however, the shadow size may also depend strongly on parametricdeviations from the Kerr metric. Based on a reconstructed image ofSagittarius A* (Sgr A* ) from a simulated one-dayobserving run of a seven-station Event Horizon Telescope (EHT) array, weemploy a Markov chain Monte Carlo algorithm to demonstrate that such anobservation can measure the angular radius of the shadow of Sgr
2015
Kazunori Akiyama, Ru-Sen Lu, Vincent L. Fish, Sheperd S. Doeleman, Avery E. Broderick, Jason Dexter, Kazuhiro Hada, Motoki Kino, Hiroshi Nagai, Mareki Honma, Michael D. Johnson, Juan C. Algaba, Keiichi Asada, Christiaan Brinkerink, Ray Blundell, Geoffrey C. Bower, Roger Cappallo, Geoffrey B. Crew, Matt Dexter, Sergio A. Dzib, Robert Freund, Per Friberg, Mark Gurwell, Paul T. P. Ho, Makoto Inoue, Thomas P. Krichbaum, Laurent Loinard, David MacMahon, Daniel P. Marrone, James M. Moran, Masanori Nakamura, Neil M. Nagar, Gisela Ortiz-Leon, Richard Plambeck, Nicolas Pradel, Rurik A. Primiani, Alan E. E. Rogers, Alan L. Roy, Jason SooHoo, Jonathan-León Tavares, Remo P. J. Tilanus, Michael Titus, Jan Wagner, Jonathan Weintroub, Paul Yamaguchi, Ken H. Young, Anton Zensus, and Lucy M. Ziurys. 2015. “230 GHz VLBI Observations of M87: Event-horizon-scale Structure during an Enhanced Very-high-energy γ-Ray State in 2012.” The Astrophysical Journal, 807. Publisher's VersionAbstract
We report on 230 GHz (1.3 mm) very long baseline interferometry (VLBI)observations of M87 with the Event Horizon Telescope using antennas onMauna Kea in Hawaii, Mt. Graham in Arizona, and Cedar Flat inCalifornia. For the first time, we have acquired 230 GHz VLBIinterferometric phase information on M87 through measurement of theclosure phase on the triangle of long baselines. Most of the measuredclosure phases are consistent with 0° as expected by physicallymotivated models for 230 GHz structure such as jet models and accretiondisk models. The brightness temperature of the event-horizon-scalestructure is ∼ 1× {10}10 K derived from the compactflux density of ∼1 Jy and the angular size of ∼40 μ {as}∼ 5.5 {R}{{s}}, which is broadly consistent with the peakbrightness of the radio cores at 1–86 GHz located within ∼{10}2 {R}{{s}}. Our observations occurred in themiddle of an enhancement in very-high-energy (VHE) γ -ray flux,presumably originating in the vicinity of the central black hole. Ourmeasurements, combined with results of multi-wavelength observations,favor a scenario in which the VHE region has an extended size of∼20–60 {R}{{s}}.
Avery E. Broderick, Ramesh Narayan, John Kormendy, Eric S. Perlman, Marcia J. Rieke, and Sheperd S. Doeleman. 2015. “The Event Horizon of M87.” The Astrophysical Journal, 805. Publisher's VersionAbstract
The 6× {{10}9} {{M}} supermassiveblack hole at the center of the giant elliptical galaxy M87 powers arelativistic jet. Observations at millimeter wavelengths with the EventHorizon Telescope have localized the emission from the base of this jetto angular scales comparable to the putative black hole horizon. The jetmight be powered directly by an accretion disk or by electromagneticextraction of the rotational energy of the black hole. However, even thelatter mechanism requires a confining thick accretion disk to maintainthe required magnetic flux near the black hole. Therefore, regardless ofthe jet mechanism, the observed jet power in M87 implies a certainminimum mass accretion rate. If the central compact object in M87 werenot a black hole but had a surface, this accretion would result inconsiderable thermal near-infrared and optical emission from thesurface. Current flux limits on the nucleus of M87 strongly constrainany such surface emission. This rules out the presence of a surface andthereby provides indirect evidence for an event horizon.
Dimitrios Psaltis, Ramesh Narayan, Vincent L. Fish, Avery E. Broderick, Abraham Loeb, and Sheperd S. Doeleman. 2015. “Event Horizon Telescope Evidence for Alignment of the Black Hole in the Center of the Milky Way with the Inner Stellar Disk.” The Astrophysical Journal, 798. Publisher's VersionAbstract
Observations of the black hole in the center of the Milky Way with theEvent Horizon Telescope at 1.3 mm have revealed a size of the emittingregion that is smaller than the size of the black-hole shadow. This canbe reconciled with the spectral properties of the source, if theaccretion flow is seen at a relatively high inclination(50°-60°). Such an inclination makes the angular momentum of theflow, and perhaps of the black hole, nearly aligned with the angularmomenta of the orbits of stars that lie within ~= 3'' from the blackhole. We discuss the implications of such an alignment for theproperties of the black hole and of its accretion flow. We argue thatfuture Event Horizon Telescope observations will not only refine theinclination of Sgr A* but also measure precisely its orientation on theplane of the sky.
J. Wagner, A. L. Roy, T. P. Krichbaum, W. Alef, A. Bansod, A. Bertarini, R. Güsten, D. Graham, J. Hodgson, R. Märtens, K. Menten, D. Muders, H. Rottmann, G. Tuccari, A. Weiss, G. Wieching, M. Wunderlich, J. A. Zensus, J. P. Araneda, O. Arriagada, M. Cantzler, C. Duran, F. M. Montenegro-Montes, R. Olivares, P. Caro, P. Bergman, J. Conway, R. Haas, J. Johansson, M. Lindqvist, H. Olofsson, M. Pantaleev, S. Buttaccio, R. Cappallo, G. Crew, S. Doeleman, V. Fish, R.-S. Lu, C. Ruszczyk, J. SooHoo, M. Titus, R. Freund, D. Marrone, P. Strittmatter, L. Ziurys, R. Blundell, R. Primiani, J. Weintroub, K. Young, M. Bremer, S. Sánchez, A. P. Marscher, R. Chilson, K. Asada, and M. Inoue. 2015. “First 230 GHz VLBI fringes on 3C 279 using the APEX Telescope.” Astronomy and Astrophysics, 581. Publisher's VersionAbstract

Aims: We report about a 230 GHz very long baseline interferometry(VLBI) fringe finder observation of blazar 3C 279 with the APEXtelescope in Chile, the phased submillimeter array (SMA), and the SMT ofthe Arizona Radio Observatory (ARO).
Methods: We installed VLBIequipment and measured the APEX station position to 1 cm accuracy(1σ). We then observed 3C 279 on 2012 May 7 in a 5 h 230 GHz VLBItrack with baseline lengths of 2800 Mλ to 7200 Mλ and afinest fringe spacing of 28.6 μas.
Results: Fringes weredetected on all baselines with signal-to-noise ratios of 12 to 55 in 420s. The correlated flux density on the longest baseline was ~0.3 Jybeam-1, out of a total flux density of 19.8 Jy. Visibilitydata suggest an emission region ≲ 38 μas in size, and at leasttwo components, possibly polarized. We find a lower limit of thebrightness temperature of the inner jet region of about 1010
Michael D. Johnson, Abraham Loeb, Hotaka Shiokawa, Andrew A. Chael, and Sheperd S. Doeleman. 2015. “Measuring the Direction and Angular Velocity of a Black Hole Accretion Disk via Lagged Interferometric Covariance.” The Astrophysical Journal, 813. Publisher's VersionAbstract
We show that interferometry can be applied to study irregular, rapidlyrotating structures, as are expected in the turbulent accretion flownear a black hole. Specifically, we analyze the lagged covariancebetween interferometric baselines of similar lengths but slightlydifferent orientations. For a flow viewed close to face-on, wedemonstrate that the peak in the lagged covariance indicates thedirection and angular velocity of the emission pattern from the flow.Even for moderately inclined flows, the covariance robustly estimatesthe flow direction, although the estimated angular velocity can besignificantly biased. Importantly, measuring the direction of the flowas clockwise or counterclockwise on the sky breaks a degeneracy inaccretion disk inclinations when analyzing time-averaged images alone.We explore the potential efficacy of our technique usingthree-dimensional, general relativistic magnetohydrodynamic simulations,and we highlight several baseline pairs for the Event Horizon Telescope(EHT) that are well-suited to this application. These results indicatethat the EHT may be capable of estimating the direction and angularvelocity of the emitting material near Sgr A*, and they suggest that arotating flow may even be utilized to improve imaging capabilities.

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