We measure the proper motion of the pulsar PSR J1745-2900 relative tothe Galactic center massive black hole, Sgr A*, using the Very LongBaseline Array (VLBA). The pulsar has a transverse velocity of 236± 11 km s^–1 at position angle 22 ± 2 degeast of north at a projected separation of 0.097 pc from Sgr A*. Giventhe unknown radial velocity, this transverse velocity measurement doesnot conclusively prove that the pulsar is bound to Sgr A* however, theprobability of chance alignment is very small. We do show that thevelocity and position are consistent with a bound orbit originating inthe clockwise disk of massive stars orbiting Sgr A* and a natal velocitykick of <~ 500 km s^–1. An origin among the isotropicstellar cluster is possible but less probable. If the pulsar remainsradio-bright, multiyear astrometry of PSR J1745-2900 can detect itsacceleration and determine the full three-dimensional orbit. We alsodemonstrate that PSR J1745-2900 exhibits the same angular broadening asSgr A* over a wavelength range of 3.6 cm to 0.7 cm, further confirmingthat the two sources share the same interstellar scattering properties.Finally, we place the first limits on the presence of awavelength-dependent shift in the position of Sgr A*, i.e., the coreshift, one of the expected properties of optically thick jet emission.Our results for PSR J1745-2900 support the hypothesis that Galacticcenter pulsars will originate from the stellar disk and deepen themystery regarding the small number of detected Galactic center pulsars.

The Event Horizon Telescope (EHT) is an earth-size aperture synthesisradio astronomy array capable of making high-resolution measurements ofsubmillimeter emission near the event horizon of supermassive blackholes. The EHT uses existing standalone submillimeter radio telescopeswhich are retrofitted to serve as VLBI stations. Current instrumentdevelopment goals include increasing the number of stations in the arrayand increasing their sensitivity. We have developed a 4 GHz bandwidthdigital backend (DBE) unit, based on the CASPER (Collaboration forAstronomy Signal Processing and Electronics Research) open source ROACH2(Reconfigurable Open Architecture Computing Hardware) platform. TheROACH2 digital backend, which we call the R2DBE, has dual channels eachsampling at a rate of 4096 MSps (megasamples-per-second), a factor of 4improvement over the previous generation system. Recording 2-bits persample, the bandwidth is equivalently stated as 16 gigabits-per-second(Gbps). This paper includes system design of the R2DBE, discusseslaboratory test results of the system using correlated noise input, andpresents field test results. The R2DBE was distributed to seven sites inearly 2015, enabling the EHT campaign in 2015 March to collect data with2 GHz bandwidth in each polarization. The 16 gigabit-per-second (Gbps)R2DBE can be scaled to create a 64 Gbps system using four R2DBEs inparallel. Thus, it enables a clear path to the EHT's goal of 4 GHzdual-polarization and dual-sideband across the array.

Near a black hole, differential rotation of a magnetized accretion diskis thought to produce an instability that amplifies weak magneticfields, driving accretion and outflow. These magnetic fields wouldnaturally give rise to the observed synchrotron emission in galaxy coresand to the formation of relativistic jets, but no observations to datehave been able to resolve the expected horizon-scale magnetic-fieldstructure. We report interferometric observations at 1.3-millimeterwavelength that spatially resolve the linearly polarized emission fromthe Galactic Center supermassive black hole, Sagittarius A*. We havefound evidence for partially ordered magnetic fields near the eventhorizon, on scales of ~6 Schwarzschild radii, and we have detected andlocalized the intrahour variability associated with these fields.

The image of the emission surrounding the black hole in the center ofthe Milky Way is predicted to exhibit the imprint of generalrelativistic (GR) effects, including the existence of a shadow featureand a photon ring of diameter ~50 μas. Structure on these scales canbe resolved by millimeter-wavelength very long baseline interferometry.However, strong-field GR features of interest will be blurred atλ >= 1.3 mm due to scattering by interstellar electrons. Thescattering properties are well understood over most of the relevantrange of baseline lengths, suggesting that the scattering may be(mostly) invertible. We simulate observations of a model image of Sgr A*and demonstrate that the effects of scattering can indeed be mitigatedby correcting the visibilities before reconstructing the image. Thistechnique is also applicable to Sgr A* at longer wavelengths.

The Event Horizon Telescope (EHT) is a project to assemble a Very LongBaseline Interferometry (VLBI) network of millimeter wavelength dishesthat can resolve strong field general relativistic signatures near asupermassive black hole. As planned, the EHT will include enough dishesto enable imaging of the predicted black hole "shadow," a feature causedby severe light bending at the black hole boundary. The center of M87, agiant elliptical galaxy, presents one of the most interesting EHTtargets as it exhibits a relativistic jet, offering the additionalpossibility of studying jet genesis on Schwarzschild radius scales.Fully relativistic models of the M87 jet that fit all existingobservational constraints now allow horizon-scale images to begenerated. We perform realistic VLBI simulations of M87 model images toexamine the detectability of the black shadow with the EHT, focusing ona sequence of model images with a changing jet mass load radius. Whenthe jet is launched close to the black hole, the shadow is clearlyvisible both at 230 and 345 GHz. The EHT array with a resolution of20-30 μas resolution (~2-4 Schwarzschild radii) is able to image thisfeature independent of any theoretical models and we show that imagingmethods used to process data from optical interferometers are applicableand effective for EHT data sets. We demonstrate that the EHT is alsocapable of tracing real-time structural changes on a few Schwarzschildradii scales, such as those implicated by very high-energy flaringactivity of M87. While inclusion of ALMA in the EHT is critical forshadow imaging, the array is generally robust against loss of a station.

We report the discovery of Faraday rotation toward radio source 3C 84,the active galactic nucleus in NGC 1275 at the core of the PerseusCluster. The rotation measure (RM), determined from polarizationobservations at wavelengths of 1.3 and 0.9 mm, is (8.7 ±2.3)× 10⁵ rad m^-2, among the largest evermeasured. The RM remained relatively constant over a 2 yr period even asthe intrinsic polarization position angle wrapped through a span of300°. The Faraday rotation is likely to originate either in theboundary layer of the radio jet from the nucleus or in the accretionflow onto the central black hole. The accretion flow probably isdisk-like rather than spherical on scales of less than a parsec,otherwise the RM would be even larger.

We demonstrate that polarimetric interferometry can be used to extractprecise spatial information about compact polarized flares of Sgr A*. Weshow that, for a faint dynamical component, a single interferometricbaseline suffices to determine both its polarization and projecteddisplacement from the quiescent intensity centroid. A second baselineenables two-dimensional reconstruction of the displacement, andadditional baselines can self-calibrate using the flare, enhancingsynthesis imaging of the quiescent emission. We apply this technique tosimulated 1.3 mm wavelength observations of a "hot spot" embedded in aradiatively inefficient accretion disk around Sgr A*. Our resultsindicate that, even with current sensitivities, polarimetricinterferometry with the Event Horizon Telescope can achieve ~5 μasrelative astrometry of compact flaring structures near Sgr A* ontimescales of minutes.

A relatively inexpensive 16 Gbps data-recording system based oncommercial off-the-shelf technology and open-source software hasrecently been developed. Combining this recorder with the paralleldevelopment of broadband Very Long Baseline Interferometer (VLBI)instrumentation is enabling dramatically improved sensitivity for bothastronomical and geodetic VLBI. In this article, we describe the VLBIsystem and the results of a demonstration experiment that illustrates anumber of cutting-edge technologies that can be deployed in the nearfuture to significantly enhance the power of the VLBI technique.

We report results from five day very long baseline interferometryobservations of the well-known quasar 3C 279 at 1.3 mm (230 GHz) in2011. The measured nonzero closure phases on triangles includingstations in Arizona, California, and Hawaii indicate that the sourcestructure is spatially resolved. We find an unusual inner jet directionat scales of ~1 pc extending along the northwest-southeast direction(P.A. = 127° ± 3°), as opposed to other (previously)reported measurements on scales of a few parsecs showing inner jetdirection extending to the southwest. The 1.3 mm structure correspondsclosely with that observed in the central region of quasi-simultaneoussuper-resolution Very Long Baseline Array images at 7 mm. The closurephase changed significantly on the last day when compared with the restof observations, indicating that the inner jet structure may be variableon daily timescales. The observed new direction of the inner jet showsinconsistency with the prediction of a class of jet precession models.Our observations indicate a brightness temperature of ~8 ×10¹⁰ K in the 1.3 mm core, much lower than that at centimeterwavelengths. Observations with better uv coverage and sensitivity in thecoming years will allow the discrimination between different structuremodels and will provide direct images of the inner regions of the jetwith 20-30 μas (5-7 light months) resolution.

Approximately 10% of active galactic nuclei exhibit relativistic jets,which are powered by the accretion of matter onto supermassive blackholes. Although the measured width profiles of such jets on large scalesagree with theories of magnetic collimation, the predicted structure onaccretion disk scales at the jet launch point has not been detected. Wereport radio interferometry observations, at a wavelength of 1.3millimeters, of the elliptical galaxy M87 that spatially resolve thebase of the jet in this source. The derived size of 5.5 ± 0.4Schwarzschild radii is significantly smaller than the innermost edge ofa retrograde accretion disk, suggesting that the M87 jet is powered byan accretion disk in a prograde orbit around a spinning black hole.

Dynamical mass measurements to date have allowed determinations of themass M and the distance D of a number of nearby supermassive blackholes. In the case of Sgr A*, these measurements are limited by a strongcorrelation between the mass and distance scaling roughly as M ~ D². Future very long baseline interferometric (VLBI)observations will image a bright and narrow ring surrounding the shadowof a supermassive black hole, if its accretion flow is optically thin.In this paper, we explore the prospects of reducing the correlationbetween mass and distance with the combination of dynamical measurementsand VLBI imaging of the ring of Sgr A*. We estimate the signal-to-noiseratio of near-future VLBI arrays that consist of five to six stations,and we simulate measurements of the mass and distance of Sgr A* usingthe expected size of the ring image and existing stellar ephemerides. Wedemonstrate that, in this best-case scenario, VLBI observations at 1 mmcan improve the error on the mass by a factor of about two compared tothe results from the monitoring of stellar orbits alone. We identify theadditional sources of uncertainty that such imaging observations have totake into account. In addition, we calculate the angular diameters ofthe bright rings of other nearby supermassive black holes and identifythe optimal targets besides Sgr A* that could be imaged by aground-based VLBI array or future space-VLBI missions allowing forrefined mass measurements.

We present the first 1.3 mm (230 GHz) very long baseline interferometrymodel image of an active galactic nucleus (AGN) jet using closure phasetechniques with a four-element array. The model image of the quasar1924-292 was obtained with four telescopes at three observatories: theJames Clerk Maxwell Telescope on Mauna Kea in Hawaii, the Arizona RadioObservatory's Submillimeter Telescope in Arizona, and two telescopes ofthe Combined Array for Research in Millimeter-wave Astronomy inCalifornia in 2009 April. With the greatly improved resolution comparedwith previous observations and robust closure phase measurement, theinner jet structure of 1924-292 was spatially resolved. The inner jetextends to the northwest along a position angle of -53° at adistance of 0.38 mas from the tentatively identified core, in agreementwith the inner jet structure inferred from lower frequencies, and makinga position angle difference of ~80° with respect to the centimeterjet. The size of the compact core is 0.15 pc with a brightnesstemperature of 1.2 × 10¹¹ K. Compared with thosemeasured at lower frequencies, the low brightness temperature may arguein favor of the decelerating jet model or particle-cascade models. Thesuccessful measurement of closure phase paves the way for imaging andtime resolving Sgr A* and nearby AGNs with the Event Horizon Telescope.

Sagittarius A*, the ~4 × 10⁶ M _sun blackhole candidate at the Galactic center, can be studied on Schwarzschildradius scales with (sub)millimeter wavelength very long baselineinterferometry (VLBI). We report on 1.3 mm wavelength observations ofSgr A* using a VLBI array consisting of the JCMT on Mauna Kea, theArizona Radio Observatory's Submillimeter Telescope on Mt. Graham inArizona, and two telescopes of the CARMA array at Cedar Flat inCalifornia. Both Sgr A* and the quasar calibrator 1924-292 were observedover three consecutive nights, and both sources were clearly detected onall baselines. For the first time, we are able to extract 1.3 mm VLBIinterferometer phase information on Sgr A* through measurement ofclosure phase on the triangle of baselines. On the third night ofobserving, the correlated flux density of Sgr A* on all VLBI baselinesincreased relative to the first two nights, providing strong evidencefor time-variable change on scales of a few Schwarzschild radii. Theseresults suggest that future VLBI observations with greater sensitivityand additional baselines will play a valuable role in determining thestructure of emission near the event horizon of Sgr A*.

Extension of very long baseline interferometry (VLBI) to observingwavelengths shorter than 1.3 mm provides exceptional angular resolution(~20 μas) and access to new spectral regimes for the study ofastrophysical phenomena. To maintain phase coherence across a globalVLBI array at these wavelengths requires that ultrastable frequencyreferences be used for the heterodyne receivers at all participatingtelescopes. Hydrogen masers have traditionally been used as VLBIreferences, but atmospheric turbulence typically limits (sub)millimeterVLBI coherence times to ~1-30 s. Cryogenic sapphire oscillators (CSOs)have better stability than hydrogen masers on these timescales and arepotential alternatives to masers as VLBI references. Here, we describethe design, implementation, and tests of a system to produce a 10 MHzVLBI frequency standard from the microwave (11.2 GHz) output of a CSO.To improve long-term stability of the new reference, the CSO was lockedto the timing signal from the Global Positioning System satellites andcorrected for the oscillator aging. The long-term performance of the CSOwas measured by comparison against a hydrogen maser in the samelaboratory. The superb short-term performance, along with the improvedlong-term performance achieved by conditioning, makes the CSO a suitablereference for VLBI at wavelengths less than 1.3 mm.

Millimeter wave very long baseline interferometry (mm-VLBI) providesaccess to the emission region surrounding Sagittarius A* (Sgr A*), thesupermassive black hole at the center of the Milky Way, on sub-horizonscales. Recently, a closure phase of 0° ± 40° wasreported on a triangle of Earth-sized baselines (SMT-CARMA-JCMT)representing a new constraint upon the structure and orientation of theemission region, independent from those provided by the previouslymeasured 1.3 mm-VLBI visibility amplitudes alone. Here, we compare thisto the closure phases associated with a class of physically motivated,radiatively inefficient accretion flow models and present predictionsfor future mm-VLBI experiments with the developing Event HorizonTelescope (EHT). We find that the accretion flow models are capable ofproducing a wide variety of closure phases on the SMT-CARMA-JCMTtriangle and thus not all models are consistent with the recentobservations. However, those models that reproduce the 1.3 mm-VLBIvisibility amplitudes overwhelmingly have SMT-CARMA-JCMT closure phasesbetween ±30°, and are therefore broadly consistent with allcurrent mm-VLBI observations. Improving station sensitivity by factorsof a few, achievable by increases in bandwidth and phasing togethermultiple antennas at individual sites, should result in physicallyrelevant additional constraints upon the model parameters and eliminatethe current 180° ambiguity on the source orientation. Whenadditional stations are included, closure phases of order45°-90° are typical. In all cases, the EHT will be able tomeasure these with sufficient precision to produce dramatic improvementsin the constraints upon the spin of Sgr A*.

Millimeter very long baseline interferometry (mm-VLBI) provides thenovel capacity to probe the emission region of a handful of supermassiveblack holes on sub-horizon scales. For Sagittarius A* (Sgr A*), thesupermassive black hole at the center of the Milky Way, this providesaccess to the region in the immediate vicinity of the horizon. Brodericket al. have already shown that by leveraging spectral and polarizationinformation as well as accretion theory, it is possible to extractaccretion-model parameters (including black hole spin) from mm-VLBIexperiments containing only a handful of telescopes. Here we repeat thisanalysis with the most recent mm-VLBI data, considering a class ofaligned, radiatively inefficient accretion flow (RIAF) models. We findthat the combined data set rules out symmetric models for Sgr A*'s fluxdistribution at the 3.9σ level, strongly favoring length-to-widthratios of roughly 2.4:1. More importantly, we find that physicallymotivated accretion flow models provide a significantly better fit tothe mm-VLBI observations than phenomenological models, at the 2.9σlevel. This implies that not only is mm-VLBI presently capable ofdistinguishing between potential physical models for Sgr A*'s emission,but further that it is sensitive to the strong gravitational lensingassociated with the propagation of photons near the black hole. Basedupon this analysis we find that the most probable magnitude, viewingangle, and position angle for the black hole spin are a = 0.0^{+0.64+ 0.86}, \theta ={68^\circ }^{+5^\circ +9^\circ }_{-20^\circ-28^\circ }, and \xi ={-52^\circ }^{+17^\circ +33^\circ }_{-15^\circ-24^\circ } east of north, where the errors quoted are the 1σ and2σ uncertainties.

The Murchison Wide-Field Array (MWA) is a low-frequency radio telescope,currently under construction, intended to search for the spectralsignature of the epoch of reionization (EOR) and to probe the structureof the solar corona. Sited in western Australia, the full MWA willcomprise 8192 dipoles grouped into 512 tiles and will be capable ofimaging the sky south of 40° declination, from 80 MHz to 300 MHzwith an instantaneous field of view that is tens of degrees wide and aresolution of a few arcminutes. A 32 station prototype of the MWA hasbeen recently commissioned and a set of observations has been taken thatexercise the whole acquisition and processing pipeline. We presentStokes I, Q, and U images from two ~4 hr integrations of a field 20°wide centered on Pictoris A. These images demonstrate the capacity andstability of a real-time calibration and imaging technique employing theweighted addition of warped snapshots to counter extreme wide-fieldimaging distortions.

Sagittarius A* is the source of near infrared, X-ray, radio, and(sub)millimeter emission associated with the supermassive black hole atthe Galactic Center. In the submillimeter regime, Sgr A* exhibitstime-variable linear polarization on timescales corresponding to <10Schwarzschild radii of the presumed 4 × 10⁶ M_sun black hole. In previous work, we demonstrated thepotential for total-intensity (sub)millimeter-wavelength very longbaseline interferometry (VLBI) to detect time-variable—andperiodic—source structure changes in the Sgr A* black hole systemusing nonimaging analyses. Here, we extend this work to include fullpolarimetric VLBI observations. We simulate full-polarization(sub)millimeter VLBI data of Sgr A* using a hot spot model that isembedded within an accretion disk, with emphasis on nonimagingpolarimetric data products that are robust against calibration errors.Although the source-integrated linear polarization fraction in themodels is typically only a few percent, the linear polarization fractionon small angular scales can be much higher, enabling the detection ofchanges in the polarimetric structure of Sgr A* on a wide variety ofbaselines. The shortest baselines track the source-integrated linearpolarization fraction, while longer baselines are sensitive topolarization substructures that are beam-diluted by connected-elementinterferometry. The detection of periodic variability in sourcepolarization should not be significantly affected even if instrumentalpolarization terms cannot be calibrated out. As more antennas areincluded in the (sub)millimeter-VLBI array, observations with fullpolarization will provide important new diagnostics to help disentangleintrinsic source polarization from Faraday rotation effects in theaccretion and outflow region close to the black hole event horizon.

The super-massive black hole candidate, Sagittarius A*, exhibitsvariability from radio to X-ray wavelengths on timescales thatcorrespond to <10 Schwarzschild radii. We survey the potential ofmillimeter wavelength very long baseline interferometry (VLBI) to detectand constrain time-variable structures that could give rise to suchvariations, focusing on a model in which an orbiting hot spot isembedded in an accretion disk. Nonimaging algorithms are developed thatuse interferometric closure quantities to test for periodicity, andapplied to an ensemble of hot spot models that sample a range ofparameter space. We find that structural periodicity in a wide range ofcases can be detected on most potential VLBI arrays using modern VLBIinstrumentation. Future enhancements of millimeter/submillimeter VLBIarrays including phased-array processors to aggregate VLBI stationcollecting area, increased bandwidth recording, and addition of new VLBIsites all significantly aid periodicity detection. The methods describedherein can be applied to other models of Sagittarius A*, including jetoutflows and magnetohydrodynamic accretion simulations.

Recent millimeter-VLBI observations of Sagittarius A* (Sgr A*) have, forthe first time, directly probed distances comparable to the horizonscale of a black hole. This provides unprecedented access to theenvironment immediately around the horizon of an accreting black hole.We leverage both existing spectral and polarization measurements and ourpresent understanding of accretion theory to produce a suite of genericradiatively inefficient accretion flow (RIAF) models of Sgr A*, which wethen fit to these recent millimeter-VLBI observations. We find that ifthe accretion flow onto Sgr A* is well described by an RIAF model, theorientation and magnitude of the black hole's spin are constrained to atwo-dimensional surface in the spin, inclination, position angleparameter space. For each of these, we find the likeliest values andtheir 1σ and 2σ errors to be a = 0^+0.4+0.7,θ = 50°^{+10° +30°}_{-10° -10°}, and ξ =-20°^{+31° +107°}_{-16° -29°}, when the resultingprobability distribution is marginalized over the others. The mostprobable combination is a = 0^+0.2+0.4, θ =90°_{-40° -50°}, and ξ ={-14°}^{+7°+11°}_{-7° -11°}, though the uncertainties on these are verystrongly correlated, and high probability configurations exist for avariety of inclination angles above 30° and spins below 0.99.Nevertheless, this demonstrates the ability millimeter-VLBIobservations, even with only a few stations, to significantly constrainthe properties of Sgr A*.