C. J. Lonsdale, R. J. Cappallo, M. F. Morales, F. H. Briggs, L. Benkevitch, J. D. Bowman, J. D. Bunton, S. Burns, B. E. Corey, L. deSouza, S. S. Doeleman, M. Derome, A. Deshpande, M. R. Gopala, L. J. Greenhill, D. E. Herne, J. N. Hewitt, P. A. Kamini, J. C. Kasper, B. B. Kincaid, J. Kocz, E. Kowald, E. Kratzenberg, D. Kumar, M. J. Lynch, S. Madhavi, M. Matejek, D. A. Mitchell, E. Morgan, D. Oberoi, S. Ord, J. Pathikulangara, T. Prabu, A. Rogers, A. Roshi, J. E. Salah, R. J. Sault, N. U. Shankar, K. S. Srivani, J. Stevens, S. Tingay, A. Vaccarella, M. Waterson, R. B. Wayth, R. L. Webster, A. R. Whitney, A. Williams, and C. Williams. 2009. “The Murchison Widefield Array: Design Overview.” IEEE Proceedings, 97, Pp. 1497-1506. Publisher's VersionAbstract
The Murchison Widefield Array (MWA) is a dipole-based aperture arraysynthesis telescope designed to operate in the 80-300 MHz frequencyrange. It is capable of a wide range of science investigations, but isinitially focused on three key science projects. These are detection andcharacterization of 3-dimensional brightness temperature fluctuations inthe 21cm line of neutral hydrogen during the Epoch of Reionization (EoR)at redshifts from 6 to 10, solar imaging and remote sensing of the innerheliosphere via propagation effects on signals from distant backgroundsources,and high-sensitivity exploration of the variable radio sky. Thearray design features 8192 dual-polarization broad-band active dipoles,arranged into 512 tiles comprising 16 dipoles each. The tiles arequasi-randomly distributed over an aperture 1.5km in diameter, with asmall number of outliers extending to 3km. All tile-tile baselines arecorrelated in custom FPGA-based hardware, yielding a Nyquist-sampledinstantaneous monochromatic uv coverage and unprecedented point spreadfunction (PSF) quality. The correlated data are calibrated in real timeusing novel position-dependent self-calibration algorithms. The array islocated in the Murchison region of outback Western Australia. Thisregion is characterized by extremely low population density and asuperbly radio-quiet environment,allowing full exploitation of theinstrumental capabilities.
Vincent L. Fish, Avery E. Broderick, Sheperd S. Doeleman, and Abraham Loeb. 2009. “Using Millimeter VLBI to Constrain RIAF Models of Sagittarius A*.” The Astrophysical Journal Letters, 692, Pp. L14-L18. Publisher's VersionAbstract
The recent detection of Sagittarius A* at λ = 1.3 mm on abaseline from Hawaii to Arizona demonstrates that millimeter wavelengthvery long baseline interferometry (VLBI) can now spatially resolveemission from the innermost accretion flow of the Galactic centerregion. Here, we investigate the ability of future millimeter VLBIarrays to constrain the spin and inclination of the putative black holeand the orientation of the accretion disk major axis within the contextof radiatively inefficient accretion flow (RIAF) models. We examine therange of baseline visibility and closure amplitudes predicted by RIAFmodels to identify critical telescopes for determining the spin,inclination, and disk orientation of the Sgr A* black hole and accretiondisk system. We find that baseline lengths near 3 Gλ have thegreatest power to distinguish amongst RIAF model parameters, and that itwill be important to include new telescopes that will form north-southbaselines with a range of lengths. If an RIAF model describes theemission from Sgr A*, it is likely that the orientation of the accretiondisk can be determined with the addition of a Chilean telescope to thearray. Some likely disk orientations predict detectable fluxes onbaselines between the continental United States and even a single 10-12m dish in Chile. The extra information provided from closure amplitudesby a four-antenna array enhances the ability of VLBI to discriminateamongst model parameters.
Sheperd Doeleman. 2008. “Approaching the event horizon: 1.3mmλ VLBI of SgrA*.” Journal of Physics Conference Series, 131. Publisher's VersionAbstract
Advances in VLBI instrumentation now allow wideband recording thatsignificantly increases the sensitivity of short wavelength VLBIobservations. Observations of the super-massive black hole candidate atthe center of the Milky Way, SgrA*, with short wavelength VLBI reducesthe scattering effects of the intervening interstellar medium, allowingobservations with angular resolution comparable to the apparent size ofthe event horizon of the putative black hole. Observations in April 2007at a wavelength of 1.3mm on a three station VLBI array have nowconfirmed structure in SgrA* on scales of just a few Schwarzschildradii. When modeled as a circular Gaussian, the fitted diameter of SgrA*is 37 μas (+16,-10; 3σ), which is smaller than the expectedapparent size of the event horizon of the Galactic Center black hole.These observations demonstrate that mm/sub-mm VLBI is poised to open anew window onto the study of black hole physics via high angularresolution observations of the Galactic Center.
Sheperd S. Doeleman, Jonathan Weintroub, Alan E. E. Rogers, Richard Plambeck, Robert Freund, Remo P. J. Tilanus, Per Friberg, Lucy M. Ziurys, James M. Moran, Brian Corey, Ken H. Young, Daniel L. Smythe, Michael Titus, Daniel P. Marrone, Roger J. Cappallo, Douglas C.-J. Bock, Geoffrey C. Bower, Richard Chamberlin, Gary R. Davis, Thomas P. Krichbaum, James Lamb, Holly Maness, Arthur E. Niell, Alan Roy, Peter Strittmatter, Daniel Werthimer, Alan R. Whitney, and David Woody. 2008. “Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre.” Nature, 455, Pp. 78-80. Publisher's VersionAbstract
The cores of most galaxies are thought to harbour supermassive blackholes, which power galactic nuclei by converting the gravitationalenergy of accreting matter into radiation. Sagittarius A* (SgrA*), thecompact source of radio, infrared and X-ray emission at the centre ofthe Milky Way, is the closest example of this phenomenon, with anestimated black hole mass that is 4,000,000 times that of the Sun. Along-standing astronomical goal is to resolve structures in theinnermost accretion flow surrounding SgrA*, where strong gravitationalfields will distort the appearance of radiation emitted near the blackhole. Radio observations at wavelengths of 3.5mm and 7mm have detectedintrinsic structure in SgrA*, but the spatial resolution of observationsat these wavelengths is limited by interstellar scattering. Here wereport observations at a wavelength of 1.3mm that set a size ofmicroarcseconds on the intrinsic diameter of SgrA*. This is less thanthe expected apparent size of the event horizon of the presumed blackhole, suggesting that the bulk of SgrA* emission may not be centred onthe black hole, but arises in the surrounding accretion flow.
Judd D. Bowman, David G. Barnes, Frank H. Briggs, Brian E. Corey, Merv J. Lynch, N. D. Ramesh Bhat, Roger J. Cappallo, Sheperd S. Doeleman, Brian J. Fanous, David Herne, Jacqueline N. Hewitt, Chris Johnston, Justin C. Kasper, Jonathon Kocz, Eric Kratzenberg, Colin J. Lonsdale, Miguel F. Morales, Divya Oberoi, Joseph E. Salah, Bruce Stansby, Jamie Stevens, Glen Torr, Randall Wayth, Rachel L. Webster, and J. Stuart B. Wyithe. 2007. “Field Deployment of Prototype Antenna Tiles for the Mileura Widefield Array Low Frequency Demonstrator.” The Astronomical Journal, 133, Pp. 1505-1518. Publisher's VersionAbstract
Experiments were performed with prototype antenna tiles for the MileuraWidefield Array Low Frequency Demonstrator (MWA LFD) to betterunderstand the wide-field, wide-band properties of their design and tocharacterize the radio-frequency interference (RFI) between 80 and 300MHz at the site in Western Australia. Observations acquired during the 6month deployment confirmed the predicted sensitivity of the antennas,sky-noise-dominated system temperatures, and phase-coherentinterferometric measurements. The radio spectrum is remarkably free ofstrong terrestrial signals, with the exception of two narrow frequencybands allocated to satellite downlinks, and rare bursts due toground-based transmissions being scattered from aircraft and meteortrails. Results indicate the potential of the MWA LFD to makesignificant achievements in its three key science objectives: epoch ofreionization science, heliospheric science, and radio transientdetection.
Y. M. Pihlström, G. B. Taylor, J. Granot, and S. Doeleman. 2007. “Stirring the Embers: High-Sensitivity VLBI Observations of GRB 030329.” The Astrophysical Journal, 664, Pp. 411-415. Publisher's VersionAbstract
We present high-sensitivity Very Long Baseline Interferometry (VLBI)observations 806 days after the γ-ray burst of 2003 March 29 (GRB030329). The angular diameter of the radio afterglow is measured to be0.347+/-0.09 mas, corresponding to 0.99+/-0.26 pc at the redshift of GRB030329 (z=0.1685). The evolution of the image size favors a uniformexternal density over an R-2 windlike density profile (atdistances of R>~1018 cm from the source), although thelatter cannot be ruled out yet. The current apparent expansion velocityof the image size is only mildly relativistic, suggesting anonrelativistic transition time of tNR~1 yr. A rebrightening,or at least a significant flattening in the flux decay, is expectedwithin the next several years as the counterjet becomes visible (thishas not yet been observed). An upper limit of <1.9c is set on theproper motion of the flux centroid.
Colin J. Lonsdale, Sheperd S. Doeleman, and Divya Oberoi. 2004. “Efficient Imaging Strategies For Next-Generation Radio Arrays.” Experimental Astronomy, 17, Pp. 345-362. Publisher's VersionAbstract
The performance goals of the Square Kilometre Array (SKA) are such thatmajor departures from prior practice for imaging interferometer arraysare required. One class of solutions involves the construction of largenumbers of stations, each composed of one or more small antennas. Theadvantages of such a “large-N” approach are alreadydocumented, but attention has recently been drawn to scalingrelationships for SKA data processing that imply excessive computingcosts associated with the use of small antennas. In this paper weexamine the assumptions that lead to such scaling laws, and argue thatin general they are unlikely to apply to the SKA situation. A variety ofstrategies for SKA imaging which exhibit better scaling behaviour arediscussed. Particular attention is drawn to field of view issues, andthe possibility of using weighting functions within an advancedcorrelator system to precisely control the field-of-view.
Sheperd S. Doeleman, Colin J. Lonsdale, Paul T. Kondratko, and C. Read Predmore. 2004. “Using VLBI to Probe the Orion KL Outflow on AU Scales.” The Astrophysical Journal, 607, Pp. 361-368. Publisher's VersionAbstract
We present the first contemporaneous 43 and 86 GHz VLBI images of thev=1, J=2-->1 and J=1-->0 SiO masers in the Orion KL nebula. Bothmaser species exhibit the same general morphology of earlier J=1-->0maser images, which appear to trace the edges of a bipolar conicaloutflow. Surprisingly, the J=2-->1 masers form farther from thecentral protostar than the J=1-->0 masers, a fact not readilyexplained by current SiO maser pumping models. The average magnitude ofoffsets between corresponding regions of the two masing transitions is
R. B. Phillips, A. H. Straughn, S. S. Doeleman, and C. J. Lonsdale. 2003. “R Cassiopeiae: Relative Strengths of SiO Masers at 43 and 86 GHz.” The Astrophysical Journal Letters, 588, Pp. L105-L108. Publisher's VersionAbstract
We present simultaneous VLBI maps of the J=1-->0 (43.1 GHz) andJ=2-->1 (86.2 GHz) silicon monoxide (SiO) v=1 masers in thecircumstellar envelope of the asymptotic giant branch star R Cassiopeiaethat have been registered to submilliarcsecond precision. Our imagesallow specific comparisons of maser strengths from volumes that areabout 1/10 AU in projected size. Bright 86.2 GHz v=1 masers in two casesoperate from the same gas volumes as the 43.1 GHz v=1 masers. Therelative strengths of the 43.1 and 86.2 GHz masers vary from roughlyunity to lower limits of several hundred to 1. The low strength ofJ=2-->1 masers relative to J=1-->0 might arise from neutraldensities of H2<=4×109 cm-3 at1.5-2 times the photospheric radius from R Cas. The SiO masers of R Casat this epoch are not consistent with emission models that employpulsation-driven shocks unless the phasing of such models can beshifted.
R. B. Phillips, G. R. Sivakoff, C. J. Lonsdale, and S. S. Doeleman. 2001. “Coordinated Millimeter VLBI Array Observations of R Cassiopeiae: 86 GHz SiO Masers and Envelope Dynamics.” The Astronomical Journal, 122, Pp. 2679-2685. Publisher's VersionAbstract
Coordinated Millimeter VLBI Array measurements at 86 GHz have imaged thesilicon monoxide masers of the nearby red giant R Cassiopeiae at twoepochs separated by approximately 1/12 the stellar pulsation period. Thev=1, J=2-->1 transition comes from a ring at 1.5 to 2 stellar radii.Doppler velocity patterns are ordered over scales up to ~1 AU but arenot consistent with simple models such as global rotation. One majormasing region disappeared in the interval between the two epochs.
S. S. Doeleman, Z.-Q. Shen, A. E. E. Rogers, G. C. Bower, M. C. H. Wright, J. H. Zhao, D. C. Backer, J. W. Crowley, R. W. Freund, P. T. P. Ho, K. Y. Lo, and D. P. Woody. 2001. “Structure of Sagittarius A* at 86 GHZ using VLBI Closure Quantities.” The Astronomical Journal, 121, Pp. 2610-2617. Publisher's VersionAbstract
At radio wavelengths, images of the compact radio source Sagittarius A*(Sgr A*) in the Galactic center are scatter broadened with aλ2 dependence due to an intervening ionized medium. Wepresent VLBI observations of Sgr A* at 86 GHz using a six station array,including the VLBA antennas at Pie Town, Fort Davis, and Los Alamos, the12 m antenna at Kitt Peak, and the millimeter arrays at Hat Creek andOwens Valley. To avoid systematic errors due to imperfect antennacalibration, the data were modeled using interferometric closureinformation. The data are best modeled by a circular Gaussian brightnessdistribution of FWHM 0.18+/-0.02 mas. The data are also shown to beconsistent with an elliptical model corresponding to the scattering of apoint source. The source structure in the north-south direction, whichis less well determined than in the east-west direction because of thelimited north-south u-v coverage of the array, is constrained to be lessthan 0.27 mas by these measurements. These results are consistent withextrapolations of intrinsic structure estimates obtained with VLBI at a7 mm wavelength, assuming the intrinsic size of Sgr A* has a greaterdependence than λ0.9 with wavelength.
Sheperd S. Doeleman, Colin J. Lonsdale, and Shannon Pelkey. 1999. “A Molecular Outflow Traced by SIO Masers in Orion KL.” The Astrophysical Journal Letters, 510, Pp. L55-L58. Publisher's VersionAbstract
We present full-resolution (~0.2 mas, ~0.1 AU) Very Long BaselineInterferometry maps of the v=1, J=1-->0 SiO masers in the Orion KLnebula. The morphology and turbulent nature of the maser emission arguesagainst models in which the masers form in an expanding and rotatingprotostellar disk. Rather, we find that the symmetry evident in the SiOmaser emission points toward its genesis in a bipolar outflow. Watermasers that also appear toward Orion KL are known to share the velocityprofile and double-peaked characteristics of the SiO maser emission yetform much farther away from the protostar. We link the water masers toour present results by proposing that both species of maser form arounda flared molecular outflow of roughly constant velocity.
Colin J. Lonsdale, Sheperd S. Doeleman, and Robert B. Phillips. 1998. “A 3 Millimeter VLBI Continuum Source Survey.” The Astronomical Journal, 116, Pp. 8-12. Publisher's VersionAbstract
A detection survey of bright extragalactic radio sources with 3 mm VLBIis described. The sample is based on a centimeter-wavelength fluxdensity limit, with account taken of the spectral shape. Of the 116sample members, 79 sources were observed and 14 were detected. Thisdetection rate is considerably lower than had been anticipated, thediscrepancy being fully attributable to attained sensitivities roughly afactor of 2 poorer than those planned, and routinely achievable withcurrent arrays. The small number of detections precludes any firmconclusions about source detectability as a function of source type,redshift, or other variable. However, the results allow us todemonstrate that, at easily attainable sensitivity levels, between 30and 50 sources would be detected in a similar survey. Because the sampledoes not include all potentially detectable sources, in particular thosewith inverted spectra above 5 GHz, these numbers represent a lower limitto the number of detectable continuum sources accessible to the current3 mm VLBI network at any given time.
Sheperd S. Doeleman, Colin J. Lonsdale, and Lincoln J. Greenhill. 1998. “VLBI Imaging of the 86 GHz SiO Maser Emission in the Circumstellar Envelope of VX Sagittarii.” The Astrophysical Journal, 494, Pp. 400-408. Publisher's VersionAbstract
We have observed and imaged the SiO maser source in the late-type starVX Sgr with a single VLBI baseline at 3.5 mm wavelength.Phase-referenced channel maps with a beamwidth of ~0.4 mas show multiplecomponents over a LSR velocity range of -1 to 24 km s-1, andmany of the maser features display velocity widths smaller than 1 kms-1. The maser spots in this 86 GHz transition (v = 1, J = 2--> 1) are arranged in an irregular ring-shaped configuration ofdiameter ~30 mas, echoing the structures found in the J = 1 --> 0transition at 43 GHz around several Mira variables and VX Sgr itself.Unlike previous observations of SiO masers, in VX Sgr we detect a clear,systematic velocity gradient of ~0.6 km s-1 AU-1among the strongest complex of masers on the southernmost portion of thering structure. This velocity gradient can be interpreted as a rigidrotation of the circumstellar envelope with v sin i ~ 13 kms-1. Maser emission is not detected on long (5000 km)baselines, which places a 0.1 mas lower limit on the size of strongmaser spots.
A. Alberdi, T. P. Krichbaum, D. A. Graham, A. Greve, M. Grewing, J. M. Marcaide, A. Witzel, R. S. Booth, L. B. Baath, F. Colomer, S. Doeleman, A. P. Marscher, A. E. E. Rogers, C. J. Schalinski, and K. Standke. 1997. “The high-frequency compact radio structure of the peculiar quasar 4C 39.25.” Astronomy and Astrophysics, 327, Pp. 513-521. Publisher's VersionAbstract
We present new high angular resolution images of the compact non-thermalradio source 4C 39.25 obtained from VLBI observations at λ1.3cm,λ7mm, and λ3mm wavelengths. These maps and Gaussianmodel-fits show that the milli-arcsecond to sub-milliarcsecond structureof 4C 39.25 consists of a complex bent core-jet structure with embeddedmoving and stationary VLBI components. Facilitated by the smallobserving beams and high angular resolutions obtained at mm-wavelengths,we measured the relative positions of the jet components with anaccuracy of a few hundred micro-arcseconds. This allows the detailedfollowup of the ongoing merging process of a westward superluminallymoving component (b_) with a stationary component a_, located at ~2.9mas east of the putative core d_. In contrast to the other components ofthe structure with steeper spectra, the westernmost component d_exhibits an inverted spectrum peaking between λ7mm andλ3mm, thus further supporting its identification as the VLBIcore, self-absorbed at longer wavelengths. From two VLBI maps obtainednearly simultaneously at λ7mm and λ1.3cm, we made thefirst spectral index map of 4C 39.25 in this wavelength regime. The maincharacteristics of the spectral index distribution of the jet arepronounced changes of the spectral index between orientations paralleland transverse to the jet axis. Near the merging components a_ and b_the spectral index steepens with increasing separation from d_. However,in the bridge of emission c_, which connects d_ with a_ and b_, thespectral index gradient has a direction transverse to the jet axis,suggesting a frequency dependent jet curvature and edge-brightening. Abrief discussion of this behaviour within current jet models ispresented.