Hartebeesthoek Radio Astronomy Observatory

Krugersdorp, South Africa

Hartebeesthoek Radio Astronomy Observatory

Krugersdorp, South Africa
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Ratner M.I.,Harvard - Smithsonian Center for Astrophysics | Bartel N.,York University | Bietenholz M.F.,York University | Bietenholz M.F.,Hartebeesthoek Radio Astronomy Observatory | And 6 more authors.
Astrophysical Journal, Supplement Series | Year: 2012

We present the principal astrometric results of the very long baseline interferometry (VLBI) program undertaken in support of the Gravity Probe B (GP-B) relativity mission. VLBI observations of the GP-B guide star, the RS CVn binary IM Pegasi (HR 8703), yielded positions at 35 epochs between 1997 and 2005. We discuss the statistical assumptions behind these results and our methods for estimating the systematic errors. We find the proper motion of IM Peg in an extragalactic reference frame closely related to the International Celestial Reference Frame 2 (ICRF2) to be -20.83 ± 0.03 ± 0.09masyr -1 in right ascension and -27.27 ± 0.03 ± 0.09masyr -1 in declination. For each component, the first uncertainty is the statistical standard error and the second is the total standard error (SE) including plausible systematic errors. We also obtain a parallax of 10.37 ± 0.07mas (distance: 96.4 ± 0.7pc), for which there is no evidence of any significant contribution of systematic error. Our parameter estimates for the 25day period orbital motion of the stellar radio emission have SEs corresponding to 0.10mas on the sky in each coordinate. The total SE of our estimate of IM Peg's proper motion is 30% smaller than the accuracy goal set by the GP-B project before launch: 0.14masyr -1 for each coordinate of IM Peg's proper motion. Our results ensure that the uncertainty in IM Peg's proper motion makes only a very small contribution to the uncertainty of the GP-B relativity tests. © 2012. The American Astronomical Society. All rights reserved..


Shapiro I.I.,Harvard - Smithsonian Center for Astrophysics | Bartel N.,York University | Bietenholz M.F.,York University | Bietenholz M.F.,Hartebeesthoek Radio Astronomy Observatory | And 6 more authors.
Astrophysical Journal, Supplement Series | Year: 2012

We describe the NASA/Stanford gyroscope relativity mission, Gravity Probe B (GP-B), and provide an overview of the following series of six astrometric and astrophysical papers that report on our radio observations and analyses made in support of this mission. The main goal of this 8.5year program of differential very long baseline interferometry astrometry was to determine the proper motion of the guide star of the GP-B mission, the RS CVn binary IM Pegasi (IM Peg; HR 8703). This proper motion is determined with respect to compact, extragalactic reference sources. The results are -20.833 ± 0.090masyr -1 and -27.267 ± 0.095masyr -1 for, respectively, the right ascension and declination, in local Cartesian coordinates, of IM Peg's proper motion, and 10.370 ± 0.074mas (i.e., 96.43 ± 0.69pc) for its parallax (and distance). Each quoted uncertainty is meant to represent an 70% confidence interval that includes the estimated contribution from systematic error. These results are accurate enough not to discernibly degrade the GP-B estimates of its gyroscopes' relativistic precessions: the frame-dragging and geodetic effects. © 2012. The American Astronomical Society. All rights reserved..


Berger E.,Harvard - Smithsonian Center for Astrophysics | Zauderer A.,Harvard - Smithsonian Center for Astrophysics | Pooley G.G.,Mullard Radio Observatory | Soderberg A.M.,Harvard - Smithsonian Center for Astrophysics | And 6 more authors.
Astrophysical Journal | Year: 2012

We present continued radio observations of the tidal disruption event SwiftJ164449.3+573451 extending to δt 216days after discovery. The data were obtained with the EVLA, AMI Large Array, CARMA, the SMA, and the VLBA+Effelsberg as part of a long-term program to monitor the expansion and energy scale of the relativistic outflow, and to trace the parsec-scale environment around a previously dormant supermassive black hole (SMBH). The new observations reveal a significant change in the radio evolution starting at δt 1 month, with a brightening at all frequencies that requires an increase in the energy by about an order of magnitude, and an overall density profile around the SMBH of ρr -3/2 (0.1-1.2 pc) with a significant flattening at r 0.4-0.6 pc. The increase in energy cannot be explained with continuous injection from an Lt -5/3 tail, which is observed in the X-rays. Instead, we conclude that the relativistic jet was launched with a wide range of Lorentz factors, obeying E(> Γj)Γ-2.5 j. The similar ratios of duration to dynamical timescale for Sw1644+57 and gamma-ray bursts (GRBs) suggest that this result may be applicable to GRB jets as well. The radial density profile may be indicative of Bondi accretion, with the inferred flattening at r 0.5 pc in good agreement with the Bondi radius for a few × 106 M ⊙ black hole. The density at 0.5 pc is about a factor of 30 times lower than inferred for the Milky Way Galactic Center, potentially due to a smaller number of mass-shedding massive stars. From our latest observations (δt 216days) we find that the jet energy is E j, iso 5 × 1053 erg (Ej 2.4 × 1051 erg for θj = 0.1), the radius is r 1.2 pc, the Lorentz factor is Γj 2.2, the ambient density is n 0.2 cm -3, and the projected angular size is r proj 25 μas, below the resolution of the VLBA+Effelsberg. Assuming no future changes in the observed evolution and a final integrated total energy of Ej 10 52 erg, we predict that the radio emission from Sw1644+57 should be detectable with the EVLA for several decades and will be resolvable with very long baseline interferometry in a few years. © 2012 The American Astronomical Society. All rights reserved.


Ransom R.R.,York University | Ransom R.R.,Okanagan College | Ransom R.R.,National Research Council Canada | Bartel N.,York University | And 7 more authors.
Astrophysical Journal, Supplement Series | Year: 2012

We used 8.4GHz very long baseline interferometry images obtained at up to 35 epochs between 1997 and 2005 to examine the radio structures of the main reference source, 3C 454.3, and two secondary reference sources, B2250+194 and B2252+172, for the guide star for the NASA/Stanford relativity mission Gravity Probe B (GP-B). For one epoch in 2004 May, we also obtained images at 5.0 and 15.4GHz. The 35 8.4GHz images for quasar 3C 454.3 confirm a complex, evolving, core-jet structure. We identified at each epoch a component, C1, near the easternmost edge of the core region. Simulations of the core region showed that C1 is located, on average, 0.18 ± 0.06mas west of the unresolved "core" identified in 43GHz images. We also identified in 3C 454.3 at 8.4GHz several additional components that moved away from C1 with proper motions ranging in magnitude between 0.9 c and 5 c. The detailed motions of the components exhibit two distinct bends in the jet axis located 3 and 5.5mas west of C1. The spectra between 5.0 and 15.4GHz for the "moving" components are steeper than those for C1. The 8.4GHz images of B2250+194 and B2252+172, in contrast to those of 3C 454.3, reveal compact structures. The spectrum between 5.0 and 15.4GHz for B2250+194 is inverted while that for B2252+172 is flat. Based on its position near the easternmost edge of the 8.4GHz radio structure, close spatial association with the 43GHz core, and relatively flat spectrum, we believe 3C 454.3 component C1 to be the best choice for the ultimate reference point for the GP-B guide star. The compact structures and inverted-to-flat spectra of B2250+194 and B2252+172 make these objects valuable secondary reference sources. © 2012. The American Astronomical Society. All rights reserved..


Bartel N.,York University | Bietenholz M.F.,York University | Bietenholz M.F.,Hartebeesthoek Radio Astronomy Observatory | Lebach D.E.,Harvard - Smithsonian Center for Astrophysics | And 7 more authors.
Astrophysical Journal, Supplement Series | Year: 2012

We made very long baseline interferometry observations at 8.4GHz between 1997 and 2005 to estimate the coordinates of the "core" component of the superluminal quasar, 3C454.3, the ultimate reference point in the distant universe for the NASA/Stanford Gyroscope Relativity Mission, Gravity Probe B (GP-B). These coordinates are determined relative to those of the brightness peaks of two other compact extragalactic sources, B2250+194 and B2252+172, nearby on the sky, and within a celestial reference frame (CRF), defined by a large suite of compact extragalactic radio sources, and nearly identical to the International Celestial Reference Frame 2 (ICRF2). We find that B2250+194 and B2252+172 are stationary relative to each other, and also in the CRF, to within 1σ upper limits of 15 and 30 μasyr -1 in α and δ, respectively. The core of 3C454.3 appears to jitter in its position along the jet direction over 0.2mas, likely due to activity close to the putative supermassive black hole nearby, but on average is stationary in the CRF within 1σ upper limits on its proper motion of 39 μasyr -1 (1.0c) and 30 μasyr -1 (0.8c) in α and δ, respectively, for the period 2002-2005. Our corresponding limit over the longer interval, 1998-2005, of more importance to GP-B, is 46 and 56 μasyr -1 in α and δ, respectively. Some of 3C454.3's jet components show significantly superluminal motion with speeds of up to 200 μasyr -1 or 5c in the CRF. The core of 3C454.3 thus provides for GP-B a sufficiently stable reference in the distant universe. © 2012. The American Astronomical Society. All rights reserved..


Bietenholz M.F.,York University | Bietenholz M.F.,Hartebeesthoek Radio Astronomy Observatory | Bartel N.,York University | Lebach D.E.,Harvard - Smithsonian Center for Astrophysics | And 5 more authors.
Astrophysical Journal, Supplement Series | Year: 2012

We present measurements of the total radio flux density as well as very long baseline interferometry images of the star, IM Pegasi, which was used as the guide star for the NASA/Stanford relativity mission Gravity ProbeB. We obtained flux densities and images from 35 sessions of observations at 8.4GHz (λ = 3.6cm) between 1997 January and 2005 July. The observations were accurately phase-referenced to several extragalactic reference sources, and we present the images in a star-centered frame, aligned by the position of the star as derived from our fits to its orbital motion, parallax, and proper motion. Both the flux density and the morphology of IM Peg are variable. For most sessions, the emission region has a single-peaked structure, but 25% of the time, we observed a two-peaked (and on one occasion perhaps a three-peaked) structure. On average, the emission region is elongated by 1.4 ± 0.4mas (FWHM), with the average direction of elongation being close to that of the sky projection of the orbit normal. The average length of the emission region is approximately equal to the diameter of the primary star. No significant correlation with the orbital phase is found for either the flux density or the direction of elongation, and no preference for any particular longitude on the star is shown by the emission region. © 2012. The American Astronomical Society. All rights reserved..


Bietenholz M.F.,Hartebeesthoek Radio Astronomy Observatory | Bietenholz M.F.,York University
Publications of the Astronomical Society of Australia | Year: 2013

Very long baseline interferometry observations of supernovae and gamma-ray bursts provide almost the only way of obtaining spatially resolved information about the sources. In particular, a determination of the expansion velocity of the forward shock, as well as the geometry of the fireball and its evolution with time are possible for relatively nearby events, provided they are radio bright. Monitoring the expansion of the shock front can provide information on the density profiles of both the circumstellar material and on the ejecta. Very long baseline interferometry observations can also potentially resolve gamma-ray burst jets which are not directed along the line of sight, providing crucial confirmation of relativistic expansion in such objects. This review gives an overview of recent results from supernovae, including the Type I b/c SNe 2011dh, 2009bb, and 2007gr, and discusses the prospects for future observations. © 2014 Astronomical Society of Australia.


Zauderer B.A.,Harvard - Smithsonian Center for Astrophysics | Berger E.,Harvard - Smithsonian Center for Astrophysics | Margutti R.,Harvard - Smithsonian Center for Astrophysics | Pooley G.G.,Mullard Radio Observatory | And 5 more authors.
Astrophysical Journal | Year: 2013

We present continued multi-frequency radio observations of the relativistic tidal disruption event Swift J164449.3+573451 (Sw 1644+57) extending to t 600 days. The data were obtained with the JVLA and AMI Large Array as part of our on-going study of the jet energetics and the density structure of the parsec-scale environment around the disrupting supermassive black hole. We combine these data with public Swift/XRT and Chandra X-ray observations over the same time-frame to show that the jet has undergone a dramatic transition starting at 500 days, with a sharp decline in the X-ray flux by about a factor of 170 on a timescale of δt/t ≲ 0.2 (and by a factor of 15 in δt/t 0.05). The rapid decline rules out a forward shock origin (direct or reprocessing) for the X-ray emission at ≲ 500 days, and instead points to internal dissipation in the inner jet. On the other hand, our radio data uniquely demonstrate that the low X-ray flux measured by Chandra at 610 days is consistent with emission from the forward shock. Furthermore, the Chandra data are inconsistent with thermal emission from the accretion disk itself since the expected temperature of 30-60 eV and inner radius of 2-10 Rs cannot accommodate the observed flux level or the detected emission at ≳ 1 keV. We associate the rapid decline with a turn off of the relativistic jet when the mass accretion rate dropped below M⊙ yr-1 (for a 3 × 106 M⊙ black hole and order unity efficiency) indicating that the peak accretion rate was about , and the total accreted mass by t 500 days is about 0.15 M . From the radio data we further find significant flattening in the integrated energy of the forward shock at t ≳ 250 days with Ej,iso 2 × 1054 erg (Ej 10 52 erg for a jet opening angle, j = 0.1) following a rise by about a factor of 15 at 30-250 days. Projecting forward, we predict that the emission in the radio and X-ray bands will evolve in tandem with similar decline rates. © 2013. The American Astronomical Society. All rights reserved.


Combrinck L.,Hartebeesthoek Radio Astronomy Observatory
62nd International Astronautical Congress 2011, IAC 2011 | Year: 2011

The international network of Satellite Laser Ranging (SLR) stations is under represented in the Southern Hemisphere. Currently there are only three Lunar Laser Ranger (LLR) stations globally, all located in the Northern Hemisphere. We are in the process of developing a combined satellite and lunar capable laser ranger utilising a one metre optical telescope. A complete description of the link budget to the Moon, required electronic hardware, optics, laser system and integrated software for this new LLR is presented. Research work undertaken utilising data from the global network is described. Future work, which will include data from the newly developed Lunar Laser Ranger, is discussed. In particular the application of SLR and LLR to evaluate General Relativity Theory (GRT) is discussed. Results obtained evaluating post-Newtonian parameters γ and β through the precise orbit determination of the LAGEOS satellites are presented. These results are brought into context with other published work utilising different instrumentation and techniques. Limitations in testing GRT are described. These limitations result from modelling deficiencies, satellite and lunar laser ranging inaccuracies and imperfect analysis strategies. Copyright ©2011 by the International Astronautical Federation. All rights reserved.


Gaylard M.,Hartebeesthoek Radio Astronomy Observatory
2011 30th URSI General Assembly and Scientific Symposium, URSIGASS 2011 | Year: 2011

The 26m radio telescope of the Hartebeesthoek Radio Astronomy Observatory (HartRAO) suffered a failure of the main bearing in the polar shaft in 2008 October. This was replaced in 2010 and the telescope returned to service. Very Long Baseline Interferometry (VLBI) restarted in 2010 August. The internet link to other continents operates at 1 GB/s, permitting e-VLBI at this data rate with the European VLBI Network (EVN). © 2011 IEEE.

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