Arthur Niell

Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre

The cores of most galaxies are thought to harbour supermassive black holes, which power galactic nuclei by converting the gravitational energy of accreting matter into radiation. Sagittarius A* (Sgr A*), the compact source of radio, infrared and X-ray emission at the centre of the Milky Way, is the closest example of this phenomenon, with an estimated black hole mass that is 4,000,000 times that of the Sun. A long-standing astronomical goal is to resolve structures in the innermost accretion flow surrounding Sgr A*, where strong gravitational fields will distort the appearance of radiation emitted near the black hole. Radio observations at wavelengths of 3.5 mm and 7 mm have detected intrinsic structure in Sgr A*, but the spatial resolution of observations at these wavelengths is limited by interstellar scattering. Here we report observations at a wavelength of 1.3 mm that set a size of microarcseconds on the intrinsic diameter of Sgr A*. This is less than the expected apparent size of the event horizon of the presumed black hole, suggesting that the bulk of Sgr A* emission may not be centred on the black hole, but arises in the surrounding accretion flow.

The high-resolution structure of the Centaurus A nucleus at 2.3 and 8.4 GHz

VLBI observations of the nucleus of Centaurus A have been made at two frequencies with an array of five Australian radio telescopes as part of the Southern Hemisphere VLBI Experiment. Observations were made at 2.3 GHz with all five antennas, while only two were employed at 8.4 GHz. At 2.3 GHz seven tracks in the (u,v) plane with coverage of 6-8 hr each were obtained, yielding significant information on the structure of the nuclear jet. At 8.4 GHz a compact unresolved core was detected as well. It is found that the source consists of the compact self-absorbed core, a jet containing a set of three knots extending from 100 to 160 mas from the core, and a very long, narrow component elongated along the same position angle as the knots. The allowable range for the position angle of the jet is 51 + or - 3 deg, in agreement with that of the radio and X-ray structure on arcsecond and arcminute scales. The jet has brightened at 2.3 GHz by about 4 Jy, a factor of nearly 3, since the early 1970s, 1.8 Jy of which has occurred in the last 2 yr with no discernable changes in structure.

Recent Progress in the VLBI2010 Development

From October 2003 to September 2005, the International VLBI Service for Geodesy and Astrometry (IVS) examined current and future requirements for geodetic VLBI, including all components from antennas to analysis. IVS Working Group 3 “VLBI 2010”, which was tasked with this effort, concluded with recommendations for a new generation of VLBI systems. These recommendations were based on the goals of achieving 1 mm measurement accuracy on global baselines, performing continuous measurements for time series of station positions and Earth orientation parameters, and reaching a turnaround time from measurement to initial geodetic results of less than 24 h. To realize these recommendations and goals, along with the need for low cost of construction and operation, requires a complete examination of all aspects of geodetic VLBI including equipment, processes, and observational strategies. Hence, in October 2005, the IVS VLBI2010 Committee (V2C) commenced work on defining the VLBI2010 system specifications. In this paper we give a summary of the recent progress of the VLBI2010 project

VLBI2010: Next generation VLBI system for geodesy and astrometry

The International VLBI Service for Geodesy and Astrometry (IVS) is well on the way to fully defining a next generation VLBI system, called VLBI2010. The goals of the new system are to achieve 1-mm position accuracy over a 24-h observing session and to carry out continuous observations, with initial results to be delivered within 24 h after taking the data. These goals require a completely new technical and conceptual design of VLBI measurements. Based on extensive simulation studies, strategies have been developed by the IVS to significantly improve its product accuracy through the use of a network of small (~12-m) fast-slewing antennas, a new method for generating high precision delay measurements, and improved methods for handling biases related to system electronics, deformations of the antenna structures, and radio source structure. To test many of the proposed strategies, NASA is sponsoring a proof-of-concept development effort using IVS antennas near Washington, DC, and Boston, MA. Furthermore, as of Feb. 2009, the construction of ten new VLBI2010 sites has already been funded, which will improve the geographical distribution of geodetic VLBI sites and provide an important step towards a global VLBI2010 network.

Radio structure at 8.4 GHz in Sagittarius A, the compact radio source at the Galactic center

VLBI observations of the compact, nonthermal radio source at the Galactic center show it to be elongated at 8.4 GHz along a position angle of 82 + or - 6 deg. The source has an axial ratio of 0.53 + or - 0.10 with a major axis of 17.4 + or - 0.5 mas. Examination of VLA maps of the Galactic center region indicate no obvious alignment with this smaller-scale elongation of the nuclear region, nor is the nuclear position angle aligned with the axis of Galactic rotation. Comparison with the size measured at frequencies from 1 to 22 GHz shows that the size follows very closely the lambda-squared dependence expected from interstellar scattering. The alongated nature of the source implies either that the scattering medium is anisotropic or that some remnant of the intrinsic structure remains visible through the scattering medium. 12 refs.

VLBI2010: A Vision for Future Geodetic VLBI

This article summarizes the results of IVS Working Group 3 ‘VLBI2010’, which was charged with creating a vision for a new geodetic VLBI instrument that will meet requirements for the coming decades. This comes at a time when problems with aging antennas, a deteriorating radio frequency environment due to interference, obsolete electronics, and high operating costs are making it difficult to achieve the required level of performance. Fortunately, recent advances in antenna manufacture, digital electronics, and data transmission technology are enabling the development of systems and modes of operation unimaginable only a few years ago, along with much reduced costs. A set of criteria to be met by a future geodetic VLBI system was established based on recommendations in reports compiled by IVS, GGOS, and NASA. These criteria are: 1 mm measurement accuracy on global baselines, continuous measurements for time series of station positions and Earth orientation parameters, and turnaround time to initial geodetic results of less than 24 hrs.

Broadband Delay System Demonstration for VLBI2010

VLBI2010 is the next generation geodetic VLBI system being developed under the auspices of the IVS. This system is envisioned to make use of comparatively small but cost-effective 12-m class antennas together with very broadband feeds (2−14 GHz) and multiple IF channels to reliably resolve RF phase. In order to demonstrate that the “broadband delay” concept is feasible, all of the components of the broadband delay system have been implemented on the Westford 18 m and MV3 5 m antennas. The combined sensitivity of these two antennas is somewhat less than that of two 12 m antennas but should be sufficient to demonstrate that the concept can be achieved. As configured the interferometer is capable of observing four 512 MHz bands within the frequency range 3.3 GHz to 11.5 GHz in both linear polarizations with an aggregate data rate of 8 gigabits per second. Observations to date show that the instrumentation functions as expected, but the sensitivities of the antennas are lower than anticipated. Efforts are underway to optimize the installation and operation of the cooled feed and receivers on both antennas.

Westford VLBI to GPS Vertical Tie and Implications for the TRF

The difference between the heights of the VLBI and GPS antennas at Westford as found in ITRF96 differs from the optical survey value by approximately 3 cm. The height of the GPS antenna is, however, a function of the minimum elevation angle cutoff with a sensitivity of approximately -3 mm/°. A sample of other Dorne Margolin choke ring antennas of the global GPS network shows similar sensitivity, while two Trimble antennas have more severe sensitivities which are of the opposite sign. Until the elevation dependent phase error can be measured in situ there is no way to determine the absolute height of the current GPS antennas. As a consequence, the scale of the Terrestrial Reference Frame cannot be determined by GPS to better than several parts per billion, and the heights of individual sites are uncertain at the several centimeter level.