Showing papers by "Richard Dodson published in 2005"

The complex wind torus and jets of PSR B1706-44

Abstract: We report on Chandra ACIS imaging of the pulsar wind nebula (PWN) of the young Vela-like PSR B1706-44, which shows the now common pattern of an equatorial wind and polar jets. The structure is particularly rich, showing a relativistically boosted termination shock, jets with strong confinement, a surrounding radio/X-ray PWN, and evidence for a quasistatic "bubble nebula." The structures trace the pulsar spin geometry and illuminate its possible relation to SNR G343.1-2.3. We also obtain improved estimates of the pulsar flux and nebular spectrum, constraining the system age and energetics.

Milli-arcsecond--scale Spectral Properties and Jet Motions in M87

Abstract: We have combined high resolution VLBI Space Observatory Programme (VSOP) data at 1.6 and 4.8 GHz with Very Long Baseline Array (VLBA) data at higher frequencies and with similar resolutions to study the spectral properties of the core of M87 with milliarcsecond resolution. The VSOP data allow a more accurate measurement of the turn-over frequency, and hence more reliable determination of associated physical parameters of the source. Comparison of the images with previously published images yields no evidence for significant motion of components in the parsec-scale jet. In addition, the brightness temperatures obtained from model-fits to the core are well below the inverse Compton limit, suggesting the radio emission we are observing is not strongly Doppler boosted. Colour version on this http URL

COSMIC: Microarcsecond Resolution with a 30 Meter Radio Telescope

Abstract: Interstellar scintillation has been conclusively demonstrated to be the principal cause of the intraday variability (IDV) observed in the centimeter-wavelength emission of many active galactic nuclei A few sources show large amplitude modulation in their flux density on a timescale of hours However, the majority of IDV sources exhibit variability on timescales of a day or more Some sources have been found to show an annual cycle in the pattern of their variability Such an annual cycle occurs because the relative speeds of the Earth and the interstellar medium change as the Earth orbits the Sun To search for these annual variations, as well as to follow the source evolution, requires a dedicated instrument; the necessary amounts of observing time are beyond the capability of the National Facility instruments Here we describe the scientific motivation for and present an outline of the COSMIC (Continuous Single-Dish Monitoring of Intraday Variability at Ceduna) project, which uses the University of Tasmania's 30 m diameter radio telescope at Ceduna, which has been monitoring the flux density of a number of the stronger southern scintillators at 665 GHz since 2003 March

Measurement of core-shifts with astrometric multi-frequency calibration

Abstract: VLBI is unique, among the space geodetic techniques, in its contribution to defining and maintaining the International Celestial Reference Frame, providing precise measurements of coordinates of extragalactic radiosources. The quest for increasing accuracy of VLBI geodetic products has lead to a deeper revision of all aspects that might introduce errors in the analysis. The departure of the observed sources from perfect, stable, compact and achromatic celestial targets falls within this category. This paper is concerned with the impact of unaccounted frequency-dependent position shifts of source cores in the analysis of dual-band S/X VLBI geodesy observations, and proposes a new method to measure them. The multi-frequency phase transfer technique developed and demonstrated by Middelberg et al. (2005) increases the high frequency coherence times of VLBI observations, using the observations at a lower frequency. Our proposed SOURCE/FREQUENCY PHASE REFERENCING method endows it with astrometric applications by adding a strategy to estimate the ionospheric contributions. Here we report on the first successful application to measure the core shift of the quasar 1038+528 A at S and X-bands, and validate the results by comparison with those from standard phase referencing techniques. In this particular case, and in general in the cm-wavelength regime, both methods are equivalent. Moreover the proposed method opens a new horizon with targets and fields suitable for high precision astrometric studies with VLBI, especially at high frequencies where severe limitations imposed by the rapid fluctuations in the troposphere prevent the use of standard phase referencing techniques.