Anastasios K. Tzioumis

On the origin of radio emission in the X-ray states of xte j1650-500 during the 2001-2002 outburst

We report on simultaneous radio and X-ray observations of the black hole candidate XTE J1650-500 during the course of its 2001-2002 outburst. The scheduling of the observations allowed us to sample the properties of XTE J1650-500 in different X-ray spectral states, namely, the hard state, the steep power-law state, and the thermal dominant state, according to the recent spectral classification of McClintock & Remillard. The hard state is consistent with a compact jet dominating the spectral energy distribution at radio frequencies; however, the current data suggest that its contribution as direct synchrotron emission at higher energies may not be significant. In that case, XTE J1650-500 may be dominated by Compton processes (either inverse Comptonization of thermal disk photons and/or synchrotron self-Compton radiation from the base of the compact jet) in the X-ray regime. We surprisingly detect a faint level of radio emission in the thermal dominant state that may be consistent with the emission of previously ejected material interacting with the interstellar medium, similar (but on a smaller angular scale) to what was observed in XTE J1550-564 by Corbel and coworkers. Based on the properties of radio emission in the steep power-law state of XTE J1650-500 and taking into account the behavior of other black hole candidates (namely, GX 339-4, XTE J1550-564, and XTE J1859+226) while in the intermediate and steep power-law states, we are able to present a general pattern of behavior for the origin of radio emission in these two states that could be important for understanding the accretion-ejection coupling very close to the black hole event horizon.

Science with the Australian Square Kilometre Array Pathfinder

The future of cm and m-wave astronomy lies with the Square Kilometre Array (SKA), a telescope under development by a consortium of 17 countries that will be 50 times more sensitive than any existing radio facility. Most of the key science for the SKA will be addressed through large-area imaging of the Universe at frequencies from a few hundred MHz to a few GHz. The Australian SKA Pathfinder (ASKAP) is a technology demonstrator aimed in the mid-frequency range, and achieves instantaneous wide-area imaging through the development and deployment of phased-array feed systems on parabolic reflectors. The large field-of-view makes ASKAP an unprecedented synoptic telescope that will make substantial advances in SKA key science. ASKAP will be located at the Murchison Radio Observatory in inland Western Australia, one of the most radio-quiet locations on the Earth and one of two sites selected by the international community as a potential location for the SKA. In this paper, we outline the ASKAP project and summarise its headline science goals as defined by the community at large.

Rapid variability and annual cycles in the characteristic timescale of the scintillating source PKS 1257-326

Rapid radio intraday variability (IDV) has been discovered in the southern quasar PKS 1257-326. Flux density changes of up to 40% in as little as 45 minutes have been observed in this source, making it, along with PKS 0405 385 and J1819+3845, one of the three most rapid IDV sources known. We have monitored the IDV in this source with the Australia Telescope Compact Array (ATCA) at 4.8 and 8.6 GHz over the course of the last year and find a clear annual cycle in the characteristic timescale of variability. This annual cycle demonstrates unequivocally that interstellar scintillation is the cause of the rapid IDV at radio wavelengths observed in this source. We use the observed annual cycle to constrain the velocity of the scattering material and the angular size of the scintillating component of PKS 1257-326. We observe a time delay, which also shows an annual cycle, between the similar variability patterns at the two frequencies. We suggest that this is caused by a small (similar to10 muas) offset between the centroids of the 4.8 and 8.6 GHz components and may be due to opacity effects in the source. The statistical properties of the observed scintillation thus enable us to resolve source structure on a scale of similar to10 muas, resolution orders of magnitude higher than current VLBI techniques allow. General implications of IDV for the physical properties of sources and the turbulent interstellar medium are discussed.

Overview of the coordinated ground-based observations of Titan during the Huygens mission

Coordinated ground-based observations of Titan were performed around or during the Huygens atmospheric probe mission at Titan on 14 January 2005, connecting the momentary in situ observations by the probe with the synoptic coverage provided by continuing ground-based programs. These observations consisted of three different categories: (1) radio telescope tracking of the Huygens signal at 2040 MHz, (2) observations of the atmosphere and surface of Titan, and (3) attempts to observe radiation emitted during the Huygens Probe entry into Titans atmosphere. The Probe radio signal was successfully acquired by a network of terrestrial telescopes, recovering a vertical profile of wind speed in Titans atmosphere from 140 km altitude down to the surface. Ground-based observations brought new information on atmosphere and surface properties of the largest Saturnian moon. No positive detection of phenomena associated with the Probe entry was reported. This paper reviews all these measurements and highlights the achieved results. The ground-based observations, both radio and optical, are of fundamental importance for the interpretation of results from the Huygens mission.

Rapid interstellar scintillation of PKS 1257-326 : Two-station pattern time delays and constraints on scattering and microarcsecond source structure

We report measurements of time delays of up to 8 minutes in the centimeter-wavelength variability patterns of the intrahour scintillating quasar PKS 1257-326, as observed between the VLA and the ATCA on three separate epochs. These time delays confirm interstellar scintillation as the mechanism responsible for the rapid variability, at the same time effectively ruling out the coexistence of intrinsic intrahour variability in this source. The time delays are combined with measurements of the annual variation in variability timescale exhibited by this source to determine the characteristic length scale and anisotropy of the quasars intensity-scintillation pattern, as well as to attempt to fit for the bulk velocity of the scattering plasma responsible for the scintillation. We find evidence for anisotropic scattering and highly elongated scintillation patterns at both 4.9 and 8.5 GHz, with an axial ratio >10 : 1, extended in a northwest direction on the sky. The characteristic scale of the scintillation pattern along its minor axis is well determined, but the high anisotropy leads to degenerate solutions for the scintillation velocity. The decorrelation of the pattern over the baseline gives an estimate of the major-axis length scale of the scintillation pattern. We derive an upper limit on the distance to the scattering plasma of no more than 10 pc.

VLBI Observations of Southern Hemisphere ICRF Sources. II. Astrometric Suitability Based on Intrinsic Structure

We present 8.4 GHz very long baseline interferometry (VLBI) observations of 48 southern hemisphere extragalactic sources from the International Celestial Reference Frame. These are the second in a series of observations intended to image all such sources in the southern hemisphere at milliarcsecond resolution and bring the total number of observed sources to 111. We use these data, together with previously published data, to quantify the magnitude of the expected effect of intrinsic source structure on astrometric bandwidth synthesis VLBI observations by calculating a structure index for the sources; the structure index yields an estimate of their astrometric quality. Approximately 35% of sources in our sample have a structure index indicative of compact or very compact structures. The remaining two-thirds of our sources are less compact and should probably be avoided in astrometric and geodetic VLBI experiments requiring the highest accuracy unless intrinsic source structure can be accounted for in the astrometric/geodetic analysis.

A LINK BETWEEN X-RAY EMISSION LINES AND RADIO JETS IN 4U 1630-47?

United States. National Aeronautics and Space Administration (Einstein Postdoctoral Fellowship Grant PF2-130097)

VLBI Observations of the Gigahertz-Peaked Spectrum Galaxy PKS 1934−638

We present 8.4 GHz VLBI observations of the gigahertz-peaked spectrum source PKS 1934-638 made with the Australian Long Baseline Array. Our observations confirm the compact double nature of the source and yield measured separations of 42.7 ± 0.4 mas at epoch 2002 July and 42.6 ± 0.3 mas at epoch 2002 November, which, when combined with previous observations, yield a rate of separation of 23 ± 10 μas yr-1. This result suggests that over a timescale of 32.1 yr, the separation of the two components has changed marginally. Nominally, this provides support to the emerging consensus that compact symmetric objects (CSOs) are young sources. Our measurement of hot spot separation has, to our knowledge, the longest temporal coverage for a CSO reported in the literature.

Optical and near-infrared spectroscopy of the black hole Swift J1753.5-0127

We report on a multiwavelength observational campaign of the black hole (BH) X-ray binary Swift J1753.5–0127 that consists of an ESO/X-shooter spectrum supported by contemporaneous Swift/X-ray Telescope+Ultra-Violet/Optical Telescope (UVOT) and Australia Telescope Compact Array data. Interstellar medium absorption lines in the X-shooter spectrum allow us to determine E(B-V)=0.45 ± 0.02 along the line of sight to the source. We also report detection of emission signatures of He ii λ 4686, Hα, and, for the first time, H i λ 10906 and Paβ. The double-peaked morphology of these four lines is typical of the chromosphere of a rotating accretion disk. Nonetheless, the paucity of disk features points toward a low level of irradiation in the system. This is confirmed through spectral energy distribution modeling, and we find that the UVOT+X-shooter continuum mostly stems from the thermal emission of a viscous disk. We speculate that the absence of reprocessing is due to the compactness of an illumination-induced envelope that fails to reflect enough incoming hard X-ray photons back to the outer regions. The disk also marginally contributes to the Compton-dominated X-ray emission and is strongly truncated, with an inner radius about 1000 times larger than the BHs gravitational radius. A near-infrared excess is present, and we associate it with synchrotron radiation from a compact jet. However, the measured X-ray flux is significantly higher than what can be explained by the optically thin synchrotron jet component. We discuss these findings in the framework of the radio-quiet versus X-ray-bright hypothesis, favoring the presence of a residual disk, predicted by evaporation models, that contributes to the X-ray emission without enhancing the radio flux.

Milliarcsecond-Accurate Astrometry of 34 Southern Hemisphere Radio Sources

Milliarcsecond-accurate radio positions in the International Celestial Reference Frame for 34 southern hemisphere extragalactic sources are reported. The positions are derived from Mark III/IV very long baseline interferometry observations made between 2004 August and 2006 April using radio telescopes located in Australia, South Africa, and Hawaii. Positions for 7 of the 34 sources have been reported previously by us but are reported here with significantly improved accuracy. These results supplement an ongoing project to increase the sky density of southern hemisphere sources in order to better define the International Celestial Reference Frame and to provide additional phase reference sources with accurate positions for use in astrophysical observations. Positions for all 34 sources are south of δ = -20° (positions for nine of the sources are south of δ = -60°) and have average formal uncertainties of 0.23 mas in α cos δ and 0.35 mas in δ.