Symposium - International Astronomical Union, 2002
This paper reviews planetary radio astronomy at low frequencies. At least at frequencies observab... more This paper reviews planetary radio astronomy at low frequencies. At least at frequencies observable from the Earth's surface, this field has been almost solely the study of Jovian magnetospheric emissions. With the discovery of extra-solar planets, and the potential for new telescopes with large collecting areas, we may well see more objects and emission mechanisms becoming detectable.
We present the first observation of the diffuse polarized synchrotron radiation of a patch (∼ 3 •... more We present the first observation of the diffuse polarized synchrotron radiation of a patch (∼ 3 • × 3 •) in the BOOMERanG field, one of the areas with the lowest CMB foreground emission. The work has been carried out with the Australia Telescope Compact Array at 1.4 GHz with 3.4 arcmin resolution and sensitivity of ∼ 0.18 mJy beam −1. The mean polarized signal has been found to be P rms = (Q 2 rms + U 2 rms) = 11.6 ± 0.6 mK, nearly one order of magnitude below than in the Galactic Plane. Extrapolations to frequencies of interest for cosmological investigations suggest that polarized synchrotron foreground noise should allow the detection of the CMB Polarization E-mode already at 32 GHz and make us confident that, at 90 GHz, it is accessible with no relevant foreground contamination. Last but not least, even the B-mode detection for T /S > 0.01 is not ruled out in such a low emission patch.
Recently Bower et al. (1999b) have reported the detection of circular polarization from the Galac... more Recently Bower et al. (1999b) have reported the detection of circular polarization from the Galactic Center black hole candidate, Sagittarius A*. We provide an independent confirmation of this detection, and provide some analysis on the possible mechanisms.
The Murchison Widefield Array (MWA) is a new low-frequency, wide field-of-view radio interferomet... more The Murchison Widefield Array (MWA) is a new low-frequency, wide field-of-view radio interferometer under development at the Murchison Radio-astronomy Observatory (MRO) in Western Australia. We have used a 32-element MWA prototype interferometer (MWA-32T) to observe two 50 • diameter fields in the southern sky in the 110 MHz to 200 MHz band in order to evaluate the performance of the MWA-32T, to develop techniques for epoch of reionization experiments, and to make measurements of astronomical foregrounds. We developed a calibration and imaging pipeline for the MWA-32T, and used it to produce ∼ 15 ′ angular resolution maps of the two fields. We perform a blind source extraction using these confusion-limited images, and detect 655 sources at high significance with an additional 871 lower significance source candidates. We compare these sources with existing lowfrequency radio surveys in order to assess the MWA-32T system performance, wide field analysis algorithms, and catalog quality. Our source catalog is found to agree well with existing low-frequency surveys in these regions of the sky and with statistical distributions of point sources derived from Northern Hemisphere surveys; it represents one of the deepest surveys to date of this sky field in the 110 MHz to 200 MHz band.
We report on radio observations of the 1999 September event of the X-ray transient V4641 Sgr (\XT... more We report on radio observations of the 1999 September event of the X-ray transient V4641 Sgr (\XTE J1819[254\SAX J1819.3[2525). This event was extremely rapid in its rise and decay across radio, optical, and X-ray wavelengths ; the X-rays rose to 12 crab within 8 hr and faded to below 0.1 crab in less than 2 hr. Radio observations were made with seven telescopes during the Ðrst day following the onset of the strong X-ray event, revealing a strong radio source that was detected for 3 further weeks by the more sensitive telescopes. The radio source was resolved even in the Ðrst Very Large Array (VLA) images (September 16.027 UT), being long with an axis ratio of at least 10 : 1. The total Ñux D0A .25 density decayed by a factor of D4 over the Ðrst day, and by September 17.94 UT the radio emission was conÐned to a slowly decaying, marginally resolved remnant located at one side of the early elongated emission. The H I absorption spectrum gives a minimum kinematic distance of about 400 pc ; various other arguments suggest that the true distance is not much greater than this. The inferred proper motions for the early extended emission day~1) correspond to v/c D 1.0È (0A .4È1A .1 3.2 (d/0.5 kpc), and this together with the radio morphology argues that this is a relativistic jet source like GRS 1915]105 and GRO J1655[40. The proper motion of the late-time remnant is at least 100 times smaller. One simple interpretation posits the ejection of a single short-lived jet segment, followed by a more slowly decaying, optically thin jet segment ejection. These two components can explain both the multifrequency radio light curves and the radio images. The most likely parameters for the fast-jet system with net-averaged proper motion of day~1, assuming d \ 0.5 kpc, are v D 0.85c and D0A .4 i D 63¡, where i is the inclination to the line of sight. The corresponding apparent velocities are 1.4c and 0.6c for the approaching and receding jets, making V4641 Sgr the closest superluminal jet source known.
We have performed monitoring observations of the 3-mm flux density toward the Galactic Center com... more We have performed monitoring observations of the 3-mm flux density toward the Galactic Center compact radio source Sgr A* with the Australia Telescope Compact Array since 2005 October. Careful calibrations of both elevationdependent and time-dependent gains have enabled us to establish the variability behavior of Sgr A*. Sgr A* appeared to undergo a high and stable state in 2006 June session, and a low and variable state in 2006 August session. We report the results, with emphasis on two detected intra-day variation events during its low states. One is on 2006 August 12 when Sgr A* exhibited a 33% fractional variation in about 2.5 hr. The other is on 2006 August 13 when two peaks separated by about 4 hr, with a maximum variation of 21% within 2 hr, were seen. The observed short timescale variations are discussed in light of two possible scenarios, i.e., the expanding plasmon model and the sub-Keplerian orbiting hot spot model. The fitting results indicate that for the adiabatically expanding plasmon model, the synchrotron cooling can not be ignored, and a minimum mass-loss rate of 9.7 × 10 −10 M ⊙ yr −1 is obtained based on parameters derived for this modified expanding plasmon model. Simultaneous multi-wavelength observation is crucial to our understanding the physical origin of rapid radio variability in Sgr A*.
We report the discovery of circularly polarised radio emission from the radio-jet X-ray binary SS... more We report the discovery of circularly polarised radio emission from the radio-jet X-ray binary SS 433 with the Australia Telescope Compact Array. The flux density spectrum of the circular polarization, clearly detected at four frequencies between 1 -9 GHz, is of the form V ∝ ν -0.9±0.1 . Multiple components in the source and a lack of very high spatial resolution do not allow a unique determination of the origin of the circular polarization, nor of the spectrum of fractional polarization. However, we argue that the emission is likely to arise in the inner regions of the binary, possibly via propagation-induced conversion of linear to circular polarization, and the fractional circular polarization of these regions may be as high as 10%. Observations such as these have the potential to investigate the composition, whether pairs or baryonic, of the ejecta from X-ray binaries.
We report multifrequency radio observations of GRO J1655−40 obtained with the Australia Telescope... more We report multifrequency radio observations of GRO J1655−40 obtained with the Australia Telescope Compact Array, the Molonglo Observatory Synthesis Telescope and the Hartebeesthoek Radio Astronomy Observatory at the time of the major hard X-ray and radio outbursts in 1994 August-September. The radio emission reached levels of the order of a few Jy and was found to be linearly polarized by up to 10%, indicating a synchrotron origin. The light curves are in good agreement with those measured with the VLA, but our closer time sampling has revealed two new short-lived events and significant deviations from a simple exponential decay. The polarization
We present a Stokes I, Q and U survey at 189 MHz with the Murchison Widefield Array 32 element pr... more We present a Stokes I, Q and U survey at 189 MHz with the Murchison Widefield Array 32 element prototype covering 2400 deg 2. The survey has a 15.6 arcmin angular resolution and achieves a noise level of 15 mJy beam −1. We demonstrate a novel interferometric data analysis that involves calibration of drift scan data, integration through the co-addition of warped snapshot images, and deconvolution of the point-spread function through forward modeling. We present a point source catalog down to a flux limit of 4 Jy. We detect polarization from only one of the sources, PMN J0351-2744, at a level of 1.8% ± 0.4%, whereas the remaining sources have a polarization fraction below 2%. Compared to a reported average value of 7% at 1.4 GHz, the polarization fraction of compact sources significantly decreases at low frequencies. We find a wealth of diffuse polarized emission across a large area of the survey with a maximum peak of ∼13 K, primarily with positive rotation measure values smaller than +10 rad m −2. The small values observed indicate that the emission is likely to have a local origin (closer than a few hundred parsecs). There is a large sky area at α 2 h 30 m where the diffuse polarized emission rms is fainter than 1 K. Within this area of low Galactic polarization we characterize the foreground properties in a cold sky patch at (α, δ) = (4 h , −27. • 6) in terms of three-dimensional power spectra.
This paper is part of a series describing the results from the Australia Telescope Hubble Deep Fi... more This paper is part of a series describing the results from the Australia Telescope Hubble Deep Field South (ATHDFS) survey obtained with the Australia Telescope Compact Array (ATCA). This survey consists of observations at 1.4, 2.5, 5.2 and 8.7 GHz, all centred on the Hubble Deep Field South. Here we present the first results from the extended observing campaign at 1.4 GHz. A total of 466 sources have been catalogued to a local sensitivity of 5σ (11 µJy rms). A source extraction technique is developed which: 1) successfully excludes spurious sources from the final source catalogues, and 2) accounts for the non-uniform noise in our image. A source catalogue is presented and the
Planets have traditionally been used as the flux density scale in this band, with Mars and Jupite... more Planets have traditionally been used as the flux density scale in this band, with Mars and Jupiter being the most appropriate for the ATCA. The MIRIAD task plboot implements a technique to use planets as primaries for a flux density scale. MIRIAD assumes a uniform black body of given brightness tempera-ture. The radio size of the planet's disk is taken as optical size: ephemerides are computed to determine the apparent size and orientation of the planetary spheroid on the sky. Given these, the expected visibility function can be com-puted.
Monthly Notices of the Royal Astronomical Society, 2005
We analyse the polarized emission at 1.4 GHz in a 3 • × 3 • area at high Galactic latitude (b ∼ −... more We analyse the polarized emission at 1.4 GHz in a 3 • × 3 • area at high Galactic latitude (b ∼ −40 •). The region, centred in (α = 5 h , δ = −49 •), was observed with the Australia Telescope Compact Array (ATCA) radio-interferometer, whose 3-30 arcmin angular sensitivity range allows the study of scales appropriate for cosmic microwave background polarization (CMBP) investigations. The angular behaviour of the diffuse emission is analysed through the E-and B-mode angular power spectra. These follow a power law C X ∝ β X with slopes β E = −1.97 ± 0.08 and β B = −1.98 ± 0.07. The emission is found to be approximately a factor 25 fainter than in Galactic plane regions. The comparison of the power spectra with other surveys indicates that this area is intermediate between strong and negligible Faraday rotation effects. A similar conclusion can be reached by analysing both the frequency and Galactic latitude behaviours of the diffuse Galactic emission of the 408-1411 MHz Leiden survey data. We present an analysis of the Faraday rotation effects on the polarized power spectra and find that the observed power spectra can be enhanced by a transfer of power from large to small angular scales. The extrapolation of the spectra to 32 and 90 GHz of the cosmic microwave background (CMB) window suggests that Galactic synchrotron emission leaves the CMBP E-mode uncontaminated at 32 GHz. The level of the contamination at 90 GHz is expected to be more than 4 orders of magnitude below the CMBP spectrum. Extrapolating to the relevant angular scales, this region also appears adequate for investigation of the CMBP B-modes for models with tensor-to-scalar fluctuation power ratio T/S > 0.01. We also identify polarized point sources in the field, providing a nine object list, which is complete down to the polarized flux limit of S p lim = 2 mJy.
Monthly Notices of the Royal Astronomical Society, 2013
We present full Stokes radio polarization observations of the quasar PKS B2126−158 (z = 3.268) fr... more We present full Stokes radio polarization observations of the quasar PKS B2126−158 (z = 3.268) from 1 to 10 GHz using the Australia Telescope Compact Array. The source has large fractional circular polarization (CP), m c ≡ |V|/I, detected at high significance across the entire band (from 15 to 90σ per 128 MHz subband). This allows us to construct the most robust CP spectrum of an active galactic nucleus (AGN) jet to date. We find m c ∝ ν +0.60 ± 0.03 from 1.5 to 6.5 GHz, with a peak of m c ∼ 1 per cent before the spectrum turns over somewhere between 6.5 and 8 GHz, above which m c ∝ ν −3.0 ± 0.4. The fractional linear polarization (LP; p) varies from 0.2 to ∼1 per cent across our frequency range and is strongly anticorrelated with the fractional CP, with a best-fitting power law giving m c ∝ p −0.24 ± 0.03. This is the first clear relation between the observed LP and CP of an AGN jet, revealing the action of Faraday conversion of LP to CP within the jet. More detailed modelling in conjunction with high spatial resolution observations are required to determine the true driving force behind the conversion (i.e. magnetic twist or internal Faraday rotation). In particular determining whether the observed Faraday rotation is internal or entirely external to the jet is key to this goal. The simplest interpretation of our observations favours some internal Faraday rotation, implying that Faraday rotation-driven conversion of LP to CP is the dominant CP generation mechanism. In this case, a small amount of vector-ordered magnetic field along the jet axis is required, along with internal Faraday rotation from the low-energy end of the relativistic electron energy spectrum in an electron-proton-dominated jet.
Monthly Notices of the Royal Astronomical Society, 2013
We present new wide-field observations of Centaurus A (Cen A) and the surrounding region at 118 M... more We present new wide-field observations of Centaurus A (Cen A) and the surrounding region at 118 MHz with the Murchison Widefield Array (MWA) 32-tile prototype, with which we investigate the spectral-index distribution of Cen A's giant radio lobes. We compare our images to 1.4 GHz maps of Cen A and compute spectral indices using temperature-temperature plots and spectral tomography. We find that the morphologies at 118 MHz and 1.4 GHz match very closely apart from an extra peak in the southern lobe at 118 MHz, which provides tentative evidence for the existence of a southern counterpart to the northern middle lobe of Cen A. Our spatially averaged spectral indices for both the northern and southern lobes are consistent with
Monthly Notices of the Royal Astronomical Society, 2006
We present results of polarization observations at 1.4 GHz of the two fields imaged by the DASI e... more We present results of polarization observations at 1.4 GHz of the two fields imaged by the DASI experiment (α = 23 h 30 m , δ = −55 • and α = 00 h 30 m , δ = −55 • , respectively). Data were taken with the Australia Telescope Compact Array with 3.4-arcmin resolution and ∼0.18-mJy beam −1 sensitivity. The emission is dominated by point sources, and we do not find evidence for diffuse synchrotron radiation even after source subtraction. This allows to estimate an upper limit of the diffuse polarized emission. The extrapolation to 30 GHz suggests that the synchrotron radiation is lower than the polarized signal measured by the DASI experiment by at least two orders of magnitude. This further supports the conclusions drawn by the DASI team itself about the negligible Galactic foreground contamination in their data set, improving by a factor of ∼5 the upper limit estimated by Leitch et al. The dominant point-source emission allows us to estimate the contamination of the cosmic microwave background (CMB) by extragalactic foregrounds. We computed the power spectrum of their contribution, and its extrapolation to 30 GHz provides a framework where the CMB signal should dominate. However, our results do not match the conclusions of the DASI team about the negligibility of point-source contamination, suggesting taking into account a source subtraction from the DASI data.
Monthly Notices of the Royal Astronomical Society, 2010
We present the full source catalogue from the Australia Telescope 20 GHz (AT20G) Survey. The AT20... more We present the full source catalogue from the Australia Telescope 20 GHz (AT20G) Survey. The AT20G is a blind radio survey carried out at 20 GHz with the Australia Telescope Compact Array (ATCA) from 2004 to 2008, and covers the whole sky south of declination 0 •. The AT20G source catalogue presented here is an order of magnitude larger than any previous catalogue of high-frequency radio sources, and includes 5890 sources above a 20 GHz flux-density limit of 40 mJy. All AT20G sources have total intensity and polarization measured at 20 GHz, and most sources south of declination −15 • also have near-simultaneous flux-density measurements at 5 and 8 GHz. A total of 1559 sources were detected in polarized total intensity at one or more of the three frequencies. The completeness of the AT20G source catalogue is 91 per cent above 100 mJy beam −1 and 79 per cent above 50 mJy beam −1 in regions south of declination −15 •. North of −15 • , some observations of sources between 14 and 20 h in right ascension were lost due to bad weather and could not be repeated, so the catalogue completeness is lower in this region. Each detected source was visually inspected as part of our quality control process, and so the reliability of the final catalogue is essentially 100 per cent. We detect a small but significant population of non-thermal sources that are either undetected or have only weak detections in low-frequency catalogues. We introduce the term Ultra-Inverted Spectrum to describe these radio sources, which have a spectral index α(5, 20) > +0.7 and which constitute roughly 1.2 per cent of the AT20G sample.
Monthly Notices of the Royal Astronomical Society, 1996
We report accurate circular polarization measurements of four bright OH megamasers (III Zw 35, 04... more We report accurate circular polarization measurements of four bright OH megamasers (III Zw 35, 04454-4838, 10039-3338 and 20100-4156) with the Australia Telescope Compact Array. No convincing Zeeman splitting was observed. We place 30' upper limits of approximately 3 mG on any uniform line-of-sight magnetic field present in the emitting regions. If the field is non-uniform, then we can place upper limits of approximately 5 mG. These limits in turn imply constraints on the physical conditions of the maser emission regions which lie in the central 100 pc of megamaser galaxies.
Monthly Notices of the Royal Astronomical Society, 2002
We report the discovery of variable circularly polarized radio emission associated with relativis... more We report the discovery of variable circularly polarized radio emission associated with relativistic ejections from GRS 1915 + 105, based on observations with the Australia Telescope Compact Array (ATCA) and the Multi-Element Radio-Linked Interferometer Network (MERLIN). Following a radio flare in 2001 January, significant and variable circular polarization, at a fractional level of 0.2-0.4 per cent, was measured with ATCA at four frequencies between 1 and 9 GHz. Following an additional outburst 65 d later in 2001 March, further ATCA observations measured a comparable sign and level of circular polarization at two frequencies. At this second epoch, contemporaneous MERLIN observations directly imaged a relativistic ejection event and allowed us confidently to associate both the circularly and the linearly polarized emission with the relativistic ejecta, allowing a detailed measurement of the full polarization properties in the optically thin phase. The fractional circular polarization spectrum appears to flatten at higher frequencies in 2001 January, when there is strong evidence for multiple components at different optical depths. While we cannot conclusively distinguish between synchrotron or propagation-induced conversion as the origin of the circularly polarized component, we do not consider that coherent or birefringent scintillation mechanisms are likely. The implication is therefore that the ejections from GRS 1915 + 105 are associated with a significant population of low-energy electrons, with associated consequences for the energetics of relativistic ejection events. We briefly compare the data for SS 433 and GRS 1915 + 105 with those for active galactic nuclei, and note that linear-to-circular polarization ratios <1 are observed in both binary sources at the lowest frequency, and ratios >1 at the higher frequencies, illustrating the role of Faraday depolarization. In addition, the 2001 January ATCA observations reveal a linear polarization 'rotator' event, probing the (variable) orientation of the magnetic field structure in the outflow.
Symposium - International Astronomical Union, 2002
This paper reviews planetary radio astronomy at low frequencies. At least at frequencies observab... more This paper reviews planetary radio astronomy at low frequencies. At least at frequencies observable from the Earth's surface, this field has been almost solely the study of Jovian magnetospheric emissions. With the discovery of extra-solar planets, and the potential for new telescopes with large collecting areas, we may well see more objects and emission mechanisms becoming detectable.
We present the first observation of the diffuse polarized synchrotron radiation of a patch (∼ 3 •... more We present the first observation of the diffuse polarized synchrotron radiation of a patch (∼ 3 • × 3 •) in the BOOMERanG field, one of the areas with the lowest CMB foreground emission. The work has been carried out with the Australia Telescope Compact Array at 1.4 GHz with 3.4 arcmin resolution and sensitivity of ∼ 0.18 mJy beam −1. The mean polarized signal has been found to be P rms = (Q 2 rms + U 2 rms) = 11.6 ± 0.6 mK, nearly one order of magnitude below than in the Galactic Plane. Extrapolations to frequencies of interest for cosmological investigations suggest that polarized synchrotron foreground noise should allow the detection of the CMB Polarization E-mode already at 32 GHz and make us confident that, at 90 GHz, it is accessible with no relevant foreground contamination. Last but not least, even the B-mode detection for T /S > 0.01 is not ruled out in such a low emission patch.
Recently Bower et al. (1999b) have reported the detection of circular polarization from the Galac... more Recently Bower et al. (1999b) have reported the detection of circular polarization from the Galactic Center black hole candidate, Sagittarius A*. We provide an independent confirmation of this detection, and provide some analysis on the possible mechanisms.
The Murchison Widefield Array (MWA) is a new low-frequency, wide field-of-view radio interferomet... more The Murchison Widefield Array (MWA) is a new low-frequency, wide field-of-view radio interferometer under development at the Murchison Radio-astronomy Observatory (MRO) in Western Australia. We have used a 32-element MWA prototype interferometer (MWA-32T) to observe two 50 • diameter fields in the southern sky in the 110 MHz to 200 MHz band in order to evaluate the performance of the MWA-32T, to develop techniques for epoch of reionization experiments, and to make measurements of astronomical foregrounds. We developed a calibration and imaging pipeline for the MWA-32T, and used it to produce ∼ 15 ′ angular resolution maps of the two fields. We perform a blind source extraction using these confusion-limited images, and detect 655 sources at high significance with an additional 871 lower significance source candidates. We compare these sources with existing lowfrequency radio surveys in order to assess the MWA-32T system performance, wide field analysis algorithms, and catalog quality. Our source catalog is found to agree well with existing low-frequency surveys in these regions of the sky and with statistical distributions of point sources derived from Northern Hemisphere surveys; it represents one of the deepest surveys to date of this sky field in the 110 MHz to 200 MHz band.
We report on radio observations of the 1999 September event of the X-ray transient V4641 Sgr (\XT... more We report on radio observations of the 1999 September event of the X-ray transient V4641 Sgr (\XTE J1819[254\SAX J1819.3[2525). This event was extremely rapid in its rise and decay across radio, optical, and X-ray wavelengths ; the X-rays rose to 12 crab within 8 hr and faded to below 0.1 crab in less than 2 hr. Radio observations were made with seven telescopes during the Ðrst day following the onset of the strong X-ray event, revealing a strong radio source that was detected for 3 further weeks by the more sensitive telescopes. The radio source was resolved even in the Ðrst Very Large Array (VLA) images (September 16.027 UT), being long with an axis ratio of at least 10 : 1. The total Ñux D0A .25 density decayed by a factor of D4 over the Ðrst day, and by September 17.94 UT the radio emission was conÐned to a slowly decaying, marginally resolved remnant located at one side of the early elongated emission. The H I absorption spectrum gives a minimum kinematic distance of about 400 pc ; various other arguments suggest that the true distance is not much greater than this. The inferred proper motions for the early extended emission day~1) correspond to v/c D 1.0È (0A .4È1A .1 3.2 (d/0.5 kpc), and this together with the radio morphology argues that this is a relativistic jet source like GRS 1915]105 and GRO J1655[40. The proper motion of the late-time remnant is at least 100 times smaller. One simple interpretation posits the ejection of a single short-lived jet segment, followed by a more slowly decaying, optically thin jet segment ejection. These two components can explain both the multifrequency radio light curves and the radio images. The most likely parameters for the fast-jet system with net-averaged proper motion of day~1, assuming d \ 0.5 kpc, are v D 0.85c and D0A .4 i D 63¡, where i is the inclination to the line of sight. The corresponding apparent velocities are 1.4c and 0.6c for the approaching and receding jets, making V4641 Sgr the closest superluminal jet source known.
We have performed monitoring observations of the 3-mm flux density toward the Galactic Center com... more We have performed monitoring observations of the 3-mm flux density toward the Galactic Center compact radio source Sgr A* with the Australia Telescope Compact Array since 2005 October. Careful calibrations of both elevationdependent and time-dependent gains have enabled us to establish the variability behavior of Sgr A*. Sgr A* appeared to undergo a high and stable state in 2006 June session, and a low and variable state in 2006 August session. We report the results, with emphasis on two detected intra-day variation events during its low states. One is on 2006 August 12 when Sgr A* exhibited a 33% fractional variation in about 2.5 hr. The other is on 2006 August 13 when two peaks separated by about 4 hr, with a maximum variation of 21% within 2 hr, were seen. The observed short timescale variations are discussed in light of two possible scenarios, i.e., the expanding plasmon model and the sub-Keplerian orbiting hot spot model. The fitting results indicate that for the adiabatically expanding plasmon model, the synchrotron cooling can not be ignored, and a minimum mass-loss rate of 9.7 × 10 −10 M ⊙ yr −1 is obtained based on parameters derived for this modified expanding plasmon model. Simultaneous multi-wavelength observation is crucial to our understanding the physical origin of rapid radio variability in Sgr A*.
We report the discovery of circularly polarised radio emission from the radio-jet X-ray binary SS... more We report the discovery of circularly polarised radio emission from the radio-jet X-ray binary SS 433 with the Australia Telescope Compact Array. The flux density spectrum of the circular polarization, clearly detected at four frequencies between 1 -9 GHz, is of the form V ∝ ν -0.9±0.1 . Multiple components in the source and a lack of very high spatial resolution do not allow a unique determination of the origin of the circular polarization, nor of the spectrum of fractional polarization. However, we argue that the emission is likely to arise in the inner regions of the binary, possibly via propagation-induced conversion of linear to circular polarization, and the fractional circular polarization of these regions may be as high as 10%. Observations such as these have the potential to investigate the composition, whether pairs or baryonic, of the ejecta from X-ray binaries.
We report multifrequency radio observations of GRO J1655−40 obtained with the Australia Telescope... more We report multifrequency radio observations of GRO J1655−40 obtained with the Australia Telescope Compact Array, the Molonglo Observatory Synthesis Telescope and the Hartebeesthoek Radio Astronomy Observatory at the time of the major hard X-ray and radio outbursts in 1994 August-September. The radio emission reached levels of the order of a few Jy and was found to be linearly polarized by up to 10%, indicating a synchrotron origin. The light curves are in good agreement with those measured with the VLA, but our closer time sampling has revealed two new short-lived events and significant deviations from a simple exponential decay. The polarization
We present a Stokes I, Q and U survey at 189 MHz with the Murchison Widefield Array 32 element pr... more We present a Stokes I, Q and U survey at 189 MHz with the Murchison Widefield Array 32 element prototype covering 2400 deg 2. The survey has a 15.6 arcmin angular resolution and achieves a noise level of 15 mJy beam −1. We demonstrate a novel interferometric data analysis that involves calibration of drift scan data, integration through the co-addition of warped snapshot images, and deconvolution of the point-spread function through forward modeling. We present a point source catalog down to a flux limit of 4 Jy. We detect polarization from only one of the sources, PMN J0351-2744, at a level of 1.8% ± 0.4%, whereas the remaining sources have a polarization fraction below 2%. Compared to a reported average value of 7% at 1.4 GHz, the polarization fraction of compact sources significantly decreases at low frequencies. We find a wealth of diffuse polarized emission across a large area of the survey with a maximum peak of ∼13 K, primarily with positive rotation measure values smaller than +10 rad m −2. The small values observed indicate that the emission is likely to have a local origin (closer than a few hundred parsecs). There is a large sky area at α 2 h 30 m where the diffuse polarized emission rms is fainter than 1 K. Within this area of low Galactic polarization we characterize the foreground properties in a cold sky patch at (α, δ) = (4 h , −27. • 6) in terms of three-dimensional power spectra.
This paper is part of a series describing the results from the Australia Telescope Hubble Deep Fi... more This paper is part of a series describing the results from the Australia Telescope Hubble Deep Field South (ATHDFS) survey obtained with the Australia Telescope Compact Array (ATCA). This survey consists of observations at 1.4, 2.5, 5.2 and 8.7 GHz, all centred on the Hubble Deep Field South. Here we present the first results from the extended observing campaign at 1.4 GHz. A total of 466 sources have been catalogued to a local sensitivity of 5σ (11 µJy rms). A source extraction technique is developed which: 1) successfully excludes spurious sources from the final source catalogues, and 2) accounts for the non-uniform noise in our image. A source catalogue is presented and the
Planets have traditionally been used as the flux density scale in this band, with Mars and Jupite... more Planets have traditionally been used as the flux density scale in this band, with Mars and Jupiter being the most appropriate for the ATCA. The MIRIAD task plboot implements a technique to use planets as primaries for a flux density scale. MIRIAD assumes a uniform black body of given brightness tempera-ture. The radio size of the planet's disk is taken as optical size: ephemerides are computed to determine the apparent size and orientation of the planetary spheroid on the sky. Given these, the expected visibility function can be com-puted.
Monthly Notices of the Royal Astronomical Society, 2005
We analyse the polarized emission at 1.4 GHz in a 3 • × 3 • area at high Galactic latitude (b ∼ −... more We analyse the polarized emission at 1.4 GHz in a 3 • × 3 • area at high Galactic latitude (b ∼ −40 •). The region, centred in (α = 5 h , δ = −49 •), was observed with the Australia Telescope Compact Array (ATCA) radio-interferometer, whose 3-30 arcmin angular sensitivity range allows the study of scales appropriate for cosmic microwave background polarization (CMBP) investigations. The angular behaviour of the diffuse emission is analysed through the E-and B-mode angular power spectra. These follow a power law C X ∝ β X with slopes β E = −1.97 ± 0.08 and β B = −1.98 ± 0.07. The emission is found to be approximately a factor 25 fainter than in Galactic plane regions. The comparison of the power spectra with other surveys indicates that this area is intermediate between strong and negligible Faraday rotation effects. A similar conclusion can be reached by analysing both the frequency and Galactic latitude behaviours of the diffuse Galactic emission of the 408-1411 MHz Leiden survey data. We present an analysis of the Faraday rotation effects on the polarized power spectra and find that the observed power spectra can be enhanced by a transfer of power from large to small angular scales. The extrapolation of the spectra to 32 and 90 GHz of the cosmic microwave background (CMB) window suggests that Galactic synchrotron emission leaves the CMBP E-mode uncontaminated at 32 GHz. The level of the contamination at 90 GHz is expected to be more than 4 orders of magnitude below the CMBP spectrum. Extrapolating to the relevant angular scales, this region also appears adequate for investigation of the CMBP B-modes for models with tensor-to-scalar fluctuation power ratio T/S > 0.01. We also identify polarized point sources in the field, providing a nine object list, which is complete down to the polarized flux limit of S p lim = 2 mJy.
Monthly Notices of the Royal Astronomical Society, 2013
We present full Stokes radio polarization observations of the quasar PKS B2126−158 (z = 3.268) fr... more We present full Stokes radio polarization observations of the quasar PKS B2126−158 (z = 3.268) from 1 to 10 GHz using the Australia Telescope Compact Array. The source has large fractional circular polarization (CP), m c ≡ |V|/I, detected at high significance across the entire band (from 15 to 90σ per 128 MHz subband). This allows us to construct the most robust CP spectrum of an active galactic nucleus (AGN) jet to date. We find m c ∝ ν +0.60 ± 0.03 from 1.5 to 6.5 GHz, with a peak of m c ∼ 1 per cent before the spectrum turns over somewhere between 6.5 and 8 GHz, above which m c ∝ ν −3.0 ± 0.4. The fractional linear polarization (LP; p) varies from 0.2 to ∼1 per cent across our frequency range and is strongly anticorrelated with the fractional CP, with a best-fitting power law giving m c ∝ p −0.24 ± 0.03. This is the first clear relation between the observed LP and CP of an AGN jet, revealing the action of Faraday conversion of LP to CP within the jet. More detailed modelling in conjunction with high spatial resolution observations are required to determine the true driving force behind the conversion (i.e. magnetic twist or internal Faraday rotation). In particular determining whether the observed Faraday rotation is internal or entirely external to the jet is key to this goal. The simplest interpretation of our observations favours some internal Faraday rotation, implying that Faraday rotation-driven conversion of LP to CP is the dominant CP generation mechanism. In this case, a small amount of vector-ordered magnetic field along the jet axis is required, along with internal Faraday rotation from the low-energy end of the relativistic electron energy spectrum in an electron-proton-dominated jet.
Monthly Notices of the Royal Astronomical Society, 2013
We present new wide-field observations of Centaurus A (Cen A) and the surrounding region at 118 M... more We present new wide-field observations of Centaurus A (Cen A) and the surrounding region at 118 MHz with the Murchison Widefield Array (MWA) 32-tile prototype, with which we investigate the spectral-index distribution of Cen A's giant radio lobes. We compare our images to 1.4 GHz maps of Cen A and compute spectral indices using temperature-temperature plots and spectral tomography. We find that the morphologies at 118 MHz and 1.4 GHz match very closely apart from an extra peak in the southern lobe at 118 MHz, which provides tentative evidence for the existence of a southern counterpart to the northern middle lobe of Cen A. Our spatially averaged spectral indices for both the northern and southern lobes are consistent with
Monthly Notices of the Royal Astronomical Society, 2006
We present results of polarization observations at 1.4 GHz of the two fields imaged by the DASI e... more We present results of polarization observations at 1.4 GHz of the two fields imaged by the DASI experiment (α = 23 h 30 m , δ = −55 • and α = 00 h 30 m , δ = −55 • , respectively). Data were taken with the Australia Telescope Compact Array with 3.4-arcmin resolution and ∼0.18-mJy beam −1 sensitivity. The emission is dominated by point sources, and we do not find evidence for diffuse synchrotron radiation even after source subtraction. This allows to estimate an upper limit of the diffuse polarized emission. The extrapolation to 30 GHz suggests that the synchrotron radiation is lower than the polarized signal measured by the DASI experiment by at least two orders of magnitude. This further supports the conclusions drawn by the DASI team itself about the negligible Galactic foreground contamination in their data set, improving by a factor of ∼5 the upper limit estimated by Leitch et al. The dominant point-source emission allows us to estimate the contamination of the cosmic microwave background (CMB) by extragalactic foregrounds. We computed the power spectrum of their contribution, and its extrapolation to 30 GHz provides a framework where the CMB signal should dominate. However, our results do not match the conclusions of the DASI team about the negligibility of point-source contamination, suggesting taking into account a source subtraction from the DASI data.
Monthly Notices of the Royal Astronomical Society, 2010
We present the full source catalogue from the Australia Telescope 20 GHz (AT20G) Survey. The AT20... more We present the full source catalogue from the Australia Telescope 20 GHz (AT20G) Survey. The AT20G is a blind radio survey carried out at 20 GHz with the Australia Telescope Compact Array (ATCA) from 2004 to 2008, and covers the whole sky south of declination 0 •. The AT20G source catalogue presented here is an order of magnitude larger than any previous catalogue of high-frequency radio sources, and includes 5890 sources above a 20 GHz flux-density limit of 40 mJy. All AT20G sources have total intensity and polarization measured at 20 GHz, and most sources south of declination −15 • also have near-simultaneous flux-density measurements at 5 and 8 GHz. A total of 1559 sources were detected in polarized total intensity at one or more of the three frequencies. The completeness of the AT20G source catalogue is 91 per cent above 100 mJy beam −1 and 79 per cent above 50 mJy beam −1 in regions south of declination −15 •. North of −15 • , some observations of sources between 14 and 20 h in right ascension were lost due to bad weather and could not be repeated, so the catalogue completeness is lower in this region. Each detected source was visually inspected as part of our quality control process, and so the reliability of the final catalogue is essentially 100 per cent. We detect a small but significant population of non-thermal sources that are either undetected or have only weak detections in low-frequency catalogues. We introduce the term Ultra-Inverted Spectrum to describe these radio sources, which have a spectral index α(5, 20) > +0.7 and which constitute roughly 1.2 per cent of the AT20G sample.
Monthly Notices of the Royal Astronomical Society, 1996
We report accurate circular polarization measurements of four bright OH megamasers (III Zw 35, 04... more We report accurate circular polarization measurements of four bright OH megamasers (III Zw 35, 04454-4838, 10039-3338 and 20100-4156) with the Australia Telescope Compact Array. No convincing Zeeman splitting was observed. We place 30' upper limits of approximately 3 mG on any uniform line-of-sight magnetic field present in the emitting regions. If the field is non-uniform, then we can place upper limits of approximately 5 mG. These limits in turn imply constraints on the physical conditions of the maser emission regions which lie in the central 100 pc of megamaser galaxies.
Monthly Notices of the Royal Astronomical Society, 2002
We report the discovery of variable circularly polarized radio emission associated with relativis... more We report the discovery of variable circularly polarized radio emission associated with relativistic ejections from GRS 1915 + 105, based on observations with the Australia Telescope Compact Array (ATCA) and the Multi-Element Radio-Linked Interferometer Network (MERLIN). Following a radio flare in 2001 January, significant and variable circular polarization, at a fractional level of 0.2-0.4 per cent, was measured with ATCA at four frequencies between 1 and 9 GHz. Following an additional outburst 65 d later in 2001 March, further ATCA observations measured a comparable sign and level of circular polarization at two frequencies. At this second epoch, contemporaneous MERLIN observations directly imaged a relativistic ejection event and allowed us confidently to associate both the circularly and the linearly polarized emission with the relativistic ejecta, allowing a detailed measurement of the full polarization properties in the optically thin phase. The fractional circular polarization spectrum appears to flatten at higher frequencies in 2001 January, when there is strong evidence for multiple components at different optical depths. While we cannot conclusively distinguish between synchrotron or propagation-induced conversion as the origin of the circularly polarized component, we do not consider that coherent or birefringent scintillation mechanisms are likely. The implication is therefore that the ejections from GRS 1915 + 105 are associated with a significant population of low-energy electrons, with associated consequences for the energetics of relativistic ejection events. We briefly compare the data for SS 433 and GRS 1915 + 105 with those for active galactic nuclei, and note that linear-to-circular polarization ratios <1 are observed in both binary sources at the lowest frequency, and ratios >1 at the higher frequencies, illustrating the role of Faraday depolarization. In addition, the 2001 January ATCA observations reveal a linear polarization 'rotator' event, probing the (variable) orientation of the magnetic field structure in the outflow.
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Papers by Robert Sault