J.-P. Macquart

Curtin University Australia, Bentley, Western Australia, Australia

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Publications (61)141.71 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Using the new wideband capabilities of the Australia Telescope Compact Array (ATCA), we obtain spectra for PKS 1718-649, a well-known gigahertz-peaked spectrum radio source. The observations, between approximately 1 and 10 GHz over three epochs spanning approximately 21 months, reveal variability both above the spectral peak at ~3 GHz and below the peak. The combination of the low and high frequency variability cannot be easily explained using a single absorption mechanism, such as free-free absorption or synchrotron self-absorption. We find that the PKS 1718-649 spectrum and its variability are best explained by variations in the free-free optical depth on our line-of-sight to the radio source at low frequencies (below the spectral peak) and the adiabatic expansion of the radio source itself at high frequencies (above the spectral peak). The optical depth variations are found to be plausible when X-ray continuum absorption variability seen in samples of Active Galactic Nuclei is considered. We find that the cause of the peaked spectrum in PKS 1718-649 is most likely due to free-free absorption. In agreement with previous studies, we find that the spectrum at each epoch of observation is best fit by a free-free absorption model characterised by a power-law distribution of free-free absorbing clouds. This agreement is extended to frequencies below the 1 GHz lower limit of the ATCA by considering new observations with Parkes at 725 MHz and 199 MHz observations with the newly operational Murchison Widefield Array. These lower frequency observations argue against families of absorption models (both free-free and synchrotron self-absorption) that are based on simple homogenous structures.
    12/2014;
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    J. Y. Koay, J. -P. Macquart
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    ABSTRACT: We investigate the feasibility of detecting and probing various components of the ionized intergalactic medium (IGM) and their turbulent properties at radio frequencies through observations of scatter broadening of compact sources. There is a strong case for conducting targeted observations to resolve scatter broadening (where the angular size scales as $\sim \nu^{-2}$) of compact background sources intersected by foreground galaxy haloes and rich clusters of galaxies to probe the turbulence of the ionized gas in these objects, particularly using Space VLBI with baselines of 350,000 km at frequencies below 800 MHz. The sensitivity of the Square Kilometre Array (SKA) allows multifrequency surveys of interstellar scintillation (ISS) of $\sim 100 \,\mu$Jy sources to detect or place very strong constraints on IGM scatter broadening down to $\sim 1\, \mu$as scales at 5 GHz. Scatter broadening in the warm-hot component of the IGM with typical overdensities of $\sim 30$ cannot be detected, even with Space VLBI or ISS, and even if the outer scales of turbulence have an unlikely low value of $\sim 1$ kpc. Nonetheless, intergalatic scatter broadening can be of order $\sim 100\, \mu$as at 1 GHz and $\sim 3\, \mu$as at 5 GHz for outer scales $\sim 1$ kpc, assuming a sufficiently high source redshift that most sight-lines intersect within a virial radius of at least one galaxy halo ($z \gtrsim 0.5$ and $z \gtrsim 1.4$ for $10^{10} {\rm M}_\odot$ and $10^{11} {\rm M}_\odot$ systems, following McQuinn (2014)). Both Space VLBI and multiwavelength ISS observations with the SKA can easily test such a scenario, or place strong constraints on the outer scale of the turbulence in such regions.
    10/2014;
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    ABSTRACT: We present a search for transient and variable radio sources at 154 MHz with the Murchison Widefield Array 32-tile prototype. Fifty-one images were obtained that cover a field of view of 1430 deg^2 centred on Hydra A. The observations were obtained over three days in 2010 March and three days in 2011 April and May. The mean cadence of the observations was 26 minutes and there was additional temporal information on day and year timescales. We explore the variability of a sample of 105 low frequency radio sources within the field. Four bright (S > 6 Jy) candidate variable radio sources were identified that displayed low levels of short timescale variability (26 minutes). We conclude that this variability is likely caused by simplifications in the calibration strategy or ionospheric effects. On the timescale of one year we find two sources that show significant variability. We attribute this variability to either refractive scintillation or intrinsic variability. No radio transients were identified and we place an upper limit on the surface density of sources rho < 7.5 x 10^-5 deg^-2 with flux densities > 5.5 Jy, and characteristic timescales of both 26 minutes and one year.
    Monthly Notices of the Royal Astronomical Society 11/2013; 438(1). · 5.52 Impact Factor
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    ABSTRACT: We describe a new tool for studying the structure and physical characteristics of ultracompact active galactic nucleus (AGN) jets and their surroundings with μas precision. This tool is based on the frequency dependence of the light curves observed for intra-day variable radio sources, where the variability is caused by interstellar scintillation. We apply this method to PKS 1257-326 to resolve the core-shift as a function of frequency on scales well below ~12 μas. We find that the frequency dependence of the position of the scintillating component is rν–0.1 ± 0.24 (99% confidence interval) and the frequency dependence of the size of the scintillating component is dν–0.64 ± 0.006. Together, these results imply that the jet opening angle increases with distance along the jet: with nd > 1.8. We show that the flaring of the jet, and flat frequency dependence of the core position is broadly consistent with a model in which the jet is hydrostatically confined and traversing a steep pressure gradient in the confining medium with and np 7. Such steep pressure gradients have previously been suggested based on very long baseline interferometry studies of the frequency dependent core shifts in AGNs.
    The Astrophysical Journal 02/2013; 765(2):142. · 6.73 Impact Factor
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    ABSTRACT: We use VLBI imaging of the interstellar scattering speckle pattern associated with the pulsar PSR 0834+06 to measure the astrometric motion of its emission. The ~ 5AU interstellar baselines, provided by interference between speckles spanning the scattering disk, enable us to detect motions with sub nanoarcsecond accuracy. We measure a small pulse deflection of ~18+/-2 km (not including geometric uncertainties), which is 100 times smaller than the native resolution of this interstellar interferometer. This implies that the emission region is small, and at an altitude of a few hundred km, with the exact value depending on field geometry. This is substantially closer to the star than to the light cylinder. Future VLBI measurements can improve on this finding. This new regime of ultra-precise astrometry may enable precision parallax distance determination of pulsar binary displacements.
    01/2013;
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    ABSTRACT: We describe a new tool for studying the structure and physical characteristics of ultracompact AGN jets and their surroundings with microarcsecond precision. This tool is based on the frequency dependence of the light curves observed for intra-day variable radio sources, where the variability is caused by interstellar scintillation. We apply this method to PKS1257-326 to resolve the core-shift as a function of frequency on scales well below ~12 microarcseconds. We find that the frequency dependence of the position of the scintillating component is r \propto \nu^{-0.1 \pm 0.24} (99% confidence interval) and the frequency dependence of the size of the scintillating component is d \propto \nu^{-0.64 \pm 0.006}. Together, these results imply that the jet opening angle increases with distance along the jet: d \propto r^{n_d}$ with n_d > 1.8. We show that the flaring of the jet, and flat frequency dependence of the core position is broadly consistent with a model in which the jet is hydrostatically confined and traversing a steep pressure gradient in the confining medium with p \propto r^{-n_p} and n_p > 7. Such steep pressure gradients have previously been suggested based on VLBI studies of the frequency dependent core shifts in AGN.
    01/2013;
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    ABSTRACT: The fraction of compact active galactic nuclei (AGNs) that exhibit interstellar scintillation (ISS) at radio wavelengths, as well as their scintillation amplitudes, have been found to decrease significantly for sources at redshifts z > 2. This can be attributed to an increase in the angular sizes of the \muas-scale cores or a decrease in the flux densities of the compact \muas cores relative to that of the mas-scale components with increasing redshift, possibly arising from (1) the space-time curvature of an expanding Universe, (2) AGN evolution, (3) source selection biases, (4) scatter broadening in the ionized intergalactic medium (IGM) and intervening galaxies, or (5) gravitational lensing. We examine the frequency scaling of this redshift dependence of ISS to determine its origin, using data from a dual-frequency survey of ISS of 128 sources at 0 < z < 4. We present a novel method of analysis which accounts for selection effects in the source sample. We determine that the redshift dependence of ISS is partially linked to the steepening of source spectral indices ({\alpha}^8.4_4.9) with redshift, caused either by selection biases or AGN evolution, coupled with weaker ISS in the {\alpha}^8.4_4.9 < -0.4 sources. Selecting only the -0.4 < {\alpha}^8.4_4.9 < 0.4 sources, we find that the redshift dependence of ISS is still significant, but is not significantly steeper than the expected (1+z)^0.5 scaling of source angular sizes due to cosmological expansion for a brightness temperature and flux-limited sample of sources. We find no significant evidence for scatter broadening in the IGM, ruling it out as the main cause of the redshift dependence of ISS. We obtain an upper limit to IGM scatter broadening of < 110\muas at 4.9 GHz with 99% confidence for all lines of sight, and as low as < 8\muas for sight-lines to the most compact, \sim 10\muas sources.
    The Astrophysical Journal 06/2012; 756(1). · 6.73 Impact Factor
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    ABSTRACT: As a critical part of the Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry (TANAMI) program, in November 2007 the Australia Telescope Compact Array (ATCA) started monitoring the radio spectra of a sample of southern hemisphere active galactic nuclei (AGN) that were selected as likely candidates for detection (as well as a control sample) by the Large Area Telescope (LAT) aboard the Fermi Gamma Ray Space Observatory. The initial sample was chosen based on properties determined from AGN detections by the Energetic Gamma Ray Experiment Telescope (EGRET). Most of the initial sample has been detected by Fermi/LAT and with the addition of new detections the sample has grown to include 226 AGN, 133 of which have data for more than one epoch. For the majority of these AGN, our monitoring program provides the only dynamic radio spectra available. The ATCA receiver suite makes it possible to observe several sources at frequencies between 4.5 and 41 GHz in a few hours, resulting in an excellent measure of spectral index at each epoch. By examining how the spectral index changes over time, we aim to investigate the mechanics of radio and gamma-ray emission from AGN jets.
    05/2012;
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    ABSTRACT: The discovery that interstellar scintillation (ISS) is suppressed for compact radio sources at z >~ 2 has enabled ISS surveys to be used as cosmological probes. We discuss briefly the potential and challenges involved in such an undertaking, based on a dual-frequency survey of ISS carried out to determine the origin of this redshift dependence.
    Proceedings of the International Astronomical Union 04/2012; 7(S285):347-348.
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    ABSTRACT: We benchmark the reliability of the Rotation Measure (RM) synthesis algorithm using the 1005 Centaurus A field sources of Feain et al. (2009). The RM synthesis solutions are compared with estimates of the polarization parameters using traditional methods. This analysis provides verification of the reliability of RM synthesis estimates. We show that estimates of the polarization parameters can be made at lower S/N if the range of RMs is bounded, but reliable estimates of individual sources with unusual RMs require unconstrainted solutions and higher S/N. We derive from first principles the statistical properties of the polarization amplitude associated with RM synthesis in the presence of noise. The amplitude distribution depends explicitly on the amplitude of the underlying (intrinsic) polarization signal. Hence it is necessary to model the underlying polarization signal distribution in order to estimate the reliability and errors in polarization parameter estimates. We introduce a Bayesian method to derive the distribution of intrinsic amplitudes based on the distribution of measured amplitudes. The theoretically-derived distribution is compared with the empirical data to provide quantitative estimates of the probability that an RM synthesis solution is correct as a function of S/N. We provide quantitative estimates of the probability that any given RM synthesis solution is correct as a function of measured polarized amplitude and the intrinsic polarization amplitude compared to the noise.
    The Astrophysical Journal 03/2012; 750(2). · 6.73 Impact Factor
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  • 01/2012;
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    ABSTRACT: The 4.9 GHz Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey detected a drop in Interstellar Scintillation (ISS) for sources at redshifts z > 2, indicating an apparent increase in angular diameter or a decrease in flux density of the most compact components of these sources, relative to their extended emission. This can result from intrinsic source size effects or scatter broadening in the Intergalactic Medium (IGM), in excess of the expected (1+z)^0.5 angular diameter scaling of brightness temperature limited sources due to cosmological expansion. We report here 4.9 GHz and 8.4 GHz observations and data analysis for a sample of 140 compact, flat-spectrum sources which may allow us to determine the origin of this angular diameter-redshift relation by exploiting their different wavelength dependences. In addition to using ISS as a cosmological probe, the observations provide additional insight into source morphologies and the characteristics of ISS. As in the MASIV Survey, the variability of the sources is found to be significantly correlated with line-of-sight H-alpha intensities, confirming its link with ISS. For 25 sources, time delays of about 0.15 to 3 days are observed between the scintillation patterns at both frequencies, interpreted as being caused by a shift in core positions when probed at different optical depths. Significant correlation is found between ISS amplitudes and source spectral index; in particular, a large drop in ISS amplitudes is observed at spectral indices of < -0.4 confirming that steep spectrum sources scintillate less. We detect a weakened redshift dependence of ISS at 8.4 GHz over that at 4.9 GHz, with the mean variance at 4-day timescales reduced by a factor of 1.8 in the z > 2 sources relative to the z < 2 sources, as opposed to the factor of 3 decrease observed at 4.9 GHz. This suggests scatter broadening in the IGM.
    The Astronomical Journal 07/2011; 142(4). · 4.97 Impact Factor
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    ABSTRACT: The extreme, intra-hour and >10% rms flux density scintillation observed in AGNs such as PKS 0405-385, J1819+3845 and PKS 1257-326 at cm wavelengths has been attributed to scattering in highly turbulent, nearby regions in the interstellar medium. Such behavior has been found to be rare. We searched for rapid scintillators among 128 flat spectrum AGNs and analyzed their properties to determine the origin of such rapid and large amplitude radio scintillation. The sources were observed at the VLA at 4.9 and 8.4 GHz simultaneously at two hour intervals over 11 days. We detected six rapid scintillators with characteristic time-scales of 10%. We found strong lines of evidence linking rapid scintillation to the presence of nearby scattering regions, estimated to be
    Astronomy and Astrophysics 01/2011; 534. · 5.08 Impact Factor
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    ABSTRACT: We have studied the archetypal Gigahertz Peaked Spectrum radio galaxy, PKS 1934 – 638, using the Australian Long Baseline Array augmented with two new telescopes that greatly improve the angular resolution of the array. These very long baseline interferometry observations represent the first scientific results from a new antenna in New Zealand and the first antenna of the Australian SKA Pathfinder. A compact double radio source, PKS 1934 – 638 has been monitored over a period of 40 years and the observation described here provides the latest datum, eight years after the previous observation, to aid in the study of the long-term evolution of the source structure. We take advantage of these new long baselines to probe PKS 1934 – 638 at the relatively low frequency of 1.4 GHz in order to examine the effects of optical depth on the structure of the radio source. Optical depth effects, resulting in the observation of frequency-dependent structure, may have previously been interpreted in terms of an expansion of the source as a function of time. Expansion and frequency-dependent effects are important to disentangle in order to estimate the age of PKS 1934 – 638. We show that frequency-dependent structure effects are likely to be important in PKS 1934 – 638 and present a simple two-dimensional synchrotron source model in which opacity effects due to synchrotron self-absorption are taken into account. Evidence for expansion of the radio source over 40 years is therefore weak with consequences for the estimated age of the radio source.
    The Astronomical Journal 10/2010; 140(5):1506. · 4.97 Impact Factor
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    J.-P. Macquart, N. Kanekar, D. A. Frail, S. M. Ransom
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    ABSTRACT: We report results from a deep high-frequency search for pulsars within the central parsec of Sgr A* using the Green Bank Telescope. The observing frequency of 15 GHz was chosen to maximize the likelihood of detecting normal pulsars (i.e. with periods of $\sim 500$\,ms and spectral indices of $\sim -1.7$) close to Sgr A*, that might be used as probes of gravity in the strong-field regime; this is the highest frequency used for such pulsar searches of the Galactic Center to date. No convincing candidate was detected in the survey, with a $10\sigma$ detection threshold of $\sim 10 \mu$Jy achieved in two separate observing sessions. This survey represents a significant improvement over previous searches for pulsars at the Galactic Center and would have detected a significant fraction ($\gtrsim 5%) of the pulsars around Sgr A*, if they had properties similar to those of the known population. Using our best current knowledge of the properties of the Galactic pulsar population and the scattering material toward Sgr A*, we estimate an upper limit of 90 normal pulsars in orbit within the central parsec of Sgr A*. Comment: 10 pages, 7 figures, accepted for publication in the ApJ.
    The Astrophysical Journal 04/2010; · 6.73 Impact Factor
  • J. P. R. Macquart, P. Hall, N. Clarke
    01/2010;
  • 01/2010;
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    ABSTRACT: We have invented a novel technique to measure the radio image of a pulsar scattered by the interstellar plasma with 0.1 mas resolution. We extend the "secondary spectrum" analysis of parabolic arcs by Stinebring et al. to very long baseline interferometry and, when the scattering is anisotropic, we are able to map the scattered brightness astrometrically with much higher resolution than the diffractive limit of the interferometer. We employ this technique to measure an extremely anisotropic scattered image of the pulsar B0834+06 at 327 MHz. We find that the scattering occurs in a compact region about 420 pc from the Earth. This image has two components, both essentially linear and nearly parallel. The primary feature, which is about 16 AU long and less than 0.5 AU in width, is highly inhomogeneous on spatial scales as small as 0.05 AU. The second feature is much fainter and is displaced from the axis of the primary feature by about 9 AU. We find that the velocity of the scattering plasma is 16 ± 10 km s–1 approximately parallel to the axis of the linear feature. The origin of the observed anisotropy is unclear and we discuss two very different models. It could be, as has been assumed in earlier work, that the turbulence on spatial scales of (~1000 km) is homogeneous but anisotropic. However, it may be that the turbulence on these scales is homogeneous and isotropic but the anisotropy is produced by highly elongated (filamentary) inhomogeneities of scale 0.05-16 AU.
    The Astrophysical Journal 12/2009; 708(1):232. · 6.73 Impact Factor

Publication Stats

577 Citations
141.71 Total Impact Points

Institutions

  • 2011–2014
    • Curtin University Australia
      • International Centre for Radio Astronomy Research (ICRAR)
      Bentley, Western Australia, Australia
  • 2010
    • International Centre for Radio Astronomy Research
      Perth City, Western Australia, Australia
  • 2007–2010
    • California Institute of Technology
      • Department of Astronomy
      Pasadena, California, United States
    • National Radio Astronomy Observatory
      Charlottesville, Virginia, United States
  • 2008
    • University of Tasmania
      • School of Mathematics & Physics
      Hobart Town, Tasmania, Australia
  • 2002–2007
    • University of Groningen
      • Kapteyn Astronomical Institute
      Groningen, Groningen, Netherlands
  • 1998–2007
    • University of Sydney
      • • Sydney Institute of Astronomy (SIfA)
      • • School of Physics
      Sydney, New South Wales, Australia
  • 2003
    • University of Adelaide
      Tarndarnya, South Australia, Australia