B. W. Stappers

Swinburne University of Technology, Melbourne, Victoria, Australia

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Publications (314)1272.84 Total impact

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    ABSTRACT: We present the results of a search for gravitational waves (GWs) from individual sources using high cadence observations of PSR B1937+21. The data were acquired from an intensive observation campaign with the Lovell telescope at Jodrell Bank, between June 2011 and May 2013. The almost daily cadence achieved, allowed us to be sensitive to GWs with frequencies up to $4.98\times10^{-6}\,\rm {Hz}$, extending the upper bound of the typical frequency range probed by Pulsar Timing Arrays. We used observations taken at three different radio frequencies with the Westerbork Synthesis Radio Telescope in order to correct for dispersion measure effects and scattering variances. The corrected timing residuals exhibited an unmodeled periodic noise with an amplitude $~150\,\rm {ns}$ and a frequency of $3.4\rm {yr}^{-1}$. As the signal is not present in the entire data set, we attributed it to the rotational behaviour of the pulsar, ruling out the possibilities of being either due to a GW or an asteroid as the cause. After removing this noise component, we placed limits on the GW strain of individual sources equaling to $h_{\rm s}=1.53\times10^{-11}$ and $h_{\rm s}=4.99\times10^{-14}$ at $10^{-7}\,\rm {MHz}$ for random and optimal sources locations respectively.
    09/2014;
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    ABSTRACT: We present broadband, low-frequency (25-80 MHz and 110-190 MHz) LOFAR observations of PSR B0943+10, with the goal of better illuminating the nature of its enigmatic mode-switching behaviour. This pulsar shows two relatively stable states: a Bright (B) and Quiet (Q) mode, each with different characteristic brightness, profile morphology, and single-pulse properties. We model the average profile evolution both in frequency and time from the onset of each mode, and highlight the differences between the two modes. In both modes, the profile evolution can be well explained by radius-to-frequency mapping at altitudes within a few hundred kilometres of the stellar surface. If both B and Q-mode emission originate at the same magnetic latitude, then we find that the change of emission height between the modes is less than 6%. We also find that, during B-mode, the average profile is gradually shifting towards later spin phase and then resets its position at the next Q-to-B transition. The observed B-mode profile delay is frequency-independent (at least from 25-80 MHz) and asymptotically changes towards a stable value of about 0.004 in spin phase by the end of mode instance, much too large to be due to changing spin-down rate. Such a delay can be interpreted as a gradual movement of the emission cone against the pulsar's direction of rotation, with different field lines being illuminated over time. Another interesting explanation is a possible variation of accelerating potential inside the polar gap. This explanation connects the observed profile delay to the gradually evolving subpulse drift rate, which depends on the gradient of the potential across the field lines.
    08/2014;
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    ABSTRACT: The radio millisecond pulsar J1713+0747 is regarded as one of the highest-precision clocks in the sky, and is regularly timed for the purpose of detecting gravitational waves. The International Pulsar Timing Array collaboration undertook a 24-hour global observation of PSR J1713+0747 in an effort to better quantify sources of timing noise in this pulsar, particularly on intermediate (1 - 24 hr) timescales. We observed the pulsar continuously over 24 hr with the Arecibo, Effelsberg, GMRT, Green Bank, LOFAR, Lovell, Nancay, Parkes, and WSRT radio telescopes. The combined pulse times-of-arrival presented here provide an estimate of what sources of timing noise, excluding DM variations, would be present as compared to an idealized root-N improvement in timing precision, where N is the number of pulses analyzed. In the case of this particular pulsar, we find that intrinsic pulse phase jitter dominates arrival time precision when the S/N of single pulses exceeds unity, as measured using the eight telescopes that observed at L-band/1.4 GHz. We present first results of specific phenomena probed on the unusually long timescale (for a single continuous observing session) of tens of hours, in particular interstellar scintillation, and discuss the degree to which scintillation and profile evolution affect precision timing. This paper presents the data set as a basis for future, deeper studies.
    08/2014;
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    ABSTRACT: Context: GeV gamma-ray pulsations from over 140 pulsars have been characterized using the Fermi Large Area Telescope, enabling improved understanding of the emission regions within the neutron star magnetospheres, and the contributions of pulsars to high energy electrons and diffuse gamma rays in the Milky Way. The first gamma-ray pulsars to be detected were the most intense and/or those with narrow pulses. Aims: As the Fermi mission progresses, progressively fainter objects can be studied. In addition to more distant pulsars (thus probing a larger volume of the Galaxy), or ones in high background regions (thus improving the sampling uniformity across the Galactic plane), we detect pulsars with broader pulses or lower luminosity. Adding pulsars to our catalog with inclination angles that are rare in the observed sample, and/or with lower spindown power, will reduce the bias in the currently known gamma-ray pulsar population. Methods: We use rotation ephemerides derived from radio observations to phase-fold gamma rays recorded by the Fermi Large Area Telescope, to then determine the pulse profile properties. Spectral analysis provides the luminosities and, when the signal-to-noise ratio allows, the cutoff energies. We constrain the pulsar distances by different means in order to minimize the luminosity uncertainties. Results: We present six new gamma-ray pulsars with an eclectic mix of properties. Three are young, and three are recycled. They include the farthest, the lowest power, two of the highest duty-cycle pulsars seen, and only the fourth young gamma-ray pulsar with a radio interpulse. We discuss the biases existing in the current gamma-ray pulsar catalog, and steps to be taken to mitigate the bias.
    07/2014;
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    ABSTRACT: The Sun is an active source of radio emission which is often associated with energetic phenomena such as solar flares and coronal mass ejections (CMEs). At low radio frequencies (<100 MHz), the Sun has not been imaged extensively because of the instrumental limitations of previous radio telescopes. Here, the combined high spatial, spectral and temporal resolution of the Low Frequency Array (LOFAR) was used to study solar Type III radio bursts at 30-90 MHz and their association with CMEs. The Sun was imaged with 126 simultaneous tied-array beams within 5 solar radii of the solar centre. This method offers benefits over standard interferometric imaging since each beam produces high temporal (83 ms) and spectral resolution (12.5 kHz) dynamic spectra at an array of spatial locations centred on the Sun. LOFAR's standard interferometric output is currently limited to one image per second. Over a period of 30 minutes, multiple Type III radio bursts were observed, a number of which were found to be located at high altitudes (4 solar radii from the solar center at 30 MHz) and to have non-radial trajectories. These bursts occurred at altitudes in excess of values predicted by 1D radial electron density models. The non-radial high altitude Type III bursts were found to be associated with the expanding flank of a CME. The CME may have compressed neighbouring streamer plasma producing larger electron densities at high altitudes, while the non-radial burst trajectories can be explained by the deflection of radial magnetic fields as the CME expanded in the low corona.
    07/2014;
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    ABSTRACT: We present radio observations of the Moon between $35$ and $80$ MHz to demonstrate a novel technique of interferometrically measuring large-scale diffuse emission extending far beyond the primary beam (global signal) for the first time. In particular, we show that (i) the Moon appears as a negative-flux source at frequencies $35<\nu<80$ MHz since it is `colder' than the diffuse Galactic background it occults, (ii) using the (negative) flux of the lunar disc, we can reconstruct the spectrum of the diffuse Galactic emission with the lunar thermal emission as a reference, and (iii) that reflected RFI (radio-frequency interference) is concentrated at the center of the lunar disc due to specular nature of reflection, and can be independently measured. Our RFI measurements show that (i) Moon-based Cosmic Dawn experiments must design for an Earth-isolation of better than $80$ dB to achieve an RFI temperature $<1$ mK, (ii) Moon-reflected RFI contributes to a dipole temperature less than $20$ mK for Earth-based Cosmic Dawn experiments, (iii) man-made satellite-reflected RFI temperature exceeds $20$ mK if the aggregate cross section of visible satellites exceeds $80$ m$^2$ at $800$ km height, or $5$ m$^2$ at $400$ km height. Currently, our diffuse background spectrum is limited by sidelobe confusion on short baselines (10-15% level). Further refinement of our technique may yield constraints on the redshifted global $21$-cm signal from Cosmic Dawn ($40>z>12$) and the Epoch of Reionization ($12>z>5$).
    07/2014;
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    ABSTRACT: In recent years, instrumentation enabling pulsar observations with unprecedentedly high fractional bandwidth has been under development which can be used to substantially improve the precision of pulsar timing experiments. The traditional template-matching method used to calculate pulse times-of-arrival (ToAs), may not function effectively on these broadband data due to a variety of effects such as diffractive scintillation in the interstellar medium, profile variation as a function of frequency, dispersion measure (DM) evolution and so forth. In this paper, we describe the channelised Discrete Fourier Transform method that can greatly mitigate the influence of the aforementioned effects when measuring ToAs from broadband timing data. The method is tested on simulated data, and its potential in improving timing precision is shown. We further apply the method to PSR J1909$-$3744 data collected at the Nan\c{c}ay Radio Telescope with the Nan\c{c}ay Ultimate Pulsar Processing Instrument. We demonstrate a removal of systematics due to the scintillation effect as well as improvement on ToA measurement uncertainties. Our method also determines temporal variations in dispersion measure, which are consistent with multi-channel timing approaches used earlier.
    07/2014;
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    ABSTRACT: We report NuSTAR observations of the millisecond pulsar - low mass X-ray binary (LMXB) transition system PSR J1023+0038 from June and October 2013, before and after the formation of an accretion disk around the neutron star. Between June 10-12, a few days to two weeks before the radio disappearance of the pulsar, the 3-79 keV X-ray spectrum was well fit by a simple power law with a photon index of Gamma=1.17 +/-0.08 (at 90% confidence) with a 3-79 keV luminosity of 7.4+/-0.4 x 10^32 erg/s. Significant orbital modulation was observed with a modulation fraction of 36+/-10%. During the October 19-21 observation, the spectrum is described by a softer power law (Gamma=1.66+/-0.06) with an average luminosity of 5.8+/-0.2 x 10^33 erg/s and a peak luminosity of ~1.2 x 10^34 erg/s observed during a flare. No significant orbital modulation was detected. The spectral observations are consistent with previous and current multi-wavelength observations and show the hard X-ray power law extending to 79 keV without a spectral break. Sharp edged, flat bottomed `dips' are observed with widths between 30-1000 s and ingress and egress time-scales of 30-60 s. No change in hardness ratio was observed during the dips. Consecutive dip separations are log-normal in distribution with a typical separation of approximately 400 s. These dips are distinct from dipping activity observed in LMXBs. We compare and contrast these dips to observations of dips and state changes in the similar transition systems PSR J1824-2452I and XSS J1227.0-4859 and discuss possible interpretations based on the transitions in the inner disk.
    The Astrophysical Journal 06/2014; 791(2). · 6.73 Impact Factor
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    ABSTRACT: The pulsar PSR J1756$-$2251 resides in a relativistic double neutron star (DNS) binary system with a 7.67-hr orbit. We have conducted long-term precision timing on more than 9 years of data acquired from five telescopes, measuring five post-Keplerian parameters. This has led to several independent tests of general relativity (GR), the most constraining of which shows agreement with the prediction of GR at the 4% level. Our measurement of the orbital decay rate disagrees with that predicted by GR, likely due to systematic observational biases. We have derived the pulsar distance from parallax and orbital decay measurements to be 0.73$_{-0.24}^{+0.60}$ kpc (68%) and < 1.2 kpc (95% upper limit), respectively; these are significantly discrepant from the distance estimated using Galactic electron density models. We have found the pulsar mass to be 1.341$\pm$0.007 M$_\odot$, and a low neutron star (NS) companion mass of 1.230$\pm$0.007 M$_\odot$. We also determined an upper limit to the spin-orbit misalignment angle of 34{\deg} (95%) based on a system geometry fit to long-term profile width measurements. These and other observed properties have led us to hypothesize an evolution involving a low mass loss, symmetric supernova progenitor to the second-formed NS companion, as is thought to be the case for the double pulsar system PSR J0737$-$3039A/B. This would make PSR J1756$-$2251 the second compact binary system providing concrete evidence for this type of NS formation channel.
    06/2014;
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    ABSTRACT: Very long baseline interferometry at millimetre/submillimetre wavelengths (mmVLBI) offers the highest achievable spatial resolution at any wavelength in astronomy. The anticipated inclusion of ALMA as a phased array into a global VLBI network will bring unprecedented sensitivity and a transformational leap in capabilities for mmVLBI. Building on years of pioneering efforts in the US and Europe the ongoing ALMA Phasing Project (APP), a US-led international collaboration with MPIfR-led European contributions, is expected to deliver a beamformer and VLBI capability to ALMA by the end of 2014 (APP: Fish et al. 2013, arXiv:1309.3519). This report focuses on the future use of mmVLBI by the international users community from a European viewpoint. Firstly, it highlights the intense science interest in Europe in future mmVLBI observations as compiled from the responses to a general call to the European community for future research projects. A wide range of research is presented that includes, amongst others: - Imaging the event horizon of the black hole at the centre of the Galaxy - Testing the theory of General Relativity an/or searching for alternative theories - Studying the origin of AGN jets and jet formation - Cosmological evolution of galaxies and BHs, AGN feedback - Masers in the Milky Way (in stars and star-forming regions) - Extragalactic emission lines and astro-chemistry - Redshifted absorption lines in distant galaxies and study of the ISM and circumnuclear gas - Pulsars, neutron stars, X-ray binaries - Testing cosmology - Testing fundamental physical constants
    06/2014;
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    ABSTRACT: We present an analysis of the emission behaviour of PSR J1107-5907, a source known to exhibit separate modes of emission, using observations obtained over approximately 10 yr. We find that the object exhibits two distinct modes of emission; a strong mode with a broad profile and a weak mode with a narrow profile. During the strong mode of emission, the pulsar typically radiates very energetic emission over sequences of ~200-6000 pulses (~60 s-24 min), with apparent nulls over time-scales of up to a few pulses at a time. Emission during the weak mode is observed outside of these strong-mode sequences and manifests as occasional bursts of up to a few clearly detectable pulses at a time, as well as low-level underlying emission which is only detected through profile integration. This implies that the previously described null mode may in fact be representative of the bottom-end of the pulse intensity distribution for the source. This is supported by the dramatic pulse-to-pulse intensity modulation and rarity of exceptionally bright pulses observed during both modes of emission. Coupled with the fact that the source could be interpreted as a rotating radio transient (RRAT)-like object for the vast majority of the time, if placed at a further distance, we advance that this object likely represents a bridge between RRATs and extreme moding pulsars. Further to these emission properties, we also show that the source is consistent with being a near-aligned rotator and that it does not exhibit any measurable spin-down rate variation. These results suggest that nulls observed in other intermittent objects may in fact be representative of very weak emission without the need for complete cessation. As such, we argue that longer (> 1 h) observations of pulsars are required to discern their true modulation properties.
    06/2014;
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    ABSTRACT: The Pulsar Arecibo L-band Feed Array (PALFA) Survey uses the ALFA 7-beam receiver to search both inner and outer Galactic sectors visible from Arecibo ($32^{\circ}\lesssim \ell \lesssim 77^{\circ}$ and $168^{\circ}\lesssim \ell \lesssim 214^{\circ}$) close to the Galactic plane ($|b|\lesssim5^{\circ}$) for pulsars. In this paper we detail a precursor survey of this region with PALFA, which observed a subset of the full region (slightly more restrictive in $\ell$ and $|b|\lesssim1^{\circ}$) and detected 45 pulsars. For both Galactic millisecond and normal pulsar populations, we compare the survey's detections with simulations to model these populations and, in particular, to estimate the number of observable pulsars in the Galaxy. We place 95\% confidence intervals of 82,000 to 143,000 on the number of detectable normal pulsars and 9,000 to 100,000 on the number of detectable millisecond pulsars in the Galactic disk. These are consistent with previous estimates. Given the most likely population size in each case (107,000 and 15,000 for normal and millisecond pulsars, respectively) we extend survey detection simulations to predict that, when complete, the full PALFA survey should have detected $1,000\substack{+330 \\ -230}$ normal pulsars and $30\substack{+200 \\ -20}$ millisecond pulsars. Identical estimation techniques predict that $490\substack{+160 \\ -115}$ normal pulsars and $12\substack{+70 \\ -5}$ millisecond pulsars would be detected by the beginning of 2014; at the time, the PALFA survey had detected 283 normal pulsars and 31 millisecond pulsars, respectively.
    05/2014;
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    ABSTRACT: Fast radio bursts (FRBs) are an emerging class of bright, highly dispersed radio pulses. Recent work by Thornton et al. (2013) has revealed a population of FRBs in the High Time Resolution Universe (HTRU) survey at high Galactic latitudes. A variety of progenitors have been proposed including cataclysmic events at cosmological distances, Galactic flare stars, and terrestrial radio frequency interference. Here we report on a search for FRBs at intermediate Galactic latitudes ($-15^{\circ}$ $< b <$ 15$^{\circ}$) in data taken as part of the HTRU survey. No FRBs were discovered in this region. Several effects such as dispersion, scattering, sky temperature and scintillation decrease the sensitivity by more than 3$\sigma$ in $\sim$20\% of survey pointings. Including all of these effects, we exclude the hypothesis that FRBs are uniformly distributed on the sky with 99\% confidence. This low probability implies that additional factors -- not accounted for by standard Galactic models -- must be included to ease the discrepancy between the detection rates at high and low Galactic latitudes. A revised rate estimate or another strong and heretofore unknown selection effect in Galactic latitude would provide closer agreement between the surveys' detection rates. The dearth of detections at low Galactic latitude disfavors a Galactic origin for these bursts.
    The Astrophysical Journal Letters 05/2014; 789(2). · 6.35 Impact Factor
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    ABSTRACT: Classifiers trained on data sets possessing an imbalanced class distribution are known to exhibit poor generalisation performance. This is known as the imbalanced learning problem. The problem becomes particularly acute when we consider incremental classifiers operating on imbalanced data streams, especially when the learning objective is rare class identification. As accuracy may provide a misleading impression of performance on imbalanced data, existing stream classifiers based on accuracy can suffer poor minority class performance on imbalanced streams, with the result being low minority class recall rates. In this paper we address this deficiency by proposing the use of the Hellinger distance measure, as a very fast decision tree split criterion. We demonstrate that by using Hellinger a statistically significant improvement in recall rates on imbalanced data streams can be achieved, with an acceptable increase in the false positive rate.
    05/2014;
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    ABSTRACT: Classifiers trained on data sets possessing an imbalanced class distribution are known to exhibit poor generalisation performance. This is known as the imbalanced learning problem. The problem becomes particularly acute when we consider incremental classifiers operating on imbalanced data streams, especially when the learning objective is rare class identification. As accuracy may provide a misleading impression of performance on imbalanced data, existing stream classifiers based on accuracy can suffer poor minority class performance on imbalanced streams, with the result being low minority class recall rates. In this paper we address this deficiency by proposing the use of the Hellinger distance measure, as a very fast decision tree split criterion. We demonstrate that by using Hellinger a statistically significant improvement in recall rates on imbalanced data streams can be achieved, with an acceptable increase in the false positive rate.
    04/2014;
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    ABSTRACT: In order to reach the sensitivity required to detect gravitational waves, pulsar timing array experiments need to mitigate as much noise as possible in timing data. A dominant amount of noise is likely due to variations in the dispersion measure. To correct for such variations, we develop a statistical method inspired by the maximum likelihood estimator and optimal filtering. Our method consists of two major steps. First, the spectral index and amplitude of dispersion measure variations are measured via a time-domain spectral analysis. Second, the linear optimal filter is constructed based on the model parameters found in the first step, and is used to extract the dispersion measure variation waveforms. Compared to current existing methods, this method has better time resolution for the study of short timescale dispersion variations, and generally produces smaller errors in waveform estimations. This method can process irregularly sampled data without any interpolation because of its time-domain nature. Furthermore, it offers the possibility to interpolate or extrapolate the waveform estimation to regions where no data is available. Examples using simulated data sets are included for demonstration.
    04/2014;
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    ABSTRACT: Glitches are sudden spin-up events that punctuate the steady spin down of pulsars and are thought to be due to the presence of a superfluid component within neutron stars. The precise glitch mechanism and its trigger, however, remain unknown. The size of glitches is a key diagnostic for models of the underlying physics. While the largest glitches have long been taken into account by theoretical models, it has always been assumed that the minimum size lay below the detectability limit of the measurements. In this paper we define general glitch detectability limits and use them on 29 years of daily observations of the Crab pulsar, carried out at Jodrell Bank Observatory. We find that all glitches lie well above the detectability limits and by using an automated method to search for small events we are able to uncover the full glitch size distribution, with no biases. Contrary to the prediction of most models, the distribution presents a rapid decrease of the number of glitches below ~0.05 $\mu$Hz. This substantial minimum size indicates that a glitch must involve the motion of at least several billion superfluid vortices and provides an extra observable which can greatly help the identification of the trigger mechanism. Our study also shows that glitches are clearly separated from all the other rotation irregularities. This supports the idea that the origin of glitches is different to that of timing noise, which comprises the unmodelled random fluctuations in the rotation rates of pulsars.
    02/2014; 440(3).
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    ABSTRACT: Millisecond radio pulsars acquire their rapid rotation rates through mass and angular momentum transfer in a low-mass X-ray binary system. Recent studies of PSR J1824-2452I and PSR J1023+0038 have observationally demonstrated this link, and they have also shown that such systems can repeatedly transition back-and-forth between the radio millisecond pulsar and low-mass X-ray binary states. This also suggests that a fraction of such systems are not newly born radio millisecond pulsars but are rather suspended in a back-and-forth state switching phase, perhaps for giga-years. XSS J12270-4859 has been previously suggested to be a low-mass X-ray binary, and until recently the only such system to be seen at MeV-GeV energies. We present radio, optical and X-ray observations that offer compelling evidence that XSS J12270-4859 is a low-mass X-ray binary which transitioned to a radio millisecond pulsar state between 2012 November 14 and 2012 December 21. Though radio pulsations remain to be detected, we use optical and X-ray photometry/spectroscopy to show that the system has undergone a sudden dimming and no longer shows evidence for an accretion disk. The optical observations constrain the orbital period to 6.913+-0.002 hr.
    02/2014; 441(2).
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    ABSTRACT: We present the first detection of carbon radio recombination line absorption along the line of sight to Cygnus A. The observations were carried out with the Low Frequency Array in the 33-57 MHz range. These low-frequency radio observations provide us with a new line of sight to study the diffuse, neutral gas in our Galaxy. To our knowledge this is the first time that foreground Milky Way recombination line absorption has been observed against a bright extragalactic background source. By stacking 48 carbon α lines in the observed frequency range we detect carbon absorption with a signal-to-noise ratio of about 5. The average carbon absorption has a peak optical depth of 2 × 10-4, a line width of 10 km s-1 and a velocity of +4 km s-1 with respect to the local standard of rest. The associated gas is found to have an electron temperature Te ˜ 110 K and density ne ˜ 0.06 cm-3. These properties imply that the observed carbon α absorption likely arises in the cold neutral medium of the Orion arm of the Milky Way. Hydrogen and helium lines were not detected to a 3σ peak optical depth limit of 1.5 × 10-4 for a 4 km s-1 channel width. Radio recombination lines associated with Cygnus A itself were also searched for, but are not detected. We set a 3σ upper limit of 1.5 × 10-4 for the peak optical depth of these lines for a 4 km s-1 channel width.
    Monthly Notices of the Royal Astronomical Society 02/2014; 437:3506-3515. · 5.52 Impact Factor
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    ABSTRACT: We present the first detection of carbon radio recombination line absorption along the line of sight to Cygnus A. The observations were carried out with the LOw Frequency ARray in the 33 to 57 MHz range. These low frequency radio observations provide us with a new line of sight to study the diffuse, neutral gas in our Galaxy. To our knowledge this is the first time that foreground Milky Way recombination line absorption has been observed against a bright extragalactic background source. By stacking 48 carbon $\alpha$ lines in the observed frequency range we detect carbon absorption with a signal-to-noise ratio of about 5. The average carbon absorption has a peak optical depth of 2$\times$10$^{-4}$, a line width of 10 km s$^{-1}$ and a velocity of +4 km s$^{-1}$ with respect to the local standard of rest. The associated gas is found to have an electron temperature $T_{e}\sim$ 110 K and density $n_{e}\sim$ 0.06 cm$^{-3}$. These properties imply that the observed carbon $\alpha$ absorption likely arises in the cold neutral medium of the Orion arm of the Milky Way. Hydrogen and helium lines were not detected to a 3$\sigma$ peak optical depth limit of 1.5$\times$10$^{-4}$ for a 4 km s$^{-1}$ channel width. Radio recombination lines associated with Cygnus A itself were also searched for, but are not detected. We set a 3$\sigma$ upper limit of 1.5$\times$10$^{-4}$ for the peak optical depth of these lines for a 4 km s$^{-1}$ channel width.
    01/2014;

Publication Stats

2k Citations
1,272.84 Total Impact Points

Institutions

  • 2010–2014
    • Swinburne University of Technology
      • Centre for Astrophysics and Supercomputing
      Melbourne, Victoria, Australia
    • University of Denver
      • Department of Physics and Astronomy
      Denver, Colorado, United States
    • Max Planck Institute for Gravitational Physics (Albert-Einstein-Institute)
      Potsdam, Brandenburg, Germany
    • University of Cape Town
      Kaapstad, Western Cape, South Africa
  • 2008–2014
    • The University of Manchester
      • Jodrell Bank Centre for Astrophysics
      Manchester, England, United Kingdom
  • 2013
    • Netherlands Institute for Space Research, Utrecht
      Utrecht, Utrecht, Netherlands
    • University of Wisconsin - Milwaukee
      • Department of Physics
      Milwaukee, Wisconsin, United States
    • Massachusetts Institute of Technology
      Cambridge, Massachusetts, United States
  • 2008–2013
    • McGill University
      • Department of Physics
      Montréal, Quebec, Canada
  • 2011
    • University of Zielona Góra
      • Institute of Astronomy
      Zielona Góra, Lubusz, Poland
    • Università degli studi di Cagliari
      • Department of Physics
      Cagliari, Sardinia, Italy
  • 2009
    • University of British Columbia - Vancouver
      • Department of Physics and Astronomy
      Vancouver, British Columbia, Canada
    • Pennsylvania State University
      University Park, Maryland, United States
    • Max Planck Institute for Radio Astronomy
      Bonn, North Rhine-Westphalia, Germany
  • 2006–2009
    • University of Groningen
      • Kernfysisch Versneller Instituut (KVI)
      Groningen, Province of Groningen, Netherlands
    • University of Sydney
      • School of Physics
      Sydney, New South Wales, Australia
  • 2003–2009
    • Netherlands Institute for Radio Astronomy
      Dwingelo, Drenthe, Netherlands
    • Universiteit Utrecht
      • Astronomical Institute
      Utrecht, Provincie Utrecht, Netherlands
  • 1999–2009
    • University of Amsterdam
      • Astronomical Institute Anton Pannekoek
      Amsterdam, North Holland, Netherlands
  • 1995–2009
    • Australian National University
      • Mount Stromlo Observatory
      Canberra, Australian Capital Territory, Australia
  • 2002–2007
    • University of Melbourne
      • School of Physics
      Melbourne, Victoria, Australia
    • University of California, Berkeley
      • Department of Astronomy
      Berkeley, California, United States
  • 2004–2005
    • Cornell University
      • Department of Astronomy
      Ithaca, NY, United States
  • 1998–2000
    • ANU College
      Slacks Creek, Queensland, Australia