Manly Astrophysics

Manly, Australia

Manly Astrophysics

Manly, Australia
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Walker M.A.,Manly Astrophysics | Demorest P.B.,U.S. National Radio Astronomy Observatory | Van Straten W.,Swinburne University of Technology
Astrophysical Journal | Year: 2013

Cyclic spectroscopy is a signal processing technique that was originally developed for engineering applications and has recently been introduced into the field of pulsar astronomy. It is a powerful technique with many attractive features, not least of which is the explicit rendering of information about the relative phases in any filtering imposed on the signal, thus making holography a more straightforward proposition. Here we present methods for determining optimum estimates of both the filter itself and the statistics of the unfiltered signal, starting from a measured cyclic spectrum. In the context of radio pulsars these quantities tell us the impulse response of the interstellar medium (ISM) and the intrinsic pulse profile. We demonstrate our techniques by application to 428 MHz Arecibo data on the millisecond pulsar B1937+21, obtaining the pulse profile free from the effects of interstellar scattering. As expected, the intrinsic profile exhibits main- and inter-pulse components that are narrower than they appear in the scattered profile; it also manifests some weak, but sharp, features that are revealed for the first time at low frequency. We determine the structure of the received electric field envelope as a function of delay and Doppler shift. Our delay Doppler image has a high dynamic range and displays some pronounced, low-level power concentrations at large delays. These concentrations imply strong clumpiness in the ionized ISM, on AU-size scales, which must adversely affect the timing of B1937+21. © 2013. The American Astronomical Society. All rights reserved..


Tuntsov A.V.,Manly Astrophysics | Walker M.A.,Manly Astrophysics | Koopmans L.V.E.,University of Groningen | Bannister K.W.,CSIRO | And 4 more authors.
Astrophysical Journal | Year: 2016

Compact radio sources sometimes exhibit intervals of large, rapid changes in their flux density, due to lensing by interstellar plasma crossing the line of sight. A novel survey program has made it possible to discover these "Extreme Scattering Events" (ESEs) in real time, resulting in a high-quality dynamic spectrum of an ESE observed in PKS 1939-315. Here we present a method for determining the column-density profile of a plasma lens, given only the dynamic radio spectrum of the lensed source, under the assumption that the lens is either axisymmetric or totally anisotropic. Our technique relies on the known, strong frequency dependence of the plasma refractive index in order to determine how points in the dynamic spectrum map to positions on the lens. We apply our method to high-frequency (4.2-10.8 GHz) data from the Australia Telescope Compact Array of the PKS 1939-315 ESE. The derived electron column-density profiles are very similar for the two geometries we consider, and both yield a good visual match to the data. However, the fit residuals are substantially above the noise level, and deficiencies are evident when we compare the predictions of our model to lower-frequency (1.6-3.1 GHz) data on the same ESE, thus motivating future development of more sophisticated inversion techniques. © 2016. The American Astronomical Society. All rights reserved.


PubMed | Manly Astrophysics, Curtin University Australia, Max Planck Institute for Radio Astronomy, Jodrell Bank Observatory and 4 more.
Type: Journal Article | Journal: Science (New York, N.Y.) | Year: 2016

Fast radio bursts (FRBs) are millisecond-duration events thought to originate beyond the Milky Way galaxy. Uncertainty surrounding the burst sources, and their propagation through intervening plasma, has limited their use as cosmological probes. We report on a mildly dispersed (dispersion measure 266.5 0.1 parsecs per cubic centimeter), exceptionally intense (120 30 janskys), linearly polarized, scintillating burst (FRB 150807) that we directly localize to 9 square arc minutes. On the basis of a low Faraday rotation (12.0 0.7 radians per square meter), we infer negligible magnetization in the circum-burst plasma and constrain the net magnetization of the cosmic web along this sightline to <21 nanogauss, parallel to the line-of-sight. The burst scintillation suggests weak turbulence in the ionized intergalactic medium.


Kettwich S.C.,Temple College | Kettwich S.C.,University of Wyoming | Anderson D.T.,University of Wyoming | Walker M.A.,Manly Astrophysics | Tuntsov A.V.,Manly Astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2015

We report laboratory measurements of the absorption coefficient of solid para-H2, within the wavelength range from 1 to 16.7 μm, at high spectral resolution. In addition to the narrow rovibrational lines of H2 which are familiar from gas-phase spectroscopy, the data manifest double transitions and broad phonon branches that are characteristic specifically of hydrogen in the solid phase. These transitions are of interest because they provide a spectral signaturewhich is independent of the impurity content of the matrix.We have used our data, in combination with a model of the ultraviolet absorptions of the H2 molecule, to construct the dielectric function of solid para-H2 over a broad range of frequencies. Our results will be useful in determining the electromagnetic response of small particles of solid hydrogen. The dielectric function makes it clear that pure H2 dust would contribute to infrared (IR) extinction predominantly by scattering starlight, rather than absorbing it, and the characteristic IR absorption spectrum of the hydrogen matrix itself will be difficult to observe.


Lin C.Y.,Australian National University | Gilbert A.T.B.,Australian National University | Walker M.A.,Manly Astrophysics
Astrophysical Journal | Year: 2011

We consider the possibility that solid molecular hydrogen is present in interstellar space. If so cosmic-rays and energetic photons cause ionization in the solid leading to the formation of H+6. This ion is not produced by gas-phase reactions and its radiative transitions therefore provide a signature of solid H2 in the astrophysical context. The vibrational transitions of H+6 are yet to be observed in the laboratory, but we have characterized them in a quantum-theoretical treatment of the molecule; our calculations include anharmonic corrections, which are large. Here we report on those calculations and compare our results with astronomical data. In addition to the H+6 isotopomer, we focus on the deuterated species (HD)+3 which is expected to dominate at low ionization rates as a result of isotopic condensation reactions. We can reliably predict the frequencies of the fundamental bands for five modes of vibration. For (HD)+3 all of these are found to lie close to some of the strongest of the pervasive mid-infrared astronomical emission bands, making it difficult to exclude hydrogen precipitates on observational grounds. By the same token these results suggest that (HD)+3 could be the carrier of the observed bands. We consider this possibility within the broader picture of interstellar medium photo-processes and we conclude that solid hydrogen may indeed be abundant in astrophysical environments. © 2011. The American Astronomical Society. All rights reserved.


Tuntsov A.V.,Manly Astrophysics | Tuntsov A.V.,Moscow State University | Bignall H.E.,Curtin University Australia | Walker M.A.,Manly Astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2013

The interstellar scattering responsible for pulsar parabolic arcs, and for intraday variability of compact radio quasars, is highly anisotropic in some cases. We numerically simulate these observed phenomena using totally anisotropic, power-law models for the electron density fluctuations which cause the scattering. By comparing our results to the scattered image of PSR B0834+06 and, independently, to dual-frequency light curves of the quasar PKS 1257-326, we constrain the nature of the scattering media on these lines of sight. We find that models with spectral indices slightly below ß = 3, including the one-dimensional Kolmogorov model, are broadly consistent with both data sets.We confirm that a single physical model suffices for both sources, with the scattering medium simply being more distant in the case of B0834+06. This reinforces the idea that intraday variability and parabolic arcs have a common cause in a type of interstellar structure which, though obscure, is commonplace. However, the implied gas pressure fluctuations are large compared to typical interstellar pressures, and the magnetic stresses are much larger still. Thus while these scattering media may be commonplace, their underlying dynamics appear quite extraordinary. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Walker M.A.,Manly Astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2013

We consider the survival of solid H2 in the diffuse interstellar medium, with application to grains which are small enough to qualify as dust. Consideration of only the thermal aspects of this problem leads to the familiar conclusion that such grains sublimate rapidly. Here we show that charging plays a critical role in determining the sublimation rate, because an electric field helps to bind molecules to the grain surface. A key aspect of the charging process is that the conduction band of solid hydrogen lies above the vacuum free-electron energy level, so low-energy electrons cannot penetrate the solid. But they are attracted by the dielectric and by positive ions in the matrix, so they become trapped in vacuum states just above the surface. This charge-separated configuration suppresses recombination and permits overall neutrality, while supporting large electric fields at the surface. Charging ceases when the potential energy just outside the electron layer coincides with the conduction band minimum. By that stage the heat of sublimation has increased tenfold, effecting a huge reduction in the sublimation rate. Consequently, hydrogen grains may be able to survive indefinitely in the diffuse interstellar medium. There are good prospects for identifying H2 grains, if they exist, as fully localized surface electrons should exhibit discrete energy levels, with a corresponding spectral line signature. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Walker M.A.,Manly Astrophysics
Astrophysical Journal | Year: 2016

In light of evidence for a high ionization rate due to low-energy cosmic rays (LECR) in diffuse molecular gas in the solar neighborhood, we evaluate their heat input to the warm ionized medium (WIM). LECR are much more effective at heating plasma than they are at heating neutrals. We show that the upper end of the measured ionization rates corresponds to a local LECR heating rate sufficient to maintain the WIM against radiative cooling, independent of the nature of the ionizing particles or the detailed shape of their spectrum. Elsewhere in the Galaxy the LECR heating rates may be higher than those measured locally. In particular, higher fluxes of LECR have been suggested for the inner Galactic disk, based on the observed hard X-ray emission, with correspondingly larger heating rates implied for the WIM. We conclude that LECR play an important and perhaps dominant role in the thermal balance of the WIM. © 2016. The American Astronomical Society. All rights reserved.


Tuntsov A.V.,Manly Astrophysics
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2014

Stellar coronae have been invoked to explain the apparently extragalactic dispersion measures observed in fast radio bursts. This paper demonstrates that the suggested plasma densities would lead to deviations from the standard dispersion curve that are inconsistent with the data. The problem is then turned around and higher order dispersion terms are connected to the moments of the density distribution along the line of sight. The deviations quantified in three observed bursts are analysed and a lower limit on the maximum electron density is obtained in one case, although with considerable uncertainty. Selection effects are then discussed and shown to be non-restrictive in relation to plasma density, except at the lowest frequencies and highest temperatures. © 2014 The Author Published by Oxford University Press on behalf of the Royal Astronomical Society.


PubMed | Manly Astrophysics, Curtin University Australia and CSIRO
Type: Journal Article | Journal: Science (New York, N.Y.) | Year: 2016

Extreme scattering events (ESEs) are distinctive fluctuations in the brightness of astronomical radio sources caused by occulting plasma lenses in the interstellar medium. The inferred plasma pressures of the lenses are ~10(3) times the ambient pressure, challenging our understanding of gas conditions in the Milky Way. Using a new survey technique, we discovered an ESE while it was in progress. Here we report radio and optical follow-up observations. Modeling of the radio data demonstrates that the lensing structure is a density enhancement and the lens is diverging, ruling out one of two competing physical models. Our technique will uncover many more ESEs, addressing a long-standing mystery of the small-scale gas structure of our Galaxy.

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