Innovation and discovery at Manly Astrophysics (2007-2017)

1. Pioneering holography of the interstellar medium
Pulsars are known to be very bright, but very compact radio sources. Because of their small size, radio spectra of pulsars show deeply modulated interference fringes resulting from multi-path scattering in the interstellar medium. These fringes evolve in time, and the resulting dynamic spectrum is a type of hologram; it contains a great deal of information about the structure and distribution of the ionised interstellar gas along the line-of-sight to the pulsar. Aiming to extract that information, we developed a novel phase retrieval algorithm which allows the electric field structure to be deduced from a measured pulsar dynamic spectrum. Building on that success, we subsequently embraced the new signal processing technique of cyclic spectroscopy, introduced into astronomy by Paul Demorest (NRAO), which is intrinsically holographic and perfect for pulsars. Holographic techniques have great potential because they provide an image of the signal paths - information that cannot be obtained from the intensities alone. These developments are revealing the structure of the ionised interstellar medium in unprecedented detail; in future they will lead to improvements in pulsar timing precision, and will enable nano-arcsecond imaging of pulsar magnetospheres.
"Interstellar holography"
Walker, M.A., Koopmans, L.V.E., Stinebring, D.R., van Straten, W., 2008 MNRAS 388, 1214
"Cyclic spectroscopy of the millisecond pulsar, B1937+21"
Walker, M.A., Demorest, P.B, van Straten, W., 2013 ApJ, 779, 99

2. Recognising a new interstellar hydrogen molecule
Quantum-mechanical calculations undertaken in the late 1990's had predicted the existence of a new, stable hydrogen molecular ion: H6+. In the early 2000's its existence was confirmed by laboratory studies of electron spin resonance in irradiated solid hydrogens. Bearing in mind that hydrogen is by far the most abundant element in the Universe, potential astronomical signatures of this new molecule are now of great interest. Colleagues Leaf Lin and Andrew Gilbert at the Australian National University undertook high-precision quantum chemical calculations of the vibrational characteristics of H6+, allowing us to compare with the spectral lines that are commonly observed from the interstellar medium. That comparison yielded a striking set of coincidences between the predicted vibrational transitions and the strong mid-infrared interstellar bands, leading us to suggest that those bands could be due to H6+. If that identification is correct then we can also conclude that solid H2 is abundant in interstellar space, because H6+ only forms in significant quantities in the condensed phase of hydrogen.
"Interstellar solid hydrogen"
Lin, C.Y, Gilbert, A.T.B, Walker, M.A., 2011 ApJ, 736, 91

3. Demonstrating that solid H2 can survive in interstellar space
Solid H2 was considered a possible component of interstellar dust until 1969, when calculations showed that it should sublimate in a matter of months. Thereafter solid H2 was excluded from consideration as a candidate dust material, despite the high abundance of the molecule. However, the sublimation calculations were undertaken for a pure solid whereas in reality interstellar dust grains are expected to acquire electrical charges. Charging affects the sublimation rate because the H2 molecules become polarised by the electric field, and additional energy is then needed to remove those molecules from the surface. We demonstrated that, because of the unusual electronic properties of solid H2 - electrons cannot penetrate the crystal, and instead they become bound in quantum states above the surface - large electric fields develop inside interstellar particles of solid H2, resulting in a very strong suppression of the sublimation rate. Charged H2 snowflakes are thus expected to be much more durable than particles of the pure solid and may be able to survive indefinitely. Charged hydrogen snowflakes are a new candidate for interstellar dust.
"A snowflake's chance in heaven"
Walker, M.A., 2013 MNRAS, 434, 2814

4. Developing a new technique to reveal plasma lensing events in progress.
Plasma lensing events (also known as "Extreme Scattering Events") were discovered in the 1980's, through their strong, time-dependent influence on the radio-waves coming from background radio quasars. The characteristics of those events imply the existence of a large population of small blobs of plasma, with such high gas densities that they challenge one of the most basic tenets of the physics of the interstellar medium - i.e. pressure equilibrium. Unfortunately the events are rare, and to find even a single example required large amounts of telescope time when using the conventional observing method, so progress was hampered by a lack of new data. Taking advantage of new, broad-band radio receiver technologies we worked with colleagues at CSIRO (Astronomy and Space Science) to develop a novel search technique that relies on the strong frequency-dependence of the plasma lens magnification. Exploiting the spectral information recorded in each observation allows us to observe infrequently - e.g. monthly, rather than daily - yielding a huge reduction in the requisite telescope time. Implementing the new approach immediately led to the discovery and detailed characterisation of a plasma lensing event.
"Real-time detection of an extreme scattering event: constraints on Galactic plasma lenses"
Bannister, K.W., Stevens, J., Tuntsov, A.,Walker, M.A., Johnston, S., Reynolds, C., Bignall, H.E., 2016 Science, 351, 354

5. Discovering that radio-wave scattering is mainly due to plasma filaments around stars.
Using the technique described in #4 enabled the same team to discover intra-hour variations in the radio quasar PKS1322-110. Although such variations are extreme, they are not unprecedented. What made the discovery important is that this quasar happens to be close (as seen on the sky) to the local, hot star Spica — the 16th brightest star in the sky. That suggests that these rare instances of extremely strong radio-wave scattering are associated with circumstellar plasma. It led us to re-examine two other cases of extreme scattering which had previously been studied in detail by others. That confirmed the hypothesis of an association with local, hot stars, with the plasma taking the form of radially oriented filaments. We found that in many ways the plasma environments of main-sequence stars resemble what is seen in the Helix planetary nebula. In that object the plasma structures form the "skins" of a vast number of tiny molecular clouds, so it now seems likely that stars typically each carry with them a similar population of molecular clouds. If so there is a great deal of additional mass, in the molecular clouds, associated with each such star, and that will have profound implications for many areas of astrophysics, including star- and planet-formation, galaxy dynamics and cosmology.
"Extreme radio-wave scattering associated with hot stars"
Walker, M.A., Tuntsov, A., Bignall, H., Reynolds, C., Bannister, K., Johnston, S., Stevens, J., Ravi, V., 2017, ApJ, 843, 15

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