Adam T. Deller

DiFX : A software correlator for very long baseline interferometry using multiprocessor computing environments

We describe the development of an FX-style correlator for very long baseline interferometry (VLBI), implemented in software and intended to run in multiprocessor computing environments, such as large clusters of commodity machines (Beowulf clusters) or computers specifically designed for high-performance computing, such as multiprocessor shared-memory machines. We outline the scientific and practical benefits for VLBI correlation, these chiefly being due to the inherent flexibility of software and the fact that the highly parallel and scalable nature of the correlation task is well suited to a multiprocessor computing environment. We suggest scientific applications where such an approach to VLBI correlation is most suited and will give the best returns. We report detailed results from the Distributed FX (DiFX) software correlator running on the Swinburne supercomputer (a Beowulf cluster of ∼300 commodity processors), including measures of the performance of the system. For example, to correlate all Stokes products for a 10 antenna array with an aggregate bandwidth of 64 MHz per station, and using typical time and frequency resolution, currently requires an order of 100 desktop- class compute nodes. Due to the effect of Moores law on commodity computing performance, the total number and cost of compute nodes required to meet a given correlation task continues to decrease rapidly with time. We show detailed comparisons between DiFX and two existing hardware-based correlators: the Australian Long Baseline Array S2 correlator and the NRAO Very Long Baseline Array correlator. In both cases, excellent agreement was found between the correlators. Finally, we describe plans for the future operation of DiFX on the Swinburne supercomputer for both astrophysical and geodetic science.

DiFX-2: A More Flexible, Efficient, Robust, and Powerful Software Correlator

Software correlation, where a correlation algorithm written in a high-level language such as C++ is run on commodity computer hardware, has become increasingly attractive for small- to medium-sized and/or bandwidth-constrained radio interferometers. In particular, many long-baseline arrays (which typically have fewer than 20 elements and are restricted in observing bandwidth by costly recording hardware and media) have utilized software correlators for rapid, cost-effective, correlator upgrades to allow compatibility with new, wider-bandwidth, recording systems and to improve correlator flexibility. The DiFX correlator, made publicly available in 2007, has been a popular choice in such upgrades and is now used for production correlation by a number of observatories and research groups worldwide. Here, we describe the evolution in the capabilities of the DiFX correlator over the past three years, including a number of new capabilities, substantial performance improvements, and a large amount of supporting infrastructure to ease use of the code. New capabilities include the ability to correlate a large number of phase centers in a single correlation pass, the extraction of phase-calibration tones, correlation of disparate but overlapping sub-bands, the production of rapidly sampled filter-bank and kurtosis data at minimal cost, and many more. The latest version of the code is at least 15% faster than the original (and in certain situations, many times this value). Finally, we also present detailed test results validating the correctness of the new code.

A strong magnetic field around the supermassive black hole at the centre of the Galaxy

Earth’s nearest candidate supermassive black hole lies at the centre of the Milky Way. Its electromagnetic emission is thought to be powered by radiatively inefficient accretion of gas from its environment, which is a standard mode of energy supply for most galactic nuclei. X-ray measurements have already resolved a tenuous hot gas component from which the black hole can be fed. The magnetization of the gas, however, which is a crucial parameter determining the structure of the accretion flow, remains unknown. Strong magnetic fields can influence the dynamics of accretion, remove angular momentum from the infalling gas, expel matter through relativistic jets and lead to synchrotron emission such as that previously observed. Here we report multi-frequency radio measurements of a newly discovered pulsar close to the Galactic Centre and show that the pulsar’s unusually large Faraday rotation (the rotation of the plane of polarization of the emission in the presence of an external magnetic field) indicates that there is a dynamically important magnetic field near the black hole. If this field is accreted down to the event horizon it provides enough magnetic flux to explain the observed emission—from radio to X-ray wavelengths—from the black hole.

Science with the Australian Square Kilometre Array Pathfinder

The future of cm and m-wave astronomy lies with the Square Kilometre Array (SKA), a telescope under development by a consortium of 17 countries that will be 50 times more sensitive than any existing radio facility. Most of the key science for the SKA will be addressed through large-area imaging of the Universe at frequencies from a few hundred MHz to a few GHz. The Australian SKA Pathfinder (ASKAP) is a technology demonstrator aimed in the mid-frequency range, and achieves instantaneous wide-area imaging through the development and deployment of phased-array feed systems on parabolic reflectors. The large field-of-view makes ASKAP an unprecedented synoptic telescope that will make substantial advances in SKA key science. ASKAP will be located at the Murchison Radio Observatory in inland Western Australia, one of the most radio-quiet locations on the Earth and one of two sites selected by the international community as a potential location for the SKA. In this paper, we outline the ASKAP project and summarise its headline science goals as defined by the community at large.

A millisecond pulsar in a stellar triple system

Gravitationally bound three-body systems have been studied for hundreds of years and are common in our Galaxy. They show complex orbital interactions, which can constrain the compositions, masses and interior structures of the bodies and test theories of gravity, if sufficiently precise measurements are available. A triple system containing a radio pulsar could provide such measurements, but the only previously known such system, PSR B1620-26 (refs 7, 8; with a millisecond pulsar, a white dwarf, and a planetary-mass object in an orbit of several decades), shows only weak interactions. Here we report precision timing and multiwavelength observations of PSR J0337+1715, a millisecond pulsar in a hierarchical triple system with two other stars. Strong gravitational interactions are apparent and provide the masses of the pulsar (1.4378(13), where is the solar mass and the parentheses contain the uncertainty in the final decimal places) and the two white dwarf companions (0.19751(15) and 0.4101(3)), as well as the inclinations of the orbits (both about 39.2°). The unexpectedly coplanar and nearly circular orbits indicate a complex and exotic evolutionary past that differs from those of known stellar systems. The gravitational field of the outer white dwarf strongly accelerates the inner binary containing the neutron star, and the system will thus provide an ideal laboratory in which to test the strong equivalence principle of general relativity.

Accretion-powered Pulsations in an Apparently Quiescent Neutron Star Binary

Accreting millisecond X-ray pulsars (AMXPs) are an important subset of low-mass X-ray binaries (LMXBs) in which coherent X-ray pulsations can be observed during occasional, bright outbursts (X-ray luminosity ). These pulsations show that matter is being channeled onto the neutron stars magnetic poles. However, such sources spend most of their time in a low-luminosity, quiescent state (L_X ≲ 10^(34) erg s^(-1)), where the nature of the accretion flow onto the neutron star (if any) is not well understood. Here we report that the millisecond pulsar/LMXB transition object PSR J1023+0038 intermittently shows coherent X-ray pulsations at luminosities nearly 100 times fainter than observed in any other AMXP. We conclude that in spite of its low luminosity, PSR J1023+0038 experiences episodes of channeled accretion, a discovery that challenges existing models for accretion onto magnetized neutron stars.

Spiral density waves in a young protoplanetary disk.

Gravitational forces are expected to excite spiral density waves in protoplanetary disks, disks of gas and dust orbiting young stars. However, previous observations that showed spiral structure were not able to probe disk midplanes, where most of the mass is concentrated and where planet formation takes place. Using the Atacama Large Millimeter/submillimeter Array, we detected a pair of trailing symmetric spiral arms in the protoplanetary disk surrounding the young star Elias 2-27. The arms extend to the disk outer regions and can be traced down to the midplane. These millimeter-wave observations also reveal an emission gap closer to the star than the spiral arms. We argue that the observed spirals trace shocks of spiral density waves in the midplane of this young disk.

On the structure of the transition disk around TW Hydrae

Context. For over a decade, the structure of the inner cavity in the transition disk of TW Hydrae has been a subject of debate. Modeling the disk with data obtained at di erent wavelengths has led to a variety of proposed disk structures. Rather than being inconsistent, the individual models might point to the di erent faces of physical processes going on in disks, such as dust growth and planet formation. Aims. Our aim is to investigate the structure of the transition disk again and to find to what extent we can reconcile apparent model di erences. Methods. A large set of high-angular-resolution data was collected from near-infrared to centimeter wavelengths. We investigated the existing disk models and established a new self-consistent radiative-transfer model. A genetic fitting algorithm was used to automatize the parameter fitting, and uncertainties were investigated in a Bayesian framework. Results. Simple disk models with a vertical inner rim and a radially homogeneous dust composition from small to large grains cannot reproduce the combined data set. Two modifications are applied to this simple disk model: (1) the inner rim is smoothed by exponentially decreasing the surface density in the inner 3 AU, and (2) the largest grains (>100 m) are concentrated towards the inner disk region. Both properties can be linked to fundamental processes that determine the evolution of protoplanetary disks: the shaping by a possible companion and the di erent regimes of dust-grain growth, respectively. Conclusions. The full interferometric data set from near-infrared to centimeter wavelengths requires a revision of existing models for the TW Hya disk. We present a new model that incorporates the characteristic structures of previous models but deviates in two key aspects: it does not have a sharp edge at 4 AU, and the surface density of large grains di ers from that of smaller grains. This is the first successful radiative-transfer-based model for a full set of interferometric data.

A PARALLAX DISTANCE AND MASS ESTIMATE FOR THE TRANSITIONAL MILLISECOND PULSAR SYSTEM J1023+0038

The recently discovered transitional millisecond pulsar system J1023+0038 exposes a crucial evolutionary phase of recycled neutron stars for multiwavelength study. The system, comprising the neutron star itself, its stellar companion, and the surrounding medium, is visible across the electromagnetic spectrum from the radio to X-ray/gamma-ray regimes and offers insight into the recycling phase of millisecond pulsar evolution. Here, we report on multiple-epoch astrometric observations with the Very Long Baseline Array (VLBA) which give a system parallax of 0.731 ± 0.022 milliarcseconds (mas) and a proper motion of 17.98 ± 0.05 mas yr–1. By combining our results with previous optical observations, we are able to use the parallax distance of 1368+42 – 39 pc to estimate the mass of the pulsar to be 1.71 ± 0.16 M ☉, and we are also able to measure the three-dimensional space velocity of the system to be 126 ± 5 km s–1. Despite the precise nature of the VLBA measurements, the remaining ~3% distance uncertainty dominates the 0.16 M ☉ error on our mass estimate.

Extremely High Precision VLBI Astrometry of PSR J0437?4715 and Implications for Theories of Gravity

Using the recently upgraded Long Baseline Array, we have measured the trigonometric parallax of PSR J0437–4715 to better than 1% precision, the most precise pulsar distance determination made to date. Comparing this VLBI distance measurement to the kinematic distance obtained from pulsar timing, which is calculated from the pulsars proper motion and apparent rate of change of orbital period, gives a precise limit on the unmodeled relative acceleration between the solar system and PSR J0437–4715, which can be used in a variety of applications. First, it shows that Newtons gravitational constant G is stable with time (Ġ/G = (−5 ± 26) × 10−13 yr−1, 95% confidence). Second, if a stochastic gravitational wave background existed at the currently quoted limit, this null result would fail ~50% of the time. Third, it excludes Jupiter-mass planets within 226 AU of the Sun in 50% of the sky (95% confidence). Finally, the ~1% agreement of the parallax and orbital period derivative distances provides a fundamental confirmation of the parallax distance method on which all astronomical distances are based.