Roland Kothes

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.

THE DISTANCE AND NEUTRAL ENVIRONMENT OF THE MASSIVE STELLAR CLUSTER WESTERLUND 1

Context. In spite of a large number of recent publications about the massive stellar cluster Westerlund 1, its distance from the Sun remains uncertain with values as low as 1.1 kpc, but largely between 4 and 5 kpc. Aims. The goal of this study is to determine a distance to Westerlund 1 independent of the characteristics of the stellar population and to study its neutral environment, using observations of atomic hydrogen. Methods. The HI observations are taken from the Southern Galactic Plane Survey to study HI absorption in the direction of the H II region created by the members of Westerlund 1 and to investigate its environment as observed in the H I line emission. A Galactic rotation curve was derived using the recently revised values for the Galactic centre distance of R ⊙ = 7.6 kpc, and the velocity of the Sun around the Galactic centre of Θ ⊙ = 214 km s -1 . This rotation curve successfully predicts the location of the Tangent point gas and the velocity of the Sagittarius Arm outside the solar circle on the far side of the Galaxy to within 4 km s -1 . Compared to the typically used values of R ⊙ = 8.5 kpc and Θ ⊙ = 220 km s -1 this reduces kinematically determined distances by more than 10%. Results. The newly determined rotation model leads us to derive a distance of 3.9 ±0.7 kpc to Westerlund 1, consistent with a location in the Scutum-Crux Arm. Included in this estimate is a very careful investigation of possible sources of error for the Galactic rotation curve. We also report on small expanding H I features around the cluster with a maximum dynamic age of 600 000 years and a larger bubble which has a minimum dynamic age of 2.5 million years. Additionally we re-calculated the kinematic distances to nearby H II regions and supernova remnants based on our new Galaxic rotation curve. Conclusions. We propose that in the early stages of the development of Wd 1 a large interstellar bubble of diameter about 50 pc was created by the cluster members. This bubble has a dynamic age similar to the age of the cluster. Small expanding bubbles, with dynamical ages ∼0.6 Myr are found around Wd 1, which we suggest consist of recombined material lost by cluster members through their winds.

Radio Continuum Surveys with Square Kilometre Array Pathfinders

In the lead-up to the Square Kilometre Array (SKA) project, several next-generation radio telescopes and upgrades are already being built around the world. These include APERTIF (The Netherlands), ASKAP (Australia), e-MERLIN (UK), VLA (USA), e-EVN (based in Europe), LOFAR (The Netherlands), MeerKAT (South Africa), and the Murchison Widefield Array. Each of these new instruments has different strengths, and coordination of surveys between them can help maximise the science from each of them. A radio continuum survey is being planned on each of them with the primary science objective of understanding the formation and evolution of galaxies over cosmic time, and the cosmological parameters and large-scale structures which drive it. In pursuit of this objective, the different teams are developing a variety of new techniques, and refining existing ones. To achieve these exciting scientific goals, many technical challenges must be addressed by the survey instruments. Given the limited resources of the global radio-astronomical community, it is essential that we pool our skills and knowledge. We do not have sufficient resources to enjoy the luxury of re-inventing wheels. We face significant challenges in calibration, imaging, source extraction and measurement, classification and cross-identification, redshift determination, stacking, and data-intensive research. As these instruments extend the observational parameters, we will face further unexpected challenges in calibration, imaging, and interpretation. If we are to realise the full scientific potential of these expensive instruments, it is essential that we devote enough resources and careful study to understanding the instrumental effects and how they will affect the data. We have established an SKA Radio Continuum Survey working group, whose prime role is to maximise science from these instruments by ensuring we share resources and expertise across the projects. Here we describe these projects, their science goals, and the technical challenges which are being addressed to maximise the science return.

RADIO POLARIMETRY OF THE ELAIS N1 FIELD: POLARIZED COMPACT SOURCES

We present deep polarimetric observations at 1420 MHz of the European Large Area ISO Survey North 1 region (ELAIS N1) as part of the Dominion Radio Astrophysical Observatory Planck Deep Fields project. By combining closely spaced aperture synthesis fields, we image a region of 7.43 deg^2 to a maximum sensitivity in Stokes Q and U of 78 μJy beam^(-1), and detect 786 compact sources in Stokes I. Of these, 83 exhibit polarized emission. We find that the differential source counts (log N-log p) for polarized sources are nearly constant down to p > 500 μJy, and that these faint polarized radio sources are more highly polarized than the strong source population. The median fractional polarization is 4.8% ± 0.7% for polarized sources with Stokes I flux density between 10 and 30 mJy, approximately 3 times larger than sources with I > 100 mJy. The majority of the polarized sources have been identified with galaxies in the Spitzer Wide Area Infrared Extragalactic Survey (SWIRE) image of ELAIS N1. Most of the galaxies occupy regions in the IRAC 5.8 μm/3.6 μm versus 8.0 μm/4.5 μm color-color diagram associated with dusty AGNs, or with ellipticals with an aging stellar population. A few host galaxies have colors that suggests significant PAH emission in the near-infrared. A small fraction, 12%, of the polarized sources are not detected in the SWIRE data. None of the polarized sources in our sample appears to be associated with an actively star-forming galaxy.

Discovery of an Energetic Pulsar Associated with SNR G76.9+1.0

We report the discovery of PSR J2022+3842, a 24 ms radio and X-ray pulsar in the supernova remnant G76.9+1.0, in observations with the Chandra X-ray telescope, the Robert C. Byrd Green Bank Radio Telescope, and the Rossi X-ray Timing Explorer (RXTE). The pulsar’s spin-down rate implies a rotation-powered luminosity u E = 1.2×10 38 erg s −1 , a surface dipole magnetic field strength Bs = 1.0×10 12 G, and a characteristic age of 8.9 kyr. PSR J2022+3842 is thus the second-most energetic Galactic pulsar known, after the Crab, as well as the most rapidly-rotating young, radio-bright pulsar known. The radio pulsations are highly dispersed and broadened by interstellar scattering, and we find that a large (�f/f ≈ 1.9 × 10 −6 ) spin glitch must have occurred between our discovery and confirmation observations. The X-ray pulses are narrow (0.06 cycles FWHM) and visible up to 20 keV, consistent with magnetospheric emission from a rotation-powered pulsar. The Chandra X-ray image identifies the pulsar with a hard, unresolved source at the midpoint of the double-lobed radio morphology of G76.9+1.0 and embedded within faint, compact X-ray nebulosity. The spatial relationship of the X-ray and radio emissions is remarkably similar to extended structure seen around the Vela pulsar. The combined Chandra and RXTE pulsar spectrum is well-fitted by an absorbed power-law model with column density NH = (1.7 ± 0.3) × 10 22 cm −2 and photon index = 1 .0 ± 0.2; it implies that the Chandra point-source flux is virtually 100% pulsed. For a distance of 10 kpc, the X-ray luminosity of PSR J2022+3842 is LX(2–10 keV) = 7.0 × 10 33 erg s −1 . Despite being extraordinarily energetic, PSR J2022+3842 lacks a bright X-ray wind nebula and has an unusually low conversion efficiency of spin-down power to X-ray luminosity,LX/ u E = 5.9 × 10 −5 .

The Distance to Supernova Remnant CTB 109 Deduced from Its Environment

We conducted a study of the environment around the supernova remnant CTB109. We found that the SNR is part of a large complex of HII regions extending over an area of 400 pc along the Galactic plane at a distance of about 3 kpc at the closer edge of the Perseus spiral arm. At this distance CTB109 has a diameter of about 24 pc. We demonstrated that including spiral shocks in the distance estimation is an ultimate requirement to determine reliable distances to objects located in the Perseus arm. The most likely explanation for the high concentration of HII regions and SNRs is that the star formation in this part of the Perseus arm is triggered by the spiral shock.

Discovery of New Faint Radio Emission on 8° to 3' Scales in the Coma Field, and Some Galactic and Extragalactic Implications

We present a deep, 8 ◦ diameter, 0.4 GHz radio image using a first time combination of the NAIC Arecibo 305-m telescope in Puerto Rico, and the wide-angle interferometer at the Dominion Radio Astrophysical Observatory at Penticton, Canada. Our observations are centered on the Coma Cluster of galaxies in the “Great Wall” of galaxies near the North Galactic Pole. The complementary nature of these two instruments enables us to produce a distortion-free image that is sensitive to radiation on scales from 8 ◦ down to that of an individual galaxy halo at the 100 Mpc distance of the Great Wall. Newly revealed patches of distributed radio “glow” are seen well above the detection limit. One prominent such area coincides with groupings of radio galaxies near the Coma cluster, and indicates intergalactic IGM magnetic fields in the range 0.2 to 0.4 µG on scales of up to ∼ 4Mpc. Other patches of diffuse emission, not previously explored at these high latitudes on arcminute scales, probably contain Galactic “cirrus”. A striking anticorrelation is found between low-level diffuse radio glow and some regions of enhanced optical galaxy surface density, suggesting that cosmological Large Scale Structure (LSS), normally defined by the baryonic (or dark) matter density, is not uniquely traced by faint continuum radio glow. Rather, intergalactic diffuse synchrotron radiation represents IGM magnetic and Cosmic ray energy density, instead of matter density. The diffuse, arcminute-level structures over a large region of sky are potentially important pathfinders to CMB foreground radiation on high multipole scales.

Two supernova remnants of low radio surface brightness discovered in the Canadian Galactic Plane Survey

Two new supernova remnants (SNRs), G85.4+0.7 and G85.9-0.6, superimposed on the radio source W 80, have been discovered in the radio continuum data from the Canadian Galactic Plane Survey(CGPS). Both SNRs consist of a thin incomplete radio shell surrounding a weak, extended X-ray source. G85.4+0.7 has a thin non-thermal shell of diameter0:4 lying within a thermal shell whose diameter is0:6. Its radio surface brightness at 1 GHz is 1 GHz 1 10 22 Watt m 2 Hz 1 sr 1 . It is located within a large H i bubble, whose systemic velocity, vLSR = 12 km s 1 , implies a distance of 3.8 kpc. Two B1 stars detected within this bubble are most likely part of the OB association which formed it. The diameter of the H i bubble is about 100 pc. The SNR has a diameter of about 30 pc and probably is the result of a type II explosion of an early B star 6300 years ago. G85.9-0.6 has a radio surface brightness of 1 GHz 2 10 22 Watt m 2 Hz 1 sr 1 .N o Hi features corresponding to the SNR have been detected. This, and the low radio and X-ray brightnesses, suggest expansion in a low-density medium. The SNR may lie in the low-density region between the local and Perseus spiral arms, at a distance of about 5 kpc. Its diameter would then be35 pc.

DA 495: An Aging Pulsar Wind Nebula

We present a radio continuum study of the pulsar wind nebula (PWN) DA 495 (G65.7+1.2), including images of total intensity and linear polarization from 408 to 10550 MHz based on the Canadian Galactic Plane Survey and observations with the Effelsberg 100 m Radio Telescope. Removal of flux density contributions from a superimposed H II region and from compact extragalactic sources reveals a break in the spectrum of DA 495 at 1.3 GHz, with a spectral index α = − 0.45 ± 0.20 below the break and α = − 0.87 ± 0.10 above it (Sν ∝ να). The spectral break is more than 3 times lower in frequency than the lowest break detected in any other PWN. The break in the spectrum is likely the result of synchrotron cooling, and DA 495, at an age of ~20,000 yr, may have evolved from an object similar to the Vela X nebula, with a similarly energetic pulsar. We find a magnetic field of ~1.3 mG inside the nebula. After correcting for the resulting high internal rotation measure, the magnetic field structure is quite simple, resembling the inner part of a dipole field projected onto the plane of the sky, although a toroidal component is likely also present. The dipole field axis, which should be parallel to the spin axis of the putative pulsar, lies at an angle of ~50° east of the north celestial pole and is pointing away from us toward the southwest. The upper limit for the radio surface brightness of any shell-type supernova remnant emission around DA 495 is Σ1GHz ~ 5.4 × 10−23 W m−2 Hz−1 sr−1 (assuming a radio spectral index of α = − 0.5), lower than the faintest shell-type remnant known to date.

The Supernova Remnant CTB 104A: Magnetic Field Structure and Interaction with the Environment

We present new, high-resolution 1420 and 408 MHz continuum images and H I and 12CO (J = 1-0) spectral line maps of the diffuse supernova remnant (SNR) CTB 104A (G93.7-0.3). Analysis of the complex continuum emission reveals no significant spectral index variations across the remnant. Three prominences around CTB 104A are found to be related to the SNR, while one extension to the east is identified as an H II region associated with a background molecular shell. Small-scale polarization and rotation measure (RM) structures are turbulent in nature, but we find a well-ordered RM gradient across the remnant, extending from southeast to northwest. This gradient does not agree with the direction of the global Galactic magnetic field but does agree with a large-scale RM anomaly inferred from RM data by Clegg et al. We show that the observed morphology of CTB 104A is consistent with expansion in a uniform magnetic field, and this is supported by the observed RM distribution. By modeling the RM gradient with a simple compression model we have determined the magnetic field strength within the remnant as B0 ≈ 2.3 μG. We have identified signatures of the interaction of CTB 104A with the surrounding neutral material and determined its distance, from the kinematics of the H I structure encompassing the radio emission, as 1.5 kpc. We also observed clear breaks in the H I shell that correspond well to the positions of two of the prominences, indicating regions where hot gas is escaping from the interior of the SNR.