James E. J. Lovell

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.

Rapid variability and annual cycles in the characteristic timescale of the scintillating source PKS 1257-326

Rapid radio intraday variability (IDV) has been discovered in the southern quasar PKS 1257-326. Flux density changes of up to 40% in as little as 45 minutes have been observed in this source, making it, along with PKS 0405 385 and J1819+3845, one of the three most rapid IDV sources known. We have monitored the IDV in this source with the Australia Telescope Compact Array (ATCA) at 4.8 and 8.6 GHz over the course of the last year and find a clear annual cycle in the characteristic timescale of variability. This annual cycle demonstrates unequivocally that interstellar scintillation is the cause of the rapid IDV at radio wavelengths observed in this source. We use the observed annual cycle to constrain the velocity of the scattering material and the angular size of the scintillating component of PKS 1257-326. We observe a time delay, which also shows an annual cycle, between the similar variability patterns at the two frequencies. We suggest that this is caused by a small (similar to10 muas) offset between the centroids of the 4.8 and 8.6 GHz components and may be due to opacity effects in the source. The statistical properties of the observed scintillation thus enable us to resolve source structure on a scale of similar to10 muas, resolution orders of magnitude higher than current VLBI techniques allow. General implications of IDV for the physical properties of sources and the turbulent interstellar medium are discussed.

A CHANDRA SURVEY OF QUASAR JETS: FIRST RESULTS

We present results from Chandra X-ray imaging and spectroscopy of a flux-limited sample of flat-spectrum radio-emitting quasars with jetlike extended structure. Twelve of 20 quasar jets are detected in 5 ks ACIS-S exposures. The quasars without X-ray jets are not significantly different from those in the sample with detected jets except that the extended radio emission is generally fainter. New radio maps are combined with the X-ray images in order to elucidate the relation between radio and X-ray emission in spatially resolved structures. We find a variety of morphologies, including long straight jets and bends up to 90°. All X-ray jets are one-sided although the radio images used for source selection often show lobes opposite the X-ray jets. The FR II X-ray jets can all be interpreted as inverse Compton scattering of cosmic microwave background photons by electrons in large-scale relativistic jets although deeper observations are required to test this interpretation in detail. Applying this interpretation to the jets as a population, we find that the jets would be aligned to within 30° of the line of sight generally, assuming that the bulk Lorentz factor of the jets is 10.

Discovery of a Subparsec Radio Counterjet in the Nucleus of Centaurus A

A subparsec scale radio counterjet has been detected in the nucleus of the closest radio galaxy, Centaurus A (NGC 5128), with VLBI imaging at 2.3 and 8.4 GHz. This is one of the first detections of a VLBI counterjet and provides new constraints on the kinematics of the radio jets emerging from the nucleus of Cen A. A bright, compact core is seen at 8.4 GHz, along with a jet extending along P.A. 51°. The core is completely absorbed at 2.3 GHz. Our images show a much wider gap between the base of the main jet and the counterjet at 2.3 GHz than at 8.4 GHz and also that the core has an extraordinarily inverted spectrum. These observations provide evidence that the innermost 0.4-0.8 pc of the source is seen through a disk or torus of ionized gas which is opaque at low frequencies due to free-free absorption.

Rapid interstellar scintillation of PKS 1257-326 : Two-station pattern time delays and constraints on scattering and microarcsecond source structure

We report measurements of time delays of up to 8 minutes in the centimeter-wavelength variability patterns of the intrahour scintillating quasar PKS 1257-326, as observed between the VLA and the ATCA on three separate epochs. These time delays confirm interstellar scintillation as the mechanism responsible for the rapid variability, at the same time effectively ruling out the coexistence of intrinsic intrahour variability in this source. The time delays are combined with measurements of the annual variation in variability timescale exhibited by this source to determine the characteristic length scale and anisotropy of the quasars intensity-scintillation pattern, as well as to attempt to fit for the bulk velocity of the scattering plasma responsible for the scintillation. We find evidence for anisotropic scattering and highly elongated scintillation patterns at both 4.9 and 8.5 GHz, with an axial ratio >10 : 1, extended in a northwest direction on the sky. The characteristic scale of the scintillation pattern along its minor axis is well determined, but the high anisotropy leads to degenerate solutions for the scintillation velocity. The decorrelation of the pattern over the baseline gives an estimate of the major-axis length scale of the scintillation pattern. We derive an upper limit on the distance to the scattering plasma of no more than 10 pc.

PMN J0134-0931: A gravitationally lensed quasar with unusual radio morphology

The radio-loud quasar J0134-0931 was discovered to have an unusual morphology during our search for gravitational lenses. In Very Large Array and MERLIN images, there are five compact components with a maximum separation of 07. All of these components have the same spectral index from 5 to 43 GHz. In a Very Long Baseline Array image at 1.7 GHz, a curved arc of extended emission joins two of the components in a manner suggestive of gravitational lensing. At least two of the radio components have near-infrared counterparts. We argue that this evidence implies that J0134-0931 is a gravitational lens, although we have not been able to devise a plausible model for the foreground gravitational potential. Like several other radio-loud lenses, the background source has an extraordinarily red optical counterpart.

The Australia Telescope Large Area Survey: spectroscopic catalogue and radio luminosity functions

The Australia Telescope Large Area Survey (ATLAS) has surveyed 7 square degrees of sky around the Chandra Deep Field South and the European Large Area ISO Survey-South 1 fields at 1.4 GHz. ATLAS aims to reach a uniform sensitivity of 10 μJy  beam −1 rms over the entire region with first data release currently reaching ∼ 30 μJy  beam −1 rms. Here we present 466 new spectroscopic redshifts for radio sources in ATLAS as part of our optical follow-up programme. Of the 466 radio sources with new spectroscopic redshifts, 142 have star-forming optical spectra, 282 show evidence for active galactic nuclei (AGN) in their optical spectra, 10 have stellar spectra and 32 have spectra revealing redshifts, but with insufficient features to classify. We compare our spectroscopic classifications with two mid-infrared diagnostics and find them to be in broad agreement. We also construct the radio luminosity function for star-forming galaxies to z = 0.5 and for AGN to z = 0.8. The radio luminosity function for star-forming galaxies appears to be in good agreement with previous studies. The radio luminosity function for AGN appears higher than previous studies of the local AGN radio luminosity function. We explore the possibility of evolution, cosmic variance and classification techniques affecting the AGN radio luminosity function. ATLAS is a pathfinder for the forthcoming Evolution Map of the Universe (EMU) survey and the data presented in this paper will be used to guide EMUs survey design and early science papers.

The VSOP 5 GHz active galactic nucleus survey. IV. The angular size/brightness temperature distribution

The VLBI Space Observatory Programme (VSOP) mission is a Japanese-led project to study radio sources with submilliarcsecond angular resolution, using an orbiting 8 m telescope on board the satellite HALCA with a global Earth-based array of telescopes. A major program is the 5 GHz VSOP Survey Program, which we supplement here with Very Long Baseline Array observations to produce a complete and flux density - limited sample. Using statistical methods of analysis of the observed visibility amplitude versus projected (u, v) spacing, we have determined the angular size and brightness temperature distribution of bright radio emission from active galactic nuclei. On average, the cores have a diameter ( full width, half-power) of 0.20 mas, which contains about 20% of the total source emission, and 14% +/- 6% of the cores are less than 0.04 mas in size. About 20% +/- 5% of the radio cores have a source frame brightness temperature T-b > 1.0 x 10(13) K, and 3% +/- 2% have T-b > 1.0 x 10(14) K. A model of the high brightness temperature tail suggests that the radio cores have brightness temperatures approximate to1 x 10(12) K and are beamed toward the observer with an average bulk motion of beta = 0.993 +/- 0.004.

PMN J1632?0033: A New Gravitationally Lensed Quasar

We report the discovery of a gravitationally lensed quasar resulting from our survey for lenses in the southern sky. Radio images of J1632-0033 with the Very Large Array and the Australia Telescope Compact Array exhibit two compact, flat-spectrum components with separation 147 and flux density ratio 13.2. Images with the Hubble Space Telescope reveal the optical counterparts to the radio components and also the lens galaxy. An optical spectrum of the bright component obtained with the first Magellan telescope reveals quasar emission lines at redshift 3.42. Deeper radio images with the Multi-Element Radio Linked Interferometry Network and the Very Long Baseline Array reveal a faint third radio component located near the center of the lens galaxy, which is either a third image of the background quasar or faint emission from the lens galaxy.

The VSOP 5 GHz Active Galactic Nucleus Survey. III. Imaging results for the first 102 sources

The VLBI Space Observatory Programme (VSOP) mission is a Japanese-led project to study radio sources with sub-milliarcsec resolution using an orbiting 8 m telescope, HALCA, along with global arrays of Earth-based telescopes. Approximately 25% of the observing time is devoted to a survey of compact active galactic nuclei (AGNs) that are stronger than 1 Jy at 5 GHz-the VSOP AGN Survey. This paper, the third in the series, presents the results from the analysis of the first 102 Survey sources. We present high-resolution images and plots of visibility amplitude versus projected baseline length. In addition, model-fit parameters to the primary radio components are listed, and from these the angular size and brightness temperature for the radio cores are calculated. For those sources for which we were able to determine the source frame core brightness temperature, a significant fraction (53 out of 98) have a source frame core brightness temperature in excess of 10(12) K. The maximum source frame core brightness temperature we observed was 1.2x10(13) K. Explaining a brightness temperature this high requires an extreme amount of relativistic Doppler beaming. Since the maximum brightness temperature one is able to determine using only ground-based arrays is of the order of 10(12) K, our results confirm the necessity of using space VLBI to explore the extremely high brightness temperature regime.