Abstracts
SKA Science Meeting
Calgary, July 19-22, 1998

The Dark Ages

Avery Meiksin, Institute for Astronomy, University of Edinburgh

Prior to the epoch of full reionization, the intergalactic medium and gravitationally collapsed systems will be detectable in 21cm radiation. Physical mechanisms that would produce a 21cm signature are discussed. These include Lya coupling of the hydrogen spin temperature to the kinetic temperature of the gas resulting from the radiation by an early generation of stars, preheating by soft x-rays from collapsing dark matter halos, and preheating by ambient Lya photons. A patchwork of either 21cm emission, or absorption against the Cosmic Microwave Background, will result. Radio surveys offer the prospect of measuring the growth of early structures in the universe and of probing the transitional epoch from a neutral universe to one that is fully ionized.

A Census of HI in Galaxies

J.M. van der Hulst, Kapteyn Institute

The Square Kilometer Array will provide unique opportunities to probe the HI content of galaxies out to large redshifts. A few examples are: (i) it will be possible to resolve L* galaxies out to a redshift of ~0.7 and measure rotation curves to determine the mass distributions of dark and luminous matter using integration times of only a few days. This will make it possible to trace the evolution of gas disks in galaxies; (ii) Beyond this redshift SKAI will still be able to detect galaxies and measure HI contents, so it will become possible to trace the evolution of HI content from the present to a redshift of about 2 using only a 14 day integration. This is sufficiently past the peak in star formation activity to be of great importance; and (iii) SKAI is extremely well fit to carry out a survey of galaxies over a large area of sky to map the large scale structure out to a depth of z=1. The expected number of detections is much larger than expected from the Sloan survey and maps the large scale structure to much greater depth were it is easier to trace the effects of different cosmological models.

HI Environment of Quasars

Jeremy Lim, Academia Sinica Institute of Astronomy & Astrophysics

The SKA should make possible exquisite studies of galaxies in neutral atomic hydrogen (HI) gas up to very high redshifts. Here we present a modest preview of what the SKA may be able to learn from HI observations of quasar host galaxies through the eyes of the VLA. The quasar phenomenon has long been thought to be triggered by the severe disruption of their otherwise normal host galaxies due to galaxy-galaxy encounters or mergers. Yet, recent high-resolution observations with the HST reveal that the larger fraction of the observed low-redshift quasars reside in apparently serene host galaxies only some of which have projected neighbouring galaxies. Here we show that the serene optical appearance of quasar host galaxies can be deceptive; by contrast, HI images reveal violent tidal disruptions not otherwise apparent. These results bridge the important transition from purely morphological studies of quasar host galaxies to dynamical studies.

Deep Continuum Fields

Peter Wilkinson, Nuffield Radio Astronomy Laboratories

The draft Science Case outlines, in broad terms,  the impact that SKAI will have on studies of ultra-faint radio sources at cosmologically significant redshifts. I will concentrate on summarising what we currently know about the faintest radio  sources and try to guess from this what SKAI might tell us at flux levels 100 times fainter.

The deepest imaging at 20cm  wavelength comes from a combination of VLA and MERLIN data on the Hubble Deep Field and Flanking Fields; the noise level achieved is ~3 microJy at a resolution of 0.2 arcsec.  Even at these flux levels only a tiny fraction (<0.5%) of HDF galaxies are detected as radio sources. The images confirm the importance of baselines >200km since most sources are unresolved with the VLA, A array beam alone. In broad terms most of these 10s of microJy sources have steepish spectra, diffuse kpc--scale morphologies and are identified with spiral galaxies. It almost certain that these  galaxies are undergoing an intense burst of star formation -- the faintest is over an order of magnitude more luminous than M82. A significant minority of sources is identified with elliptical galaxies and show AGN characteristics. There may be sources in the HDF with no optical identification; these would be candidate dust--shrouded galaxies i.e. I band dropouts.

Summary of the "Sub-microJansky Radio Sky" Workshop

Andrew Hopkins, C. Jackson, L. Cram, A. Green, D. Manchester, L. Staveley-Smith, R. Norris and D. Jauncey.

A number of separate studies are presently investigating how best to design the 1kT to carry out the kinds of observations desired by the astronomical community. We present a summary of one of these studies, a workshop called the "Sub-microJansky Radio Sky," held at ATNF, Sydney, on 17 June 1998. This workshop addressed the nature of the radio sky at the very faint flux densities likely to be attainable by the 1kT. In particular, each speaker investigated a separate population of radio sources and how the expected appearance of that population at such faint flux densities would dictate how to refine some of the design constraints for the 1kT.

Observations of Extragalactic Supernova Remnants in Starburst Galaxies

Alan Pedlar, Nuffield Radio Astronomy Laboratories

I will review the use of supernova remnants (SNR) to investigate the starburst phenomenon in galaxies. These observations can give a direct measure of the supernova rate, and hence, via the initial mass function, the star formation rates in these galaxies. In addition, through absorption measurements, the SNR can be used to probe the ionised and neutral component of the starburst ISM on parsec scales. A further aspect of this research is that each starburst acts as a laboratory for the statistical study of well-defined samples of SNR.

The work is currently strongly limited by brightness sensitivity and hence only a few nearby starbursts have been studied in depth. I will illustrate the sensitivity problem with reference to recent MERLIN and VLBI observations and indicate the areas in which SKAI would greatly benefit this area of research.

Polarimetric and Continuum Observations of Faint Sources

John R. Dickel, University of Illinois

I discuss two particular types of projects to challenge the SKAI/1kT.

VLA observations of M31*, the black hole candidate in the center of M31, show that it can vary by over a factor of two on timescales from weeks to years. Theories predict that the variations can occur in days or less and that the radio emission should vary oppositely to that at x-ray wavelengths (e.g. Melia, 1992, ApJ, 398, L95). Because it takes at least 24 hours of integration time to measure this source - twice it has been undetected at < 15 microJy - current instruments are not capable of checking the theoretical predictions of the variability timescales nor of determining the spectral index of the source. The proposed sensitivity of the SKAI/1kT should provide accurate measurements in a few hours. We will be able, for the first time, to get simultaneous light curves of the source at two radio wavelengths to see specifically how the brightness changes and allow accurate modelling of the emission processes.

An important parameter for understanding the particle acceleration processes in Galactic and extragalactic synchrotron sources is the variation in continuum spectral index across a source. Experience has shown that because the smooth and fine scale structures can have different spectra, it is vital to cover a full range of spatial frequencies at each radio frequency when making spectral index measurements and also to have as identical as possible coverage at each frequency. Either the filling factor of the array should be large or the configuration of elements must be adjusted so that good baseline scaling can be obtained between 3 or 4 set frequencies.

Pushing the Envelope: The Radio Afterglows of Gamma-ray Bursters

Dale Frail, National Radio Astronomy Observatory

I will review what progress has been made in the understanding of gamma-ray bursters, concentrating on where radio observations have contributed to this field. The full potential of the radio observations has yet to be realized. To do so will require new and improved capabilities in existing or planned radio telescopes.

Masers, Megamasers, and the 1kT

Ray Norris. Australia Telescope National Facility

The interest in masers stems primarily from their use as tools to investigate the kinematics of regions containing them.

The most spectacular such use was the discovery that the H2O masers in NGC4258 are confined to a thin molecular disk, only 0.5 pc in diameter, surrounding the central engine. This discovery has provided the best evidence to date for the existence of massive Black Holes (MBH) in active galactic nuclei (AGN). As a result, H2O megamasers are now becoming one of the most powerful tools available to us for probing the inner parsecs of active galaxies, for example providing a mass estimate, accurate to a few percent, of the MBH, and exploring the turn-on of the radio jet a fraction of a pc from the MBH. The 1kT will allow us to detect and study such megamasers up to a redshift 0f 0.5, and thus allow us to explore the properties of the MBH as a function of galaxy type and evolution.

Other maser work has concentrated on the life and death-processes of stars, using masers in the interstellar and circumstellar environment to map the kinematics and properties of the accreting or outflowing gas. The 1kT will enable us to extend this work to nearby galaxies, so we can compare star formation processes in our own galaxy with those in starburst galaxies such as NGC253. A by-product of studying OH/IR stars will be to make a direct measurement of distance to nearby galaxies without any of the usual assumptions involving standard candles or standard rulers.

Use of ARISE with the SKA

J.S. Ulvestad - NRAO

A future Space VLBI mission, ARISE, is under consideration by NASA for launch after 2008. A 25-m telescope would be launched, operating at frequencies (nominally) between 8 GHz and 86 GHz. At the lower frequencies, the combination of ARISE and SKA will give the most sensitive Space VLBI baseline available in the foreseeable future. This talk will briefly describe ARISE and some of its capabilities in combination with the SKA.

SKA and its Optical and IR Counterparts

N. Duric, University of New Mexico

The full power of SKA will be realised not in isolation but in conjunction with its analogues in the optical and IR domain. A number of optical and IR interferometers are being built and/or are planned to be operational on time scale comparable to that of SKA. Together with SKA these interferometers will provide an unprecedented view of compact astrophysical objects. The science that can be derived from multi-frequency, high-resolution observations will be discussed.

Low Frequency Radio Astronomy with the SKA

Namir E. Kassim, Naval Research Laboratory
William C. Erickson, University of Tasmania

Even though cosmic radio waves were discovered at long wavelengths, the limited angular resolution imposed on radio interferometers by the ionosphere has left the low frequency region of the electromagnetic spectrum largely unexplored. The new 74 MHz VLA system has now established that self-cal can effectively remove the ionospheric phase fluctuations, which opens the possibility of exploring this region at high resolution for the first time. I will discuss some of the interesting questions which can be explored at these frequencies by a SKA which includes a long wavelength, broad-band capability. Such a capability would offer good and unique research and could be built at a relatively low cost.

Is the Square Km Array useful for Recombination Lines below 500 MHz?

K.R. Anantharamaiah, NRAO, USA and Raman Research Institute, India

Since it would be much less expensive to realise a Square Km Array which operates only at low-frequencies, I explore the usefulness of such an array for studies of galactic and extragalactic radio recombination lines at frequencies below 500 MHz. I will also point to other very useful studies such as of Pulsars and of the Deuterium line that can be made with such an array.

Observations of Galactic Supernova Remnants with the SKA

Brad Wallace, Herzberg Institute of Astrophysics

Known galactic SNRs range in size from about 1' to several degrees, and in intensity from among the strongest radio sources to the faintest detectable. As such, the study of Galactic SNRs stretches the abilities of any single radio telescope. While the proposed SKA is no different in this regard, it does promise to advance many research areas related to Galactic SNRs.

Present observations of Galactic SNRs, for example, have been largely limited to wavelengths between 6 and 90 cm, and even in this restricted wavelength range only a handful of remnants (typically the brightest) have been well imaged. The proposed high-resolution, wide-field, imaging capabilities of the SKA at long wavelengths will allow better determinations of the spectral indices of Galactic SNRs and, more importantly, the spectral index variations within remnants to better understand particle acceleration mechanisms and to search for thermal material associated with the remnants.

Similarly, only the brightest Galactic SNRs have been imaged at high resolution. The ability of the SKA to image structures at sub-arcsecond resolution will allow unprecedented radio wavelength studies of the interaction between pulsars and their remnants, the structure of compact knots, and searches for young and/or distant remnants.

Imaging Stellar Surfaces

Jeremy Lim, Academia Sinica Institute of Astronomy & Astrophysics

Imaging the surfaces of other stars is one of the major frontiers of stellar astronomy, and promises to revolutionize our understanding of stellar astrophysics.  Accomplishing this at radio wavelengths requires very high sensitivity at high angular resolutions, requirements which are fundamental to the technical capabilities of the SKA.  Here we present a preview of the exciting results SKA is likely to obtain from imaging the surfaces of red giant and supergiant stars, based on recent VLA imaging observations of the red supergiant Betelgeuese.  These observations reveal that the radio surface of Betelgeuse at 7-mm is asymmetric, and that the temperature of the atmosphere decreases steadily from the optical photospheric temperature with height.  We discuss the implications of these results for the mechanisms thought responsible for extending Betelgeuse's atmosphere, as well as driving its prodigious mass-loss.  These results stress the importance of multi-frequency observations to mm-wavelengths for studying the surfaces of red giant and supergiant stars.

New Perspectives in Cool Star Astronomy with the SKA

Manuel Guedel, Paul Scherrer Institute, Switzerland

Radio astronomy has identified a bewildering variety of radio emissions from magnetically active stars, although most emissions are difficult to characterise in the context of solar physics. Some active stars are sources of both steady gyrosynchrotron emission and extremely luminous coherent microwave radiation, features that are unknown on the Sun. Simple estimates show that present-day stellar radio astronomy is in a ``tip-of-the-iceberg'' situation: Quite likely we do not see typical solar-type emissions in active stars, but either detect the extremes or some different phenomena. A typical nearby solar analogue can hardly be detected with present instrumentation even during flares. This presentation discusses what we may detect in terms of solar phenomena with the SKA in nearby stars. Most of the crucial phenomena being studied by solar astronomers will be routinely observable on a number of targets. In particular, coherent emissions of various types, small and short gyrosynchrotron bursts, or chromospheric and TR emission of non-flaring coronae will become easily accessible. The high sensitivity will allow us to obtain high S/N light curves that will be important to search for micro-variability and to investigate acceleration event statistics.

Novae

Tim O'Brien,  John Moores University

The study of nova outbursts has a bearing on many areas of astrophysics including evolution of binary star systems, accretion disks, thermonuclear processes, common-envelope phases, mass-loss via winds, clumping, dust formation and hydrodynamics. Our understanding of these events is severely limited by their unpredictability and transient nature.

I will review previous radio observations of classical novae, including recent results obtained with MERLIN and the VLA, and describe the limitations imposed by current sensitivities. I will then assess the potential impact of the SKA on observations of novae as a whole and on a few individual objects.

Stellar winds, with sensitivity

Sean M. Dougherty,  Univ. of Calgary/DRAO

Radio interferometer arrays such as the VLA and MERLIN have sufficient sensitivity to image a small number of stellar wind systems with brightness temperatures ~10^4 K at a resolution of ~50 mas. This size scale corresponds to 50 AU at a distance of 1kpc, typical of the radii of the stellar winds of massive stars, and the component separation of a number of binary systems containing giant or massive stars.

During the past decade a number of exciting discoveries in this area of study have been made: the detection of white dwarf/red giant wind interactions in D-type symbiotic systems; the wind-interaction origin of the non-thermal emission in WR+O star binary systems; clumpy winds around LBV's; detection of the non-spherical nature of the stellar winds around the rapidly rotating Be stars.

Unfortunately, only a small number of stellar wind systems can be successfully imaged with present-day arrays, and even then, imaging of these objects is constrained to study of plasma that has travelled some distance from the point of origin. The proposed sensitivity and resolution of the SKA will provide a number of advances in this field, including high dynamic range images of the presently observed sources allowing the study of wind clumping, the capability to image on the size scales of the interactions that gives rise to the stellar wind phenomena, and a far larger number of detectable sources for identifying characteristics of a population of systems. This talk will outline some of the recent highlights in this area of study, the limitations imposed by present-day sensitivity, and describe a number of these advances that will be made with the SKA.

Probing Protostellar Objects with the SKA

Lewis B.G. Knee,  Herzberg Institute of Astrophysics

The study of star formation is a major science driver for the new generation of millimetre/submillimetre arrays currently under development. In this talk I will present some initial thoughts on how the SKA too can be used to study protostars and their environments. The emphasis will be on the study of phenomena for which the SKA will be the premier instrument, such as detailed high resolution studies of protostellar environs using the ammonia lines in the 18 GHz to 28 GHz band, study of the structure and time evolution of ionized protostellar jets in the continuum, the nonthermal emission from young stars and their jets, and studies of the kinematics of ionized gas using centimetre wavelength atomic recombination lines.

Pulsar observing strategies with the 1kT

Matthew Bailes, Swinburne University of Technology

The huge collecting area of the 1kT gives it the potential to become a fantastic instrument for the study of pulsars where one is usually sensitivity, not confusion limited. However, a careful consideration of the observational constraints suggest that designs based on paraboloids would encounter real difficulties in attempting to discover and time a large number of pulsars, both in our Galaxy and in the SMC/LMC system. There is a conflict between the diameter of the individual elements and the time required to discover and time a large number of pulsars that is not easily overcome without invoking more radical technologies. For the bulk of pulsar discoveries and timing, frequencies above a few GHz are not necessary.

DSN Science with the SKA

T.B.H.Kuiper, M.J.Klein, G.M. Resch, and M. Slade

Jet Propulsion Laboratory, California Institute of Technology

The NASA Deep Space Network science activities could be greatly enhanced by a very large aperture cm wavelength radio telescope. In this paper we discuss the impact a SKA will have on research which uses the DSN to measure cm wavelength radiation emitted by or reflected from celestial objects, that is, radio and radar astronomy. We describe the desired SKA characteristics to support such research. DSN research areas not mentioned in the current draft of "The Science Case" include the kinematics of star formation, pre-biotic chemistry, radar of planetary surfaces, moons and rings, radio astronomy of planetary atmospheres and magnetospheres, and some types of astrometry. Scientific analysis of the telecommunications and transponder signals returned by remote spacecraft will be treated in a separate paper on the SKA as a deep space telecommunications instrument.

Asteroids and Planetary Satellites with the SKA

Russell O. Redman (NRC/HIA, Victoria)

The SKA would be a powerful new tool at 20 GHz to study the surfaces of small solid bodies throughout the Solar system, from near-Earth asteroids out to the Kuiper Belt. At least hundreds and probably thousands of asteroids should be detectable. Thermal radiation from these objects emerges from deep beneath the surface where the temperature of the material is the time-average of the surface temperature. This temperature is an important quantity to know when building thermal models of the object and can only be measured accurately for a few objects with current telescopes. It will be possible to image the surfaces of large numbers of main-belt asteroids and the larger planetary satellites. By gathering data over several days, enough of the UV plane could be filled at each rotational phase to construct sequences of images of these objects as they rotate. From these images it may be possible to estimate cratering rates in the asteroid belt itself, and to estimate the time elapsed since the last major impact rearranged the surface features of the larger asteroids. Small companions like Dactyl, the moonlet orbiting Ida, should be detectable, and the superb angular resolution offered by the SKA will allow us to study the dynamics of their orbits.

Technical Developments

The Science-Engineering Interface

Lawrence Cram, University of Sydney

The science program envisaged for the Square Kilometre Array leads directly to several quite firm specifications, such as the A/Teff ratio, and the maximum required diameter of the core of the array. However, the science program also implies many other specifications that are not so readily quantified, such as the imaging rate available in observations designed to reach a specified dynamic range or S/N ratio, or the sensitivity in a field of view containing extended HI sources with a spectrum of sizes. Engineering trade-offs will lead to significant compromises in some of these specifications, and therefore it is important to develop tools that allow the scientific impact of complex engineering optimisations to be assessed by the potential users. The task is further complicated by the reasonable expectation that imaging software (used post-facto) will be able to rectify many of the small errors that are inevitable in a real system, allowing relaxation of some engineering parameters and consequent improvements in the cost/benefit equations. I plan to discuss options for linking the science program to engineering studies using telescope simulation software to help address some of these issues. 

Interference Suppression in Redundant Arrays

Ray Norris, Australia Telescope National Facility

Some possible configurations of the 1kT, such as large tiled arrays, will probably involve massively redundant arrays. Clearly, it will not be possible, or even desirable, to record data from all baselines, and some form of fractal or hierarchical sub-netting is likely to be adopted. However interfering sources tend to be in the near field of individual interferometers, and will have a different response to different members of a redundant set of interferometers, whereas astronomical sources will be in the far field, and will have identical responses to all members of the set. This difference can be used to select a configuration to maximise interference suppression.

Research Results on Array Antenna Systems at NFRA

Arnold van Ardenne, NFRA

To investigate the feasibility of the phased array concept for the next generation of radio telephones, NFRA has embarked on a technical R&D program since 1995. We have started on a 5 year program which involves the prototyping of arrays with increasing size and complexity e.g. level of integration each to be build over a 1.5 years paralleled with a number of research programs with longer time horizons. To date, we have finished the first step i.e. a small "10" (actually 8) element array including digital beamforming some results of which will be presented.  Also, first results will be shown of the second "100" (actually 64 active and 128 passive element) array now in the build-up phase and we will introduce the next step i.e. 1000 element array. Also, we will show some results off the parallel programs with some emphasis on integration aspects.

Technical Progress on Array Feeds and RFI Cancellation

Rick Fisher and Rich Bradley, NRAO, Green Bank

This presentation is a brief summary of the status of two projects. Last summer we were successful at synthesising a focal plane map at 1400 MHz on the 140-ft telescope using a cross-correlated array. We are now in the process of improving the noise figure of this array, and we are planning to investigate both direct combination and correlation methods for forming close-spaced beam arrays.

We reported the results of a single-channel adaptive RFI canceller in a poster paper at the Sydney 1kT Workshop. Work is now underway to extend this to a multi-channel, dual-polarisation instrument to investigate the possibility of cancelling many signals within a passband simultaneously and to follow up on our conjecture that multi-path RFI depolarisation may be limiting the effectiveness of a single polarisation cancelling device. We have a few more lab measurements to report.

Array Receiver Development at OSU

Steve Ellingson, The Ohio State University ElectroScience Laboratory

OSU is currently developing a small-scale array system for experiments in SKA technology. We are particularly interested in horizon-to-horizon continuous (no scanning) surveillance, which is useful for detection of transient events, retroactive discovery, and SETI. However, this mode of operation requires massive processing throughput. This talk outlines two receiver architectures that we have developed to address this problem. In both approaches the goal is to acquire 1.5 MHz or more of bandwidth from the analogue IF output associated with each sensor in the array, render it in digital form, and move it through a DSP analysis engine as efficiently as possible. The pros and cons of these approaches, including technical and economic considerations, are discussed.

Critical Technologies for the Large Adaptive Reflector

Bruce Veidt, Herzberg Institute of Astrophysics

The Large Adaptive Reflector (LAR) is a radically different reflector antenna based on a long focal length adaptive primary reflector and an airborne feed system. This talk will discuss two of the key technologies necessary to make the LAR work: the triple-tethered aerostat and the antenna feed system. I will describe the procedure we are developing to model the aerostat performance. I will also show how the feed size and desired frequency coverage define design parameters such as focal length.

Extending the Long Wavelength Range and Size of the LAR

Tom Legg, Herzberg Institute of Astrophysics

The long wavelength limit of an LAR type of telescope is set by the actuators. These need to reduce the focal length at long wavelengths and thereby allow efficient feeding of the telescope. Because the surface is already adjustable, both the long wavelength limit and the maximum telescope diameter can be substantially increased by individually controlling the surface panels so that they form a stepped Fresnel surface for wavelengths longer than 21 cm. Consideration of stepped surfaces as interferometer elements shows that there is a bandwidth limitation that is fairly innocuous. The Fresnel surfaces could allow observations at 1m wavelength with an actuator travel of 1.7m, or at 2m with a travel of 2.7m.