The Australian Square Kilometre Array Pathfinder (ASKAP) will give us an unprecedented opportunity to investigate the transient sky at radio wavelengths. In this paper we present VAST, an ASKAP survey for Variables and Slow Transients. VAST will exploit the wide-field survey capabilities of ASKAP to enable the discovery and investigation of variable and transient phenomena from the local to the cosmological, including flare stars, intermittent pulsars, X-ray binaries, magnetars, extreme scattering events, interstellar scintillation, radio supernovae, and orphan afterglows of gamma-ray bursts. In addition, it will allow us to probe unexplored regions of parameter space where new classes of transient sources may be detected. In this paper we review the known radio transient and variable populations and the current results from blind radio surveys. We outline a comprehensive program based on a multi-tiered survey strategy to characterise the radio transient sky through detection and monitoring of transient and variable sources on the ASKAP imaging timescales of 5 s and greater. We also present an analysis of the expected source populations that we will be able to detect with VAST.

/pdf/vast-an-askap-survey-for-variables-and-slow-transients-2xx0qn0lof.pdf

VAST: An ASKAP Survey for Variables and Slow Transients

The Australian Square Kilometre Array Pathfinder (ASKAP) will give us an unprecedented opportunity to investigate the transient sky at radio wavelengths. In this paper we present VAST, an ASKAP survey for Variables and Slow Transients. VAST will exploit the wide-field survey capabilities of ASKAP to enable the discovery and investigation of variable and transient phenomena from the local to the cosmological, including flare stars, intermittent pulsars, X-ray binaries, magnetars, extreme scattering events, interstellar scintillation, radio supernovae and orphan afterglows of gamma ray bursts. In addition, it will allow us to probe unexplored regions of parameter space where new classes of transient sources may be detected. In this paper we review the known radio transient and variable populations and the current results from blind radio surveys. We outline a comprehensive program based on a multi-tiered survey strategy to characterise the radio transient sky through detection and monitoring of transient and variable sources on the ASKAP imaging timescales of five seconds and greater. We also present an analysis of the expected source populations that we will be able to detect with VAST.

/pdf/vast-an-askap-survey-for-variables-and-slow-transients-1vfa8zd8p6.pdf

Electron density turbulence in the Galaxy causes radio sources to be broadened angularly by amounts that depend strongly on the direction of the source. Recent scattering measurements using very long baseline interferometry (VLBI)1 and interplanetary scintillations (IPS)2 of extragalactic sources, and interstellar scintillations (ISS) of pulsars3 are used here to constrain the distribution of scattering material in the Galaxy. Although ubiquitous, scattering material appears to be highly concentrated in a thin disk of ∼100 pc thickness and in clumps of 1–10 pc size probably associated with H II regions, stellar wind bubbles, and/or supernova shocks. Typical transverse velocities of scattering material are much smaller than typical pulsar velocities. We predict the scattering angle as a function of galactic latitude. The results imply a maximum observable surface brightness for incoherent sources of electron synchrotron radiation.

Radio wave scattering in the galactic disk

The noise in millisecond pulsar (MSP) timing data can include contributions from observing instruments, the interstellar medium, the solar wind, solar system ephemeris errors, and the pulsars themselves. The noise environment must be accurately characterized in order to form the null hypothesis from which signal models can be compared, including the signature induced by nanohertz-frequency gravitational waves (GWs). Here we describe the noise models developed for each of the MSPs in the Parkes Pulsar Timing Array (PPTA) third data release, which have been used as the basis of a search for the isotropic stochastic GW background. We model pulsar spin noise, dispersion measure variations, scattering variations, events in the pulsar magnetospheres, solar wind variability, and instrumental effects. We also search for new timing model parameters and detected Shapiro delays in PSR J0614−3329 and PSR J1902−5105. The noise and timing models are validated by testing the normalized and whitened timing residuals for Gaussianity and residual correlations with time. We demonstrate that the choice of noise models significantly affects the inferred properties of a common-spectrum process. Using our detailed models, the recovered common-spectrum noise in the PPTA is consistent with a power law with a spectral index of γ = 13/3, the value predicted for a stochastic GW background from a population of supermassive black hole binaries driven solely by GW emission.

https://iopscience.iop.org/article/10.3847/2041-8213/acdd03/pdf

The Gravitational-wave Background Null Hypothesis: Characterizing Noise in Millisecond Pulsar Arrival Times with the Parkes Pulsar Timing Array

Interstellar scintillation was detected from a source in or near the supernova remnant G78.1 + 1.8, implying an angular size ≲10−6 arc s, a brightness temperature >1018 K, and a distance of ∼3 kpc. The source does not seem to be a pulsar and is therefore the first source other than a pulsar to show interstellar scintillation. We place limits on the scintillating flux for 15 other galactic and extragalactic objects.

Interstellar scintillation and compact sources in supernova remnants

A search for interstellar scintillation (ISS) of low-frequency variable radio sources is reported. Observations of 28 confirmed or suspected low-frequency variables, 21 nonvariable sources, and two pulsars were made at 408 MHz. As expected, the pulsars showed ISS, but scintillation was not detected in any other source. A typical upper limit to the rms modulation due to ISS is 150 mJy, giving lower limits to the apparent angular diameter of about 10 to the -6th arcsec. The possibilities that a true point source is broadened to an apparent angular diameter greater than or roughly equal to 10 to the -6th arcsec by scattering local to the source or in a general intergalactic medium are discussed.

P. E. Hardee

Papers

Search for microarcsecond structure in low-frequency variable radio sources.