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.

💡 Research Summary

The paper presents VAST (Variables and Slow Transients), a large‑scale survey using the Australian Square Kilometre Array Pathfinder (ASKAP) to systematically explore the radio transient and variable sky on timescales of five seconds and longer. ASKAP’s key strengths— a 30 deg² instantaneous field of view enabled by Phased‑Array Feeds, a collecting area of ~4 000 m², a system temperature of ~50 K, and a 300 MHz bandwidth covering 700–1800 MHz— translate into an RMS sensitivity of 640 µJy beam⁻¹ in a 10‑second integration and 47 µJy beam⁻¹ in one hour. These capabilities make ASKAP uniquely suited to maximise the survey metric A·Ω·T·Δt (collecting area × sky coverage × total observing time × time resolution), which has limited previous blind radio transient surveys.

VAST is organized into three tiered components:

1. VAST‑Wide – surveys 10 000 deg² each day to an RMS of 0.5 mJy beam⁻¹, with a 1‑hour stacked image and daily repeats. Its primary aim is to catch rare, bright events such as orphan gamma‑ray‑burst (GRB) afterglows, extreme scattering events (ESEs), and high‑luminosity AGN flares, while also building a statistical sample of more common variables.

2. VAST‑Deep – covers the same sky area but integrates to a much deeper RMS of 50 µJy beam⁻¹ (1 hr) and 12 hr stacked images. This depth enables detection of faint, previously inaccessible populations: low‑luminosity core‑collapse supernovae, heavily dust‑obscured explosions, and potentially entirely new classes of slow transients.

3. VAST‑Galactic – focuses on the Galactic plane and the Large/Small Magellanic Clouds (≈750 deg²) to 0.1 mJy beam⁻¹, targeting stellar flares, cataclysmic variables, X‑ray binaries, magnetars, intermittent pulsars, and rotating radio transients (RRATs).

The scientific motivation is grouped into four physical categories: explosions, propagation effects, accretion‑driven variability, and magnetic‑field‑driven events. For explosions, VAST will provide a statistically robust census of GRB orphan afterglows (allowing beaming‑independent energy estimates) and of radio‑bright core‑collapse supernovae, including those missed optically because of dust. Propagation studies will exploit ESEs and interstellar scintillation to probe the small‑scale structure of the ionised interstellar medium and the intergalactic medium, potentially detecting the elusive baryonic component of the cosmic web. Accretion‑driven variability will be examined through long‑term monitoring of AGN and radio‑quiet quasars, yielding insights into black‑hole growth, jet physics, and feedback processes. Magnetic‑field‑driven phenomena such as flaring M‑dwarfs, magnetars, and intermittent pulsars will be captured in unprecedented numbers, shedding light on particle acceleration and magnetospheric dynamics.

A particularly timely aspect is the synergy with gravitational‑wave (GW) observatories. The large instantaneous sky coverage of ASKAP makes it an ideal instrument for searching electromagnetic counterparts to GW events (e.g., neutron‑star mergers) whose localisation regions are expected to be 10–100 deg². VAST’s cadence and sensitivity are well matched to the predicted radio afterglow evolution of such mergers.

Using population models (e.g., Chandra & Frail 2012) the authors estimate that VAST‑Wide will detect dozens of GRB orphan afterglows and hundreds of radio supernovae per year, while VAST‑Deep could uncover several hundred faint transients of unknown nature. The survey will also generate a catalog of thousands of AGN variability events and a statistically meaningful sample of ESEs.

Data processing is addressed in detail. A real‑time pipeline will produce 5‑second snapshot images, perform source extraction, and generate difference images to flag variability. Machine‑learning classifiers will separate genuine astrophysical transients from radio‑frequency interference and imaging artefacts. Detected candidates will be distributed via VOEvent alerts for rapid multi‑wavelength follow‑up, and a commensal mode will allow VAST to run alongside other ASKAP projects, increasing overall efficiency.

In summary, VAST leverages ASKAP’s unprecedented combination of field of view, sensitivity, and cadence to open a new discovery space in radio time‑domain astronomy. It will dramatically improve upon previous blind surveys (e.g., LOFAR, VLA, GMRT) in terms of sky coverage, depth, and temporal resolution, enabling both the systematic study of known variable populations and the discovery of entirely new classes of slow radio transients.