The ASKAP Variables and Slow Transients (VAST) Extragalactic Survey - Data Release 1

Abstract

The Variables and Slow Transients (VAST) Survey on the Australian SKA Pathfinder (ASKAP) is designed to systematically explore the dynamic radio sky, detecting sources that vary on timescales from minutes to several years. In this paper, we present Data Release 1 of the VAST Extragalactic Survey, which targets slowly evolving synchrotron transients in the southern sky. The observations were carried out between June 2023 and May 2025, comprising 2945 images of 276 fields spanning $\sim 12300\ \mathrm{deg}^2$, observed at 888 MHz with a typical rms sensitivity of 0.24 mJy $\rm{beam}^{-1}$ and 12-20 arcsec resolution. Each field was revisited approximately every two months, yielding 10 or 11 observations per field. The VAST pipeline extracts the light curves for all the observed sources, and additional filters are implemented to improve the reliability of the resulting light curve database. The light curve database contains 0.5 million sources and 6.4 million individual measurements, publicly available through the CSIRO data access portal. An untargeted variability search yields 117 astrophysical variables, including 27 pulsars, 40 radio stars (10 newly detected at radio wavelengths), 44 active galactic nuclei, two optically identified supernovae, one supernova candidate, one brown dwarf, and two sources without multi-wavelength counterparts that are yet to be identified. This data release provides the first large-scale, high-cadence, uniform view of long-term radio variability in the extragalactic sky and lays the groundwork for future population studies of radio transients with ASKAP.

Figures (23)

• Figure 1: The VAST Extragalactic Survey footprint, showing the number of observations of each field. The sky map is plotted with J2000 equatorial coordinates in the Mollweide projection. The VAST Galactic survey is plotted in grey for reference. Typically, each field has been observed 10-11 times to date. There is a single field (navy) that has only been observed three times, before it was removed from the survey footprint, as it contains 3C273 (64 Jy), which causes poor image quality.
• Figure 2: Summary of VAST Extragalactic observations, giving the number of fields in each epoch. From 2024 the strict 2-month cadence was removed, meaning that a full reobservation of all 276 fields was often spread over multiple epochs. The greyed out epochs are not included in the VAST Extragalactic DR1 (see \ref{['app:data_obs_issues']} for details).
• Figure 3: A VAST image () from epoch 74 with two cutouts. Cutout A: a 1 deg image centred on (J2000) (12:49:26.5, -55:17:52.87) containing several bright sources, including the large radio galaxy 2MASX J21512991-5520124. Cutout B: a 0.3 deg image centred on (J2000) (21:36:18.9, -58:00:12.68) containing a range of source morphologies. The central part of the full image, shown by the black dashed line, indicates the coverage included in the post-processed data products.
• Figure 4: A flow chart describing the high-level data flow of VAST observations. Each block refers to a section describing that step in more detail. The purple/blue steps result in the data products as available on CASDA quickly after an observation. The pink and orange steps are additional steps as introduced in this work.
• Figure 5: Astrometric offset between RACS-low2 and VAST before post-processing (in blue) and after post-processing (in orange). Each dot represents a single observation of a VAST source cross-matched to the RACS-calibrator sample (see text). The gradient in the orange datapoints represents the point density. The green dashed line shows the typical $2.5 \times 2.5 \hbox{$^{\prime\prime}$}$ pixel size in VAST images.