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8th of October 2021
A VAST Pilot Survey varriable of unknown origin
The ASKAP Variables and Slow Transients Survey (VAST) is designed to detect highly variable and transient radio sources on timescales from 5 seconds to ∼5 years. Murphy et al. have recently presented initial results from the VAST Phase I Pilot Survey. A total of 113 fields were observed for a 12 minutes integration time, with between 5 and 13 repeats, and cadences between 1 day and 8 months. An initial search revealed 28 highly variable and/or transient sources. Previous ADAPs have shown examples of pulsars and stars: the third class of variable sources were the "others". The figure above shows the light curve for one of these. (The X-axis is Modified Julian Day number: MJD 58600 corresponds to 27 April 2019 and MJD 59100 is 9 September 2020.)

A total of 14 highly variable sources were found that were not associated with a known pulsar or star. The left panel above shows the VAST Stokes I (total intensity) image for the epoch with the maximum flux density. The middle panel shows the Stokes V (circular polarisation) image for the same epoch where positive flux density corresponds to right handed circular polarisation and negative to left handed. The ellipse in the lower left corner of each radio image shows the FWHM of the restoring beam. The right panels show Stokes I contours at 30, 60, and 90 per cent of the peak Stokes I flux density overlaid on an RGB image of optical data from the Dark Energy Survey with red=i-band, green=r-band, blue=g-band. None of the "other" sources were detected in circular polarisation, although a number of the limits are not very constraining, and most, like the example above, were not detected at optical wavelengths. While several could be associated with Active Galactic Nuclei or galaxies, a number of others, such as the example above, remain unclear and are the subject of ongoing study.

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