Radio variable and transient sources on minute time-scales in the ASKAP pilot surveys

We present results from a radio survey for variable and transient sources on 15-min time-scales, using the Australian SKA Pathfinder (ASKAP) pilot surveys. The pilot surveys consist of 505 h of observations conducted at around 1 GHz observing frequency, with a total sky coverage of 1476 deg(2). Each observation was tracked for approximately 8 - 10 h, with a typical rms sensitivity of & SIM;30 & mu;Jy beam(-1) and an angular resolution of & SIM;12 arcsec. The variability search was conducted within each 8 - 10 h observation on a 15-min time-scale. We detected 38 variable and transient sources. Seven of them are known pulsars, including an eclipsing millisecond pulsar, PSR J2039-5617. Another eight sources are stars, only one of which has been previously identified as a radio star. For the remaining 23 objects, 22 are associated with active galactic nuclei or galaxies (including the five intra-hour variables that have been reported previously), and their variations are caused by discrete, local plasma screens. The remaining source has no multiwavelength counterparts and is therefore yet to be identified. This is the first large-scale radio survey for variables and transient sources on minute time-scales at a sub-mJy sensitivity level. We expect to discover & SIM;1 highly variable source per day using the same technique on the full ASKAP surveys.