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The ASKAP Survey
for Variables
and Slow Transients (VAST) and
the Rapid ASKAP
Continuum Survey (RACS) are being used to search for radio sources
that vary over time-scales of minutes, days, months or even
years. These surveys take repeated short (12-15 minute) snapshots of
the sky to see what changes. While searching for transients we ended
up following in the paths of the ancients and re-discovered a couple
of local favourites which are compiled in the image above. Planets,
the name itself derived from
an ancient Greek word
meaning "wanderers", move across the sky and so can appear as
transients. Mercury lives in the fast lane and would normally be
blurred over the course of a 12 minute ASKAP observation but when
tracked we can clearly see this tiny planet. Venus is a fast mover but
also bright - in this instance we see the trail as it moves relative to the
background sky in
just 12 minutes. The moon, also a fast mover, is big, bright, and
rather annoyingly masks out background sources. In contrast, the outer
planets take a more leisurely approach to their wanderings and appear
as transients over periods of months. When we look
at Jupiter
at radio wavelengths what we actually see are its radiation belts
- these appear as source on either side of Jupiter. Saturn appears as
a simple dot because unlike in optical radio doesn't see the
ring. Neptune is very distant and quite weak and cannot be seen in a
single 12-minute observation. However, VAST observed Neptune a total
of 13 times, by accounting for movement of the planet and then
stacking these together the final image is just sensitive enough to
detect Neptune. Note that Mars and Uranus were only briefly observed
by RACS/VAST and were too weak to be detected.
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