We have shown previously that a broad correlation between the peak radio
luminosity and the variability time-scales, approximately L ~ t^5, exists for
variable synchrotron emitting sources and that different classes of
astrophysical source occupy different regions of luminosity and time-scale
space. Based on those results, we investigate whether the most basic
information available for a newly discovered radio variable or transient -
their rise and/or decline rate - can be used to set initial constraints on the
class of events from which they originate. We have analysed a sample of ~ 800
synchrotron flares, selected from light-curves of ~ 90 sources observed at 5-8
GHz, representing a wide range of astrophysical phenomena, from flare stars to
supermassive black holes. Selection of outbursts from the noisy radio
light-curves has been done automatically in order to ensure reproducibility of
results. The distribution of rise/decline rates for the selected flares is
modelled as a Gaussian probability distribution for each class of object, and
further convolved with estimated areal density of that class in order to
correct for the strong bias in our sample. We show in this way that comparing
the measured variability time-scale of a radio transient/variable of unknown
origin can provide an early, albeit approximate, classification of the object,
and could form part of a suite of measurements used to provide early
categorisation of such events. Finally, we also discuss the effect
scintillating sources will have on our ability to classify events based on
their variability time-scales.Comment: Accepted for publication in MNRA
