  The Gaia DR2 sample of short-timescale variable candidates results from the
investigation of the first 22 months of Gaia photometry for a subsample of
sources at the Gaia faint end. For this exercise, we limited ourselves to the
case of suspected rapid periodic variability. Our study combines
fast-variability detection through variogram analysis, high-frequency search by
means of least-squares periodograms, and empirical selection based on the
investigation of specific sources seen through the Gaia eyes (e.g. known
variables or visually identified objects with peculiar features in their light
curves). The progressive definition and validation of this selection criterion
also benefited from supplementary ground-based photometric monitoring of a few
preliminary candidates, performed at the Flemish Mercator telescope (Canary
Islands, Spain) between August and November 2017. We publish a list of 3,018
short-timescale variable candidates, spread throughout the sky, with a
false-positive rate up to 10-20% in the Magellanic Clouds, and a more
significant but justifiable contamination from longer-period variables between
19% and 50%, depending on the area of the sky. Although its completeness is
limited to about 0.05%, this first sample of Gaia short-timescale variables
recovers some very interesting known short-period variables, such as
post-common envelope binaries or cataclysmic variables, and brings to light
some fascinating, newly discovered variable sources. In the perspective of
future Gaia data releases, several improvements of the short-timescale
variability processing are considered, by enhancing the existing variogram and
period-search algorithms or by classifying the identified candidates.
Nonetheless, the encouraging outcome of our Gaia DR2 analysis demonstrates the
power of this mission for such fast-variability studies, and opens great
perspectives for this domain of astrophysics.
