The short timescale suspected periodic selection criteria relies on the analysis of
known constant and variable sources from OGLE catalogues. In order to validate the
analysis, sources from other catalogues of variable stars such as Catalina, LINEAR,
ASAS, AAVSO, etc as well as other resources from the literature are crossmatched with
the Gaia data using the Simbad crossmatch tool. Finally, visual inspection of
candidate light-curves together with complementary follow-up of some short period
variable candidates enabled us to further refine the selection criteria and clean the
suspected short period sample.
By applying the preliminary short-timescale selection criteria to all Gaia sources
with CCD photometry available, having more than 20 FoV transits in ,
and a magnitude between 16.5 and 20 mag (which is the range where the
variogram detection criterion has been validated), 16 703 sources are selected as
preliminary short period candidates. Visual inspection of light-curves of a few
hundred randomly selected examples enables to identify several unexpected and
probably spurious behaviours, such as light-curves switching between two
discrete magnitude level, or sources exhibiting incompatible behaviours in ,
To filter out such spurious variability, cleaning of the sample based on the
candidates’ environment over the sky
(in a similar way as to what is done by Wevers et al.2018), removing e.g.
candidates possibly contaminated by bright nearby sources, have been necessary.
An additional time series cleaning operator has also been applied, specific to the
short timescale analysis and based on the expected amplitude of the variation in
the band, to remove the possibly remaining and outliers.
Finally, thanks to extra-cuts on the number of observations, skewness, median
variogram ratio and correlation values in , and bands, the remaining
spurious variable candidates have been efficiently excluded.
At this stage, some further validation and black-listing of the short timescale
candidates sources has been necessary.
First, a few tens of sources in the sample are reported as showing excess flux
features in + compared to , which have been removed.
Additionally, a few hundred candidates are overlapping with the bona fide
eclipsing binaries sample provided by the eclipsing binaries work-package (whose
analysis were performed as a test case, but whose results were not made
public for Gaia DR2) to CU4 for further analysis and characterization. The publication
of new eclipsing binaries identified and characterized from Gaia data is planned
only from Data Release 3 and onwards. Hence those few hundred sources are
excluded from the published short timescale candidates list.
Finally, after applying all the filtering and refinements described in the
previous and current sections, the published list of short timescale, suspected
periodic candidates should contain 3018 bona fide sources. This list includes
about 138 known variables from the literature catalogues used for quality
assessment and validation, with about three quarters of them being period
variables with periods below d. All the non-periodic variable and constant
sources from these catalogues have been removed from the published short
timescale suspected periodic candidates sample. Hence, there is a contamination
of about 19% of the sample from longer period variables. However, those sources
have periods around a few days, and relatively high amplitudes, hence not being
short period variables per se, but whose detection at the short timescale level
When compared to all the OGLE short period variables processed as part of the
global short timescale variability search for Gaia DR2, the completeness of the short
timescale suspected periodic candidates sample published is assessed around
Further contamination estimation is performed, using the OGLE photometric
database: the Gaia DR2 short timescale sample of 3018 sources is crossmatched
with this OGLE catalogue in the Magellanic Clouds, then the OGLE and Gaia time
series are compared to check if the features observed in the later are compatible
with the former. From this analysis, the real contamination from spurious or
non-periodic variability is assessed around 10–20% is those regions.
More details on the Gaia DR2 short timescale analysis results, efficiency and
quality, are available in Roelens et al. (2018).