10.6 Time series and variability

Author(s): Tatiana Muraveva

Gaia DR2 comprises the catalogues of variable stars of different types, such as RR Lyrae stars, Cepheids, Long Period Variables (hereafter LPVs), short timescale variables and stars with rotational modulations, and information on their main properties. A number of tests have been implemented in order to check the internal data coherence, relevance of the estimated parameters and compatibility with external catalogues. Moreover, the effect of variability on the photometric data has been studied.

A first batch of tests were developed in order to check the internal consistency of the data. Namely, the tests validate that:

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  numbers of variable stars exported by the variability processing and analysis unit are consistent with numbers of variables in the correspondent GACS tables

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  sources provided in variability tables are also presented in gaia_source table (Section 14.1.1)

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  sources provided in variability tables are also presented in vari_time_series_statistics table (Section 14.3.9)

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  sources provided in variability tables are marked as “Variable” in gaia_source table

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  the same source does not have contradicting types of variability

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  main parameters are within the expected range.

Conclusions: These tests revealed that numbers of variable stars exported by the variability processing and analysis unit on the pre-DR2 data were not consistent with the numbers presented in the corresponding tables on GACS. This was an expected issue since additional filters have been applied to the variable dataset. The tests showed also that some sources have more than one type of variability. Namely, a small number of Cepheids, RR Lyrae stars and rotational modulations have been also classified as short timescale variables. This overlap between different types of variability is acceptable, since the class of the short timescale variables includes all sources varying with the period less than one day. It was also found that some main parameters are out of the expected range for the samples of RR Lyrae stars, Cepheids and LPVs. This issue is described in the the variability processing and analysis unit’s documentation.

Another batch of tests were dedicated to the comparison of the obtained results with the external catalogues. The Gaia DR2 includes samples of RR Lyrae stars, Cepheids, LPVs and short timescale variables, for which classification and main parameters are available from the OGLE Catalogue (Udalski et al. 2008; Soszyński et al. 2017). The tests were used to validate:

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  classification of variables is in agreement with the OGLE classification

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  main parameters such as periods and average $G$ magnitudes are coherent with the OGLE data.

Conclusions: This batch of tests showed that for a small number of variables Gaia classification does not match the OGLE classification. Moreover, for a number of variable stars estimated main parameters differ from the parameters provided in the OGLE Catalogue. Distributions of the differences in periods for RR Lyrae stars and $G$ magnitudes for Cepheids are shown Figure 10.35 and Figure 10.36, respectively. The reason of discrepancy in periods for some RR Lyrae stars is a small number of epochs used to calculate the periods. The discrepancy between the mean magnitudes provided by Gaia and OGLE for Cepheids is expected since both surveys are operating in different passbands and transformation of OGLE $V$ and $I$ magnitudes to $G$ band is not perfect. Finally, the reason of inconsistency between Gaia and OGLE periods for a sample of the short timescale variables is that the period search method for this type of variables is still in progress of development, hence, the period information should be taken with caution.Numbers of the affected sources decreased significantly during the final validation. The final numbers of sources that have the classification and main parameters inconsistent with the OGLE catalogues, are relatively small, hence, no major problems regarding the comparison with external catalogues have been found.

Finally, a batch of tests were developed to check if the intensity-averaged $G$ magnitudes of RR Lyrae stars and Cepheids provided in variability tables (int_average_g) are in agreement with the mean $G$ magnitudes provided in gaia_source table (phot_g_mean_mag). It was found that the mean magnitudes are in disagreement for a small number of sources. The reason is that the algorithm applied to calculate mean phot_g_mean_mag magnitudes is still not optimal for variable sources due to a large spread of the data points.

Recommendation: users of the Gaia catalogue are encouraged to apply the intensity-average $G$ magnitudes provided in the variability tables for variable sources when they are available.