In December 2014 a first reduction of the photometry acquired by Gaia during 28 days of Ecliptic Pole Scanning Law (EPSL) and 3 days of Nominal Scanning Law (NSL) was delivered to CU7, the Coordination Unit in charge of performing the analysis of the variable sources observed by Gaia. In the week of 19-25 March 2015, the full chain of the CU7 pipeline ran on the EPSL+NSL dataset (about 800,000 sources which have more than 20 Field-of-View transits), starting from the general Variability Detection, general Characterization, proceeding through the global Classification and ending with the detailed checks and typecasting of the Specific Objects Study (SOS). The time-series photometry of 1242 sources in the South Ecliptic Pole (SEP), which covers an external region of the Large Magellanic Cloud (LMC), was fed into the Cepheid&RR Lyrae SOS pipeline, which returned a rich harvest of LMC RR Lyrae stars (more than 800, including some hundreds of new discoveries, with typical average magnitudes around G~19.5 mag and periods in the range of about 0.15  to 1.0 days) and a few Cepheids at the short period/faint end of the LMC Cepheids period-luminosity distribution. Some of them are new Cepheids discovered by Gaia.

The figure above shows the G-band light curves of five of these Cepheids folded according to the periods determined by the CU7 Cepheid&RR Lyrae SOS pipeline. They have average G magnitudes in the range of 18.3 to 18.7 mag and periods ranging from 0.3 to 1.93 days, hence partially overlapping with the RR Lyrae stars.

The light curves consist (from top to bottom) of  56, 44, 39, 49 and 31 observations, each corresponding to 9 individual CCD observations, spread over a total time span of 32 days. A few (1-3) observations with estimated uncertainties larger than 0.05 mag were excluded from the plots. Median uncertainties of the measurements are around 10-15 mmag.

The sample in the figure includes one first-overtone classical Cepheid (upper panel), 3 candidate anomalous Cepheids (central three panels) and a candidate Type2 Cepheid (bottom panel). The classification in sub-types provided by the Cepheid&RR Lyrae SOS pipeline will be refined as improved BP and RP time-series become available for these sources.

The Cepheids in the four upper panels were classified as Cepheids either in the OGLE III (Soszynski et al., 2008, Acta Astron., 58, 163) or in the EROS-2 (Kim et al. 2014, A&A, 566, A43) catalogues of LMC variable stars. OGLE III specifically confirms our classification as first-overtone classical Cepheid for the source in the upper panel of the figure. Conversely, the Cepheid in the lower panel has a counterpart in the EROS-2 catalogue which Kim et al. (2014) classify as an eclipsing binary, while Gaia data show the star as a Cepheid with no obvious sign of binary component.

Photometry for brighter Cepheids known in the LMC (whose magnitude distribution peaks at G~15.5 mag) was not produced with this first data reduction, but will soon become available with the reprocessing of the full EPSL data stream.

The nominal scanning law yieds a different cadence than achieved with EPSL data. Nevertheless, the examples above can be used to assess the scientific performance. On average Gaia observes every source 70 times during the nominal 5 year operational period, providing a roughly similar phase coverage for short period variables.

These five faint Cepheids demonstrate the great potential of Gaia's photometry and the excellent performance of the CU7 pipeline.

Credits: ESA/Gaia/DPAC/CU5/DPCI/CU7/INAF-OABo/INAF-OACn Gisella Clementini, Vincenzo Ripepi, Silvio Leccia, Laurent Eyer, Lorenzo Rimoldini, Isabelle Lecoeur-Taibi, Nami Mowlavi, Dafydd Evans, Geneva CU7/DPCG and the whole CU7 team. The photometric data reduction was done with the PhotPipe pipeline at DPCI; processing data were received from the IDT pipeline at DPCE.

[Published: 28/05/2015]