CoRoT light curves of Blazhko RR Lyrae stars

Context. The CoRoT - Convection Rotation and planetary Transits - space mission provides a unique opportunity to monitor RR Lyrae stars with excellent time-sampling, unprecedented photometric precision, and a long time base of 150 days. Aims. The pulsation characteristics of RR Lyrae stars rely on robust physics, but we still lack a firm quantitative understanding of the physical mechanisms driving the Blazhko modulation and the long-term changes in their pulsation behavior. We use the high-precision space data of an unknown RR Lyrae star CoRoT ID 0105288363 observed during a second long run centered on the Galaxy - LRc02 -, to improve our understanding of the pulsation properties of RR Lyrae stars. Methods. The CoRoT data were corrected using a jump and trend filtering code. We applied different period-finding techniques including Period04, MuFrAn, PDM, and SigSpec. Amplitude and phase modulation were investigated using an analytical function method as well as traditional O-C diagrams. Results. For the first time, we detect significant cycle-to-cycle changes in the Blazhko modulation, which appear to be analogous to those predicted by Stothers - owing to the suppression of turbulent convection - to explain this phenomenon. We discuss the clear correlations between the phase and the amplitude of the bump, and the skewness and acuteness of the light curve during different Blazhko cycles. We find that these quantities are strongly anticorrelated with the fundamental pulsation period. This provides a strong support to the slow convective cycle model suggested by Stothers. We also detect a long-term modulation period in the maximum brightness spectrum. A more extended coverage of the long-term modulation is required to constrain its period. Seventh-order side peaks of the pulsation multiplet structure are also visible with the left-side peak amplitudes being higher than those of the right. This has never previously been detected. Future theoretical investigations are required to understand on a quantitative basis the complex behavior of the Blazhko effect. In particular, we still lack firm constraints of the physical mechanisms driving both phase and amplitude modulations during consecutive Blazhko cycles and their correlation, if any, with the long-term modulation. © 2011 ESO.