Digging deeper for RR Lyrae stars with low modulation amplitudes

With the goal of searching for very low modulation amplitudes among fundamental mode RR Lyrae stars and assessing their incidence rate, we performed a survey of 36 stars observed by the Kepler satellite over the entire four-year period of its mission. The search was conducted by a task-oriented code, designed to find low-amplitude signals in the presence of high-amplitude components and instrumental systematics. We found six new modulated stars and negated one earlier claimed star, thereby increasing the number of known Blazhko stars from 18 to 24 and yielding an observed occurrence rate of 67% for the Kepler field. Five of the new stars have the lowest modulation amplitudes found so far, with ∼250 ppm Fourier side-lobe amplitudes near the fundamental mode frequency. Because of the small sample size in the Kepler field, we extended the survey to 12 campaign fields observed by K2, the two-wheeled mission of Kepler. From the 1061 stars, we identified 514 Blazhko stars. After correcting for the short duration of the time spent on each field and for the noise dependence of the detections, we arrived at an underlying occurrence rate of ∼75% – likely a lower limit for the true rate of Blazhko stars in the K2 fields.