Statistical Properties of Superflares on Solar-type Stars: Results Using All of the Kepler Primary Mission Data

Solar flares are energetic explosions in the solar atmosphere, and superflares are the flares having the energy 10 - 10⁶ times larger than that of the largest solar flare. Recently, many superflares on solar-type (G-type main-sequence; effective temperature is 5100 – 6000 K) stars were found in the initial 500 days data obtained by the Kepler spacecraft (Maehara et al. 2012; Shibayama et al. 2013). Notsu et al. (2019) conducted precise measurements and binarity check on the basis of spectroscopic observations and the Gaia-DR2 data. As a result, the number of Sun-like (effective temperature is 5600 – 6000 K and rotation period is over 20 days) superflare stars significantly decreased.

We report the latest statistical analyses of superflares on solar-type stars using all of the Kepler primary mission data and Gaia-DR2 catalog. We updated the flare detection method by using highpass filter to remove rotational variations caused by starspots. We also examined the sample biases on the frequency of superflares, taking into account gyrochronology and flare detection completeness. The sample size of solar-type stars and Sun-like stars are ~4 and ~12 times, respectively, compared with Notsu et al. (2019). As a result, we found 2341 superflares on 265 solar-type stars, and 26 superflares on 15 Sun-like stars. This enabled us to have a more well-established view on the statistical properties of superflares. The observed upper limit of the flare energy decreases as the rotation period increases in solar-type stars. The frequency of superflares decreases as the stellar rotation period increases. The maximum energy we found on Sun-like stars is 4×10³⁴ erg. Our analysis of Sun-like stars suggest that the Sun can cause superflares with energies of 7×10³³ erg (~X700-class flares) and ~1×10³⁴ erg (~X1000-class flares) once every ~3,000 years and ~6,000 years, respectively (Okamoto et al. 2021).