Do superflares occur on the Sun? — Latest results using all the Kepler 4-year data and Gaia-DR2 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. It had been thought that superflares cannot occur on slowly-rotating solar-type (G-type main-sequence) stars like the Sun. Recently, many superflares on solar-type stars were found in the initial 500 days data obtained by the Kepler spacecraft (Maehara et al. 2012 Nature; Shibayama et al. 2013 ApJS). Notsu et al. (2019 ApJ) conducted precise measurements of the stellar parameters and binarity check on the basis of spectroscopic observations and the Gaia-DR2 data. As a result, the number of Sun-like (slowly-rotating solar-type) superflare stars significantly decreased. Then in our latest study (Okamoto, Notsu, Maehara et al. in prep), we searched for superflares using all the Kepler 4-year primary mission data covering ∼1500 days, adding the targets newly identified as solar-type stars. We updated the flare detection method by using high-pass filter to remove rotational variations caused by starspots. We also took into account the effect of sample biases on the frequency of superflares, by considering gyrochronology and flare detection completeness. As a result, the number of superflares on Sun-like stars in this study greatly increased by ∼12 times compared with Notsu et al. (2019), which enabled us to discuss more well-established view on statistical properties of superflares on Sun-like stars. As a result, the upper limit of the flare energy decreases as the rotation period (stellar age) increases in solar-type stars, while flare energy can be explained by the magnetic energy stored around starspots. These can be consistent with the result that the starspot coverage decrease as the rotation period increases. Frequency of superflares decreases as the stellar age increases, and flare frequency as a function of flare energy shows power-law distributions (dN/dE ~ E^α with α ≳ -2). As a result, Sun-like stars can cause superflares with energies up to about 5×10³⁴ erg once every ∼5000 years, and this strongly supports the possiblity of superflares on the Sun.