Solar flares are sometimes accompanied by coronal mass ejections (CMEs). This suggests that these stars are either sub-giant stars or binary systems, not main sequence stars like our Sun. 2014) reported that some superflare stars with a photometric period of 10 to 20 days have large rotational velocities of v sin i ∼ 10 to 20 km/s. The average strength of the magnetic field on the surface of these stars, by using the absorption line of Ca II at λ 8,542 Å, are estimated to be 1 to 20 G which is comparable to or 1 order of magnitude higher than that of the Sun (approximately 2 G). 2014) reported that the rotational velocities of two superflare stars, KIC 9766237 and KIC 9944137, are comparable to that of our Sun, and no hint of binarity was detected. Although most of G-type main sequence stars exhibiting superflares are rapidly rotating stars, the temperature and rotation period of some superflare stars are close to those of the Sun. The bolometric energy released by the superflare ranges from 10 33 to 10 36 erg which is 10 to 10 4 times larger than that of the largest solar flares (10 32 erg). Recently, many superflares on solar-type stars (G-type main sequence stars) have been discovered by the Kepler space telescope ( Maehara et al. On the other hand, much larger flares called ‘superflares’ have been observed on a wide variety of stars including solar-type stars ( Landini et al. 2012), and the occurrence frequency of such flare is about once in 10 years. The total bolometric energy released by the largest solar flares is estimated to be the order of 10 32 erg (Emslie et al. The frequency-energy distribution of solar flares can be fitted by a simple power-law function with an index of −1.5 to −1.9 in the flare energy range between 10 24 and 10 32 erg (e.g., Aschwanden et al. The occurrence frequency of solar flares decreases as the flare energy increases. Solar flares are eruptive events in the solar atmosphere caused by the magnetic reconnection (e.g., Shibata and Magara 2011). This can be explained if we assume the time scale of flares is determined by the Alfvén time. We also found that the duration of superflares ( τ) increases with the flare energy ( E) as τ ∝ E 0.39 ± 0.03. The upper limit of energy released by superflares is basically comparable to a fraction of the magnetic energy stored near starspots which is estimated from the photometry. The average occurrence rate of superflares with the energy of 10 33 erg which is equivalent to X100 solar flares is about once in 500 to 600 years. Using these new data combined with the results from the data with 30-min sampling, we found that the occurrence frequency (dN/dE) of superflares as a function of flare energy ( E) shows the power-law distribution (dN/dE ∝ E − α) with α ∼−1.5 for 10 33< E<10 36 erg which is consistent with the previous results. Some superflares show multiple peaks with the peak separation of the order of 100 to 1,000 s which is comparable to the periods of quasi-periodic pulsations in solar and stellar flares. We found 187 superflares on 23 solar-type stars whose bolometric energy ranges from the order of 10 32 to 10 36 erg. We searched for superflares on solar-type stars using Kepler data with 1-min sampling in order to detect superflares with a short duration.
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