By extending our previous study by Maehara et al .
( 31 ) , we searched for superflares on G-type dwarfs ( solar type stars ) using Kepler data for a longer period ( 500 days ) than that ( 120 days ) in our previous study .
As a result , we found 1547 superflares on 279 G-type dwarfs , which are much more than previous 365 superflares on 148 stars .
Using these new data , we studied the statistical properties of occurrence frequency of superflares , and basically confirmed the previous results , i.e. , the occurrence frequency ( dN / dE ) of superflares vs flare energy ( E ) shows power-law distribution with dN / dE \propto E ^ { - \alpha } , where \alpha \sim 2 .
It is interesting that this distribution is roughly on the same line as that for solar flares .
In the case of the Sun-like stars ( with surface temperature 5600-6000K and slowly rotating with a period longer than 10 days ) , the occurrence frequency of superflares with energy of 10 ^ { 34 } -10 ^ { 35 } erg is once in 800-5000 years .
We also studied long term ( 500 days ) stellar brightness variation of these superflare stars , and found that in some G-type dwarfs the occurrence frequency of superflares was extremely high , \sim 57 superflares in 500 days ( i.e. , once in 10 days ) .
In the case of Sun-like stars , the most active stars show the frequency of one superflares ( with 10 ^ { 34 } erg ) in 100 days .
There is evidence that these superflares have extremely large starspots with a size about 10 times larger than that of the largest sunspot .
We argue that the physical origin of extremely high occurrence frequency of superflares in these stars may be attributed to the existence of extremely large starspots .