Superflares on Solar Type Stars Observed with Kepler Takuya Shibayama1 ([email protected]), Hiroyuki Maehara2, Shota Notsu1, Yuta Notsu1, Satoshi Honda3, Daisaku Nogami1, Kazunari Shibata1 1 Kyoto University, Kwasan Observatory ; 2 University of Tokyo, Kiso Observatory ; 3 Hyogo University, Nishi-Harima Observatory

Space weather study becomes increasingly important for our electronic civilization. One of the main cause of space environmental disturbance is large solar flare. The largest Brightening area of flare is small compared to that of the stellar solar flare that our civilization have experienced is Carrington event in 1859, and the disk, therefore, detection of flare event from full disc integrated total energy of the flare is of the order of 1032 erg. luminosity needs high precision photometry. Kepler spacecraft is a We study superflares (whose total energy is more than 1033 erg) on solar type stars with space telescope, whose aim is discovering exoplanet with Kepler space telescope and detected 1,547 superflares on 279 solar type stars. Typical planetary transit. For that reason, Kepler observed high precision frequency of superflare of 1034 erg on solar type star is once in 800 years. Our study photometry of many stars. The number of G-dwarf in the sample implies the probability that our Sun exhibits superflare. is about 80,000. We detected sudden brightening and, finally, Large solar white light flare Artistic illustration of superflare discovered 1547 superflares on 279 G-dwarfs. Figure 1 shows Solar type star : 5100 K < T < 6000 K, log g > 4.0, superflare frequency vs. energy of superflare. The power-law index eff Light Curve of a Superflare Sun-like Star Sun-like star : 5600 K < Teff < 6000 K, log g > 4.0, Prot >10 day of the distribution is similar to that of solar flares. Most of 0.004 superflare stars show quasi-periodic brightness variation during Superflare Superflare hole observation period. This modulation is thought to be caused 0.002 by star spot and stellar rotation. We estimated star spot area and 0 stellar rotation period from the amplitude and the period of brightness variation. The typical amplitude of the brightness -0.002 variation is 1 %, therefore, the typical radius of stellar spot is one -0.004 KIC10471412, Teff = 5771 K, log g = 4.1, Prot = 15.1 day tenth of stellar radius, and the rotational period ranges from less 15 days 0.3 % Normalized Brightness -0.006 than 1 day to more than 20 days. The existence of large stellar 0 50 100 150 200 spot on superflare stars is consistent with the high activity. Julian Day – 2455040 (day) Similar Statistics of Superflare JD-2455040 Enlarge 0.004 0.008 Kepler space telescope observes photometric light curve of 0.004 0.006 0.003 about 80,000 solar type stars. For comparison with solar flare, 0.002 0.002 0.004 0.001 we define “Sun-like stars” (5600 K < Teff < 6000 K, log g > 4.0, 0 0 P >10 day). 44 superflares are observed on 19 Sun-like stars. 0.3 % 0.002 -0.2 0 0.2 0.4 rot -0.002 The superflare frequency distribution on Sun-like stars is 0 -0.004 roughly on the same slope as that of solar flare, and locates Solar TSI to the scale -0.002 Normalized Brightness between that of solar maximum and minimum. Stars more Normalized brightness -0.006 -0.004 0 2 4 6 8 10 similar to the Sun (Prot >20 day) have superflare (Table below) 0 10 20 30 40 Day Days from flare peak Light curve of the Sun in 2003 (SORCE). Enlarged light curve of a superflare on

] Nanoflares (EUV)

1 −20 − 10 Darkening caused by sunspot, which is the superflare star above. The amplitude similar to stellar light curve. This implies of this superflare is about 0.3 % of year Microflares (soft X-ray) 1

− existence of stellar spot on superflare stellar brightness. Typical amplitude of 10−25

star stars. Most of superflare stars show spot- superflare is 1 % of stellar brightness.

1 Solar flares − Solar maximum (2001) like brightness variation. The typical This is larger than that of solar flare (hard X-ray) (GOES X-ray) 10−30 amplitude is 1%, therefore, the typical spot detected from total solar irradiance size of superflare stars is 10% of stellar (0.03%). radius. We estimated stellar rotation period 10−35 Solar type stars Solar minimum (2008) from brightness variation period. (GOES X-ray) Flare frequency [erg 10−40 Sun-like stars Superflare on the Sun? 24 26 28 30 32 34 36 38 10 10 10 10 10 10 10 10 Our study suggests a occurrence of superflare on the Sun. Typical frequency Flare energy [erg] of superflare of 1034 erg on Sun-like stars is once in 2000 years. From

Superflares on Sun-like Stars with Prot > 20 days theoretical point of view, our Sun can store enough magnetic energy to cause superflare of 1034 erg within one solar cycle (Shibata et al. 2013). Observationally, large cosmic ray events in 7th and 9th century are found from tree ring (Miyake et al.). Although the source of this cosmic ray is under discussion, this event can be caused by solar superflare. The frequency of these events is consistent with superflare frequency of this study. In 1989, X15 solar flare caused large scale black out in Quebec, Canada and broke a transformer in USA. If solar superflare occur in this era of electronic communication, estimated damage is very large. Black out and communication failure all over the world and break down of almost all artificial satellite are Number of Flares, Flare Stars and Sample, and Fraction of Flare Stars possible.

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