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

Soshi Okamoto,Yuta Notsu,Satoshi Honda,Daisaku Nogami,Kazunari Shibata,Kai Ikuta,Kosuke Namekata,Hiroyuki Maehara

doi:10.3847/1538-4357/abc8f5

Abstract

Abstract We report the latest statistical analyses of superflares on solar-type (G-type main-sequence; effective temperature is 5100–6000 K) stars using all of the Kepler primary mission data and Gaia Data Release 2 catalog. We updated the flare detection method from our previous studies by using a high-pass 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 sizes of solar-type and Sun-like stars (effective temperature is 5600–6000 K and rotation period is over 20 days in solar-type stars) are ∼4 and ∼12 times, respectively, compared with Notsu et al. As a result, we found 2341 superflares on 265 solar-type stars and 26 superflares on 15 Sun-like stars; the former increased from 527 to 2341 and the latter from three to 26 events compared with our previous study. 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 suggests that the Sun can cause superflares with energies of ∼7 × 1033 erg (∼X700-class flares) and ∼1 × 1034 erg (∼X1000-class flares) once every ∼3000 and ∼6000 yr, respectively.

Highlights

Flares are explosions in the stellar atmosphere with intense release of magnetic energy stored around starspots (e.g., Shibata & Magara 2011)
We report the latest statistical analyses of superflares on solar-type (G-type main-sequence; effective temperature is 5100 – 6000 K) stars using all of the Kepler primary mission data, and Gaia-DR2 (Data Release 2) catalog
In our previous studies (Maehara et al 2012,2015,&2017; Shibayama et al 2013; Notsu et al 2013b,2015b,&2019), we investigated the statistical properties of solar-type and Sun-like superflare stars using photometric data of Kepler space telescope

Summary

Introduction

Flares are explosions in the stellar atmosphere with intense release of magnetic energy stored around starspots (e.g., Shibata & Magara 2011). Young rapidly-rotating stars, close binary stars, and dMe stars show much higher levels of magnetic activities than the Sun (e.g., Gershberg 2005; Reid & Hawley 2005; Benz & Gudel 2010; Kowalski et al 2010; Osten et al 2016; Linsky 2019; Namekata et al 2020b). They often causes “superflares”, which have total bolometric energies 101 – 106 times more energetic than the largest solar flares (∼ 1032 erg; Emslie et al 2012). Many solar-type (G-type main-sequence) stars including slowly-rotating Sun-like stars that showed superflares have been recently reported by using Kepler space telescope (Maehara et al 2012&2015; Shibayama et al 2013; Candelaresi et al 2014; Wu et al 2015; Balona 2015; Davenport 2016; Van Doorsselaere et al 2017; Notsu et al 2019) and by TESS (Tu et al 2020; Doyle et al 2020) 1

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