EDEN: Flare Activity of the Nearby Exoplanet-hosting M Dwarf Wolf 359 Based on K2 and EDEN Light Curves

Abstract We report the flare activity of Wolf 359, the fifth closest star to the Sun and a candidate exoplanet-hosting M dwarf. The star was a target of the Kepler/K2 mission and was observed by the EDEN project, a global network of 1–2 m class telescopes for detection and characterization of rocky exoplanets in the habitable zones of late-M dwarfs within 50 light year from the solar system. In the combination of the archived K2 data and our EDEN observations, a total of 872 flares have been detected, 861 with the K2 (860 in the short-cadence and 18 in the long-cadence data, with 17 long-cadence events having short-cadence counterparts) and 11 with EDEN. Wolf 359 has relatively strong flare activity even among flaring M dwarfs, in terms of the flare activity indicator (FA) defined as the integrated flare energy relative to the total stellar bolometric energy, where FA = ∑ E f /∫ L bol dt ∼ 8.93 × 10 −5 for the long-cadence flares, whereas for K2 short cadence and EDEN flares, the FA values are somewhat larger, FA ≈ 6.67 × 10 −4 and FA ≈ 5.25 × 10 −4 , respectively. Such a level of activity, in accordance with the rotation period ( P rot ), suggests the star to be in the saturation phase. The size of the starspots is estimated to be at least 1.87% ± 0.59% of the projected disk area of Wolf 359. We find no correlation of FA with the stellar rotational phase. Our analysis indicates a flare frequency distribution in a power-law form of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="italic">dN</mml:mi> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mi mathvariant="italic">dE</mml:mi> <mml:mo>∝</mml:mo> <mml:msup> <mml:mrow> <mml:mi>E</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mi>α</mml:mi> </mml:mrow> </mml:msup> </mml:math> with α = 2.13 ± 0.14, equivalent to an occurrence rate of flares E f ≥ 10 31 erg about once per day and of superflares with E f ≥ 10 33 erg approximately 10 times per year. These superflares may impact the habitability of system in multiple ways, the details of which are topics for future investigations.