Fast optical flares from M dwarfs detected by a one-second-cadence survey with Tomo-e Gozen

  • Masataka Aizawa
    Tsung-Dao Lee Institute, Shanghai Jiao Tong University , Shengrong Road 520, 201210 Shanghai, China
  • Kojiro Kawana
    Department of Physics, Graduate School of Science, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
  • Kazumi Kashiyama
    Department of Physics, Graduate School of Science, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
  • Ryou Ohsawa
    Institute of Astronomy, Graduate School of Science, The University of Tokyo , 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
  • Hajime Kawahara
    Research Center for the Early Universe, Graduate School of Science, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
  • Fumihiro Naokawa
    Department of Physics, Graduate School of Science, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
  • Tomoyuki Tajiri
    Department of Earth and Planetary Science, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
  • Noriaki Arima
    Institute of Astronomy, Graduate School of Science, The University of Tokyo , 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
  • Hanchun Jiang
    School of Astronomy and Space Science, Nanjing University , Nanjing 210093, China
  • Tilman Hartwig
    Department of Physics, Graduate School of Science, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
  • Kotaro Fujisawa
    Department of Physics, Graduate School of Science, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
  • Toshikazu Shigeyama
    Research Center for the Early Universe, Graduate School of Science, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
  • Ko Arimatsu
    The Hakubi Center/Astronomical Observatory, Graduate School of Science , Kyoto University Kitashirakawa-oiwake-cho, Sakyo-ku, Kyoto, Kyoto 606-8502, Japan
  • Mamoru Doi
    Research Center for the Early Universe, Graduate School of Science, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
  • Toshihiro Kasuga
    National Astronomical Observatory of Japan , 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
  • Naoto Kobayashi
    Institute of Astronomy, Graduate School of Science, The University of Tokyo , 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
  • Sohei Kondo
    Kiso Observatory, Institute of Astronomy, Graduate School of Science, The University of Tokyo , 10762-30 Mitake, Kiso-machi, Kiso-gun, Nagano 397-0101, Japan
  • Yuki Mori
    Kiso Observatory, Institute of Astronomy, Graduate School of Science, The University of Tokyo , 10762-30 Mitake, Kiso-machi, Kiso-gun, Nagano 397-0101, Japan
  • Shin-ichiro Okumura
    Japan Spaceguard Association, Bisei Spaceguard Center , 1716-3 Okura, Bisei-cho, Ibara, Okayama 714-1411, Japan
  • Satoshi Takita
    Kiso Observatory, Institute of Astronomy, Graduate School of Science, The University of Tokyo , 10762-30 Mitake, Kiso-machi, Kiso-gun, Nagano 397-0101, Japan
  • Shigeyuki Sako
    Institute of Astronomy, Graduate School of Science, The University of Tokyo , 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan

抄録

<jats:title>Abstract</jats:title> <jats:p>We report on a one-second-cadence wide-field survey for M-dwarf flares using the Tomo-e Gozen camera mounted on the Kiso Schmidt telescope. We detect 22 flares from M3–M5 dwarfs with a rise time of 5 s ≲ trise ≲ 100 s and an amplitude of 0.5 ≲ ΔF/F⋆ ≲ 20. The flare light-curves mostly show steeper rises and shallower decays than those obtained from the Kepler one-minute cadence data and tend to have flat peak structures. Assuming a blackbody spectrum with a temperature of 9000–15000 K, the peak luminosities and energies are estimated to be 1029 erg s−1 ≲ Lpeak ≲ 1031 erg s−1 and 1031 erg ≲ Eflare ≲ 1034 erg, which constitutes the bright end of fast optical flares for M dwarfs. We confirm that more than $90\%$ of the host stars of the detected flares are magnetically active based on their Hα-emission-line intensities obtained by LAMOST. An estimated occurrence rate of detected flares is ∼0.7 per day per active star, indicating they are common in magnetically active M dwarfs. We argue that the flare light-curves can be explained by the chromospheric compression model: the rise time is broadly consistent with the Alfvén transit time of a magnetic loop with a length scale of lloop ∼ 104 km and a field strength of 1000 gauss, while the decay time is likely determined by the radiative cooling of the compressed chromosphere down near to the photosphere with a temperature of ≳ 10000 K. These flares from M dwarfs could be a major contamination source for a future search of fast optical transients of unknown types.</jats:p>

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