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

Author:

Aizawa Masataka1ORCID,Kawana Kojiro2ORCID,Kashiyama Kazumi2345ORCID,Ohsawa Ryou67ORCID,Kawahara Hajime38ORCID,Naokawa Fumihiro23,Tajiri Tomoyuki8,Arima Noriaki69ORCID,Jiang Hanchun10,Hartwig Tilman2411ORCID,Fujisawa Kotaro2ORCID,Shigeyama Toshikazu39ORCID,Arimatsu Ko12ORCID,Doi Mamoru361314,Kasuga Toshihiro15ORCID,Kobayashi Naoto671314ORCID,Kondo Sohei7,Mori Yuki7,Okumura Shin-ichiro16ORCID,Takita Satoshi7,Sako Shigeyuki61314ORCID

Affiliation:

1. Tsung-Dao Lee Institute, Shanghai Jiao Tong University , Shengrong Road 520, 201210 Shanghai, China

2. Department of Physics, Graduate School of Science, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

3. Research Center for the Early Universe, Graduate School of Science, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

4. Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU,WPI), The University of Tokyo , 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583, Japan

5. Astronomical Institute, Graduate School of Science, Tohoku University , Aoba-ku, Sendai, Miyagi 980-8578, Japan

6. Institute of Astronomy, Graduate School of Science, The University of Tokyo , 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan

7. Kiso Observatory, Institute of Astronomy, Graduate School of Science, The University of Tokyo , 10762-30 Mitake, Kiso-machi, Kiso-gun, Nagano 397-0101, Japan

8. Department of Earth and Planetary Science, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

9. Department of Astronomy, Graduate School of Science, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

10. School of Astronomy and Space Science, Nanjing University , Nanjing 210093, China

11. Institute for Physics of Intelligence, School of Science, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

12. The Hakubi Center/Astronomical Observatory, Graduate School of Science , Kyoto University Kitashirakawa-oiwake-cho, Sakyo-ku, Kyoto, Kyoto 606-8502, Japan

13. UTokyo Organization for Planetary Space Science, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

14. Collaborative Research Organization for Space Science and Technology, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

15. National Astronomical Observatory of Japan , 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

16. Japan Spaceguard Association, Bisei Spaceguard Center , 1716-3 Okura, Bisei-cho, Ibara, Okayama 714-1411, Japan

Abstract

Abstract 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.

Funder

Japan Society for the Promotion of Science

Chinese Academy of Sciences

National Development and Reform Commission

Publisher

Oxford University Press (OUP)

Subject

Space and Planetary Science,Astronomy and Astrophysics

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