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

Abstract

We report 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 rise times and amplitudes ranging from $5\sec \lesssim t_{\mathrm{rise}}\lesssim 100\sec$ and $0.5\lesssim \Delta F/F_{\star }\lesssim 20$, respectively. 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 temperatures of $9,000-15,000\mathrm{K}$, the peak luminosities and bolometric energies are estimated to be $10^{29}\mathrm{erg}\mathrm{se}{\mathrm{c}}^{-1}\lesssim L_{\mathrm{peak}}\lesssim 10^{31}\mathrm{erg}\mathrm{se}{\mathrm{c}}^{-1}$ and $10^{31}\mathrm{erg}\lesssim E_{\mathrm{bol}}\lesssim 10^{34}\mathrm{erg}$, 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 H$\alpha$ emission line intensities obtained by LAMOST. The estimated occurrence rate of the detected flares is $\sim 0.7$ per day per an 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 $l_{\mathrm{loop}}\sim 10^4\mathrm{km}$ and a field strength of $1,000\mathrm{G}$, while the decay time is likely determined by the radiative cooling of the compressed chromosphere down to near the photosphere with a temperature of $\gtrsim 10,000\mathrm{K}$. These flares from M dwarfs could be a major contamination source for a future search of fast optical transients of unknown types.

39 pages, 13 figures, accepted for publication in PASJ

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