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arXiv · 2102.04751

Low-frequency monitoring of flare star binary CR Draconis: Long-term electron-cyclotron maser emission

J. R. Callingham·B. J. S. Pope·A. D. Feinstein·H. K. Vedantham·T. W. Shimwell·P. Zarka·C. Tasse·L. Lamy·K. Veken·S. Toet·J. Sabater·P. N. Best·R. J. van Weeren·H. J. A. Röttgering·T. P. Ray
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Abstract

Recently detected coherent low-frequency radio emission from M dwarf systems shares phenomenological similarities with emission produced by magnetospheric processes from the gas giant planets of our Solar System. Such beamed electron-cyclotron maser emission can be driven by a star-planet interaction or a breakdown in co-rotation between a rotating plasma disk and a stellar magnetosphere. Both models suggest that the radio emission could be periodic. Here we present the longest low-frequency interferometric monitoring campaign of an M dwarf system, composed of twenty-one $\approx$8 hour epochs taken in two series of observing blocks separated by a year. We achieved a total on-source time of 6.5 days. We show that the M dwarf binary CR Draconis has a low-frequency 3$\sigma$ detection rate of 90$^{+5}_{-8}$% when a noise floor of $\approx$0.1 mJy is reached, with a median flux density of 0.92 mJy, consistent circularly polarised handedness, and a median circularly polarised fraction of 66%. We resolve three bright radio bursts in dynamic spectra, revealing the brightest is elliptically polarised, confined to 4 MHz of bandwidth centred on 170 MHz, and reaches a flux density of 205 mJy. The burst structure is mottled, indicating it consists of unresolved sub-bursts. Such a structure shares a striking resemblance with the low-frequency emission from Jupiter. We suggest the near-constant detection of high brightness temperature, highly-circularly-polarised radiation that has a consistent circular polarisation handedness implies the emission is produced via the electron-cyclotron maser instability. Optical photometric data reveal the system has a rotation period of 1.984$\pm$0.003 days. We observe no periodicity in the radio data, but the sampling of our radio observations produces a window function that would hide the near two-day signal.

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J. R. Callingham, B. J. S. Pope, A. D. Feinstein, H. K. Vedantham, T. W. Shimwell, P. Zarka, C. Tasse, L. Lamy, K. Veken, S. Toet, J. Sabater, P. N. Best, R. J. van Weeren, H. J. A. Röttgering, T. P. Ray. 2021-02-09. Low-frequency monitoring of flare star binary CR Draconis: Long-term electron-cyclotron maser emission. https://doi.org/10.1051/0004-6361%2F202039144

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