Abstract
We study the origin of unusually persistent microwave and X-ray radiation from the ultracool dwarf TVLM 513-46546. It is shown that the source of ≈1 keV X-ray emission is not the entire corona of the brown dwarf, but a population of several hundreds of coronal magnetic loops, with 10 MK plasma heated upon dissipation of the electric current generated by the photospheric convection. Unlike models, which assume a large-scale magnetic structure of the microwave source, our model suggests that the microwave radiation comes from hundreds of magnetic loops quasi-uniformly distributed over the dwarf’s surface. We propose a long-term operating mechanism of acceleration of electrons generating gyrosynchrotron radio emission caused by oscillations of electric current in the magnetic loops as an equivalent RLC circuit. The second population of magnetic loops—the sources of microwave radiation, is at the same time a source of softer (≈0.2 keV) X-ray emission.
Highlights
The brown dwarf TVLM 513-46546 is currently one of the most studied among ultracool stars of spectral types M > 7, L, T, and Y
We show that the sources of radio and X-ray emission of TVLM 513-46546 consist of a few ×100 coronal magnetic loops quasi-uniformly distributed over the dwarf surface
We propose the heating mechanism of coronal loop plasma driven by dissipation of electric current generated by photospheric convection and the mechanism of long-term particle acceleration in the atmosphere of a brown dwarf supporting the persistent nature of the radio flux
Summary
The brown dwarf TVLM 513-46546 is currently one of the most studied among ultracool stars of spectral types M > 7, L, T, and Y. In addition to the surprising radio to X-ray luminosity ratio (LR/LX ≈ 3.3 × 10−12 Hz−1), by a factor of 104 larger than expected [2,4], a persistent (on a multiyear timescale) quiescent nonthermal radio emission from the brown dwarf TVLM 513-46546 is observed, which assumes a continuous source of accelerated electrons. The current models of the microwave [1] and multiwavelength radiation sources in TVLM 513-46546 [2] provide no explanation for the nature of the accelerator of electrons producing the quiescent nonthermal gyrosynchrotron emission for several years. We propose the heating mechanism of coronal loop plasma driven by dissipation of electric current generated by photospheric convection and the mechanism of long-term particle acceleration in the atmosphere of a brown dwarf supporting the persistent nature of the radio flux. The magnetic loops responsible for the persistent microwave emission can be the sources of the “soft” X-ray component
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