A Mini-survey of Ultracool Dwarfs at 4.9 GHz

Abstract

A selection of ultracool dwarfs are known to be radio active, with both gyrosynchrotron emission and the electron cyclotron maser instability being given as likely emission mechanisms. To explore whether ultracool dwarfs previously undetected at 8.5 GHz may be detectable at a lower frequency. We select a sample of fast rotating ultracool dwarfs with no detectable radio activity at 8.5 GHz, observing each of them at 4.9 GHz. From the 8 dwarfs in our sample, we detect emission from 2MASS J07464256+2000321, with a mean flux level of 286 $\pm$ 24 $\mu Jy$. The light-curve of 2MASS J07464256+2000321, is dominated towards the end of the observation by a very bright, $\approx$100 % left circularly polarized burst during which the flux reached 2.4 mJy. The burst was preceded by a raise in the level of activity, with the average flux being $\approx$ 160 $\mu Jy$ in the first hour of observation rising to $\approx$ 400 $\mu Jy$ in the 40 minutes before the burst. During both periods, there is significant variability. The detection of 100% circular polarization in the emission at 4.9 GHz points towards the electron cyclotron maser as the emission mechanism. However, the observations at 4.9 GHz and 8.5 GHz were not simultaneous, thus the actual fraction of dwarfs capable of producing radio emission, as well as the fraction of those that show periodic pulsations is still unclear, as indeed are the relative roles played by the electron cyclotron maser instability versus gyrosynchrotron emission, therefore we cannot assert if the previous non-detection at 8.5 GHz was due to a cut-off in emission between 4.9 and 8.4 GHz, or due to long term variability.

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• The Second Arecibo Search for 5 GHz Radio Flares from Ultracool Dwarfs
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• The Discovery of Solar-like Activity Cycles Beyond the End of the Main Sequence?
• Radio-flaring Ultracool Dwarf Population Synthesis
• Low-resolution optical spectra of ultracool dwarfs with OSIRIS/GTC
• Multi-Epoch VLBI of L Dwarf Binary 2MASS J0746+2000AB: Precise Mass Measurements and Confirmation of Radio Emission from Both Components
• The Occurrence Rate of Quiescent Radio Emission for Ultracool Dwarfs using a Generalized Semi-Analytical Bayesian Framework
• Kinetic Simulation of the Electron-Cyclotron Maser Instability: Relaxation of Electron Horseshoe Distributions
• The Potential of Detecting Radio-flaring Ultracool Dwarfs at L band in the FAST Drift-scan Survey