Summary: Astronomy & Astrophysics manuscript no. secondrev2.1 c ESO 2008
June 11, 2008
A Mini-survey of Ultracool Dwarfs at 4.9 GHz
A. Antonova1, J.G. Doyle1, G. Hallinan2, S. Bourke2, A. Golden2
Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland
Computational Astrophysics Laboratory, I.T. Building, National University of Ireland, Galway, Ireland
Context. 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.
Aims. To explore whether ultracool dwarfs previously undetected at 8.5 GHz may be detectable at a lower frequency.
Methods. We select a sample of fast rotating ultracool dwarfs with no detectable radio activity at 8.5 GHz, observing each of them at
Results. From the 8 dwarfs in our sample, we detect emission from 2MASS J07464256+2000321, with a mean flux level of 286 ±
24 µJy. The light-curve of 2MASS J07464256+2000321, is dominated towards the end of the observation by a very bright, 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 160 µJy in the first hour of observation rising to 400 µJy in the 40 minutes before the burst. During both
periods, there is significant variability.
Conclusions. The detection of 100% circular polarization in the emission at 4.9 GHz points towards the electron cyclotron maser