1. Division of Physics, Mathematics & Astronomy 105-24, California Institute of Technology, Pasadena, California 91125, USA
2. Department of Physics, New Mexico Tech, PO Box 3394 c/s, Socorro, New Mexico 87801, USA
3. Department of Physics, Oberlin College, Oberlin, Ohio 44074, USA
4. Department of Physics, Carleton College, 300 N. College St., Northfield, Minnesota 55057, USA
5. National Radio Astronomy Observatory, PO Box O, Socorro, New Mexico 87801, USA
6. Department of Physics & Astronomy, University of Denver, 2112 E. Wesley Ave., Denver, Colorado 80208, USA
7. Department of Physics & Astronomy, University of New Mexico, 800 Yale Blvd NE, Albuquerque, New Mexico 87131, USA
8. Department of Astronomy, 525 Davey Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16082, USA
9.  Department of Physics & Astronomy, 177 Chem-Physics building, University of Kentucky, Lexington, Kentucky 40506, USA
10. Department of Physics, Amherst College, Amherst, Massachusetts 01002-5000, USA
11. Department of Astronomy, University of Maryland, College Park, Maryland 20742, USA
12. Department of Physics, Southwestern University, SU Box 7263, Georgetown, Texas 78626, USA
13. Department of Physics & Astronomy, Agnes Scott College, Decatur, Georgia 30030, USA
14. Department of Astronomy, New Mexico State University, MSC 4500 PO Box 3001, Las Cruces, New Mexico 88003, USA

Correspondence to: E. Berger¹ Correspondence and requests for materials should be addressed to E.B. (e-mail: Email: ejb@astro.caltech.edu).

Abstract

Brown dwarfs are not massive enough to sustain thermonuclear fusion of hydrogen at their centres, but are distinguished from gas-giant planets by their ability to burn deuterium¹. Brown dwarfs older than 10 Myr are expected to possess short-lived magnetic fields² and to emit radio and X-rays only very weakly from their coronae. An X-ray flare was recently detected³ on the brown dwarf LP944-20, whereas previous searches^{4, 5, 6, 7} for optical activity (and one X-ray search¹) yielded negative results. Here we report the discovery of quiescent and flaring radio emission from LP944-20, with luminosities several orders of magnitude larger than predicted by the empirical relation^{8, 9} between the X-ray and radio luminosities that has been found for many types of stars. Interpreting the radio data within the context of synchrotron emission, we show that LP944-20 has an unusually weak magnetic field in comparison to active M-dwarf stars^{10, 11}, which might explain the previous null optical^{4, 5, 6, 7} and X-ray¹ results, as well as the strength of the radio emissions compared to those at X-ray wavelengths.

1. Division of Physics, Mathematics & Astronomy 105-24, California Institute of Technology, Pasadena, California 91125, USA
2. Department of Physics, New Mexico Tech, PO Box 3394 c/s, Socorro, New Mexico 87801, USA
3. Department of Physics, Oberlin College, Oberlin, Ohio 44074, USA
4. Department of Physics, Carleton College, 300 N. College St., Northfield, Minnesota 55057, USA
5. National Radio Astronomy Observatory, PO Box O, Socorro, New Mexico 87801, USA
6. Department of Physics & Astronomy, University of Denver, 2112 E. Wesley Ave., Denver, Colorado 80208, USA
7. Department of Physics & Astronomy, University of New Mexico, 800 Yale Blvd NE, Albuquerque, New Mexico 87131, USA
8. Department of Astronomy, 525 Davey Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16082, USA
9.  Department of Physics & Astronomy, 177 Chem-Physics building, University of Kentucky, Lexington, Kentucky 40506, USA
10. Department of Physics, Amherst College, Amherst, Massachusetts 01002-5000, USA
11. Department of Astronomy, University of Maryland, College Park, Maryland 20742, USA
12. Department of Physics, Southwestern University, SU Box 7263, Georgetown, Texas 78626, USA
13. Department of Physics & Astronomy, Agnes Scott College, Decatur, Georgia 30030, USA
14. Department of Astronomy, New Mexico State University, MSC 4500 PO Box 3001, Las Cruces, New Mexico 88003, USA

Correspondence to: E. Berger¹ Correspondence and requests for materials should be addressed to E.B. (e-mail: Email: ejb@astro.caltech.edu).