Letter to the Editor

A Chandra X-ray detection of the L dwarf binary Kelu-1

Simultaneous Chandra and Very Large Array observations

¹ ISDC, Ch. d'Ecogia 16, 1290 Versoix, Switzerland e-mail: Marc.Audard@obs.unige.ch
² Observatoire de Genève, University of Geneva, Ch. des Maillettes 51, 1290 Sauverny, Switzerland
³ Astronomy Department, University of Maryland, College Park, MD, USA
⁴ Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, CO 80309-0389, USA
⁵ Paul Scherrer Institut, Villigen & Würenlingen, 5232 Villigen PSI, Switzerland
⁶ Department of Physics and Astronomy, 223 Sharp Lab, University of Delaware, Newark, DE 19716, USA

Received: 15 June 2007
Accepted: 6 July 2007

Abstract

Context.Magnetic activity in ultracool dwarfs, as measured in X-rays and Hα, shows a steep decline after spectral type M7-M8. So far, no L dwarf has been detected in X-rays. In contrast, L dwarfs may have higher radio activity than M dwarfs.

Aims.We observe L and T dwarfs simultaneously in X-rays and radio to determine their level of magnetic activity in the context of the general decline of magnetic activity with cooler effective temperatures.

Methods.The field L dwarf binary Kelu-1 was observed simultaneously with Chandra  and the Very Large Array.

Results.Kelu-1AB was detected in X-rays with   10²⁵ erg s^-1, while it remained undetected in the radio down to a limit of   10¹³ erg s^-1 Hz^-1. We argue that, whereas the X-ray and Hα emissions decline in ultracool dwarfs with decreasing effective temperature, the radio luminosity stays (more or less) constant across M and early-L dwarfs. The radio surface flux or the luminosity may better trace magnetic activity in ultracool dwarfs than the ratio of the luminosity to the bolometric luminosity.

Conclusions.Deeper radio observations (and at short frequencies) are required to determine if and when the cut-off in radio activity occurs in L and T dwarfs, and what kind of emission mechanism takes place in ultracool dwarfs.