A Chandra X-ray detection of the L dwarf binary Kelu-1 -: Simultaneous Chandra and very large array observations

Context. Magnetic activity in ultracool dwarfs, as measured in X-rays and Ha, 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 L-X = 2.9(-1.3)(+1.8) x 10(25) erg s(-1), while it remained undetected in the radio down to a 3 sigma limit of L-R <= 1.4 x 10(13) erg s(-1) Hz(-1). We argue that, whereas the X-ray and Ha 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.