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Magnetic activity in ultracool dwarfs, as measured in X-rays and H$\alpha$,shows a steep decline after spectral type M7-M8. So far, no L dwarf has beendetected in X-rays. In contrast, L dwarfs may have higher radio activity than Mdwarfs. We observe L and T dwarfs simultaneously in X-rays and radio todetermine their level of magnetic activity in the context of the generaldecline of magnetic activity with cooler effective temperatures. The field Ldwarf binary Kelu-1 was observed simultaneously with Chandra and the Very LargeArray. Kelu-1AB was detected in X-rays with $L_{\rm X} = 2.9_{-1.3}^{+1.8}\times 10^{25}$ erg/s, while it remained undetected in the radio down to a $3\sigma$ limit of $L_{\rm R} \leq 1.4 \times 10^{13}$ erg/s/Hz. We argue that,whereas the X-ray and H$\alpha$ emissions decline in ultracool dwarfs withdecreasing effective temperature, the radio luminosity stays (more or less)constant across M and early-L dwarfs. The radio surface flux or the luminositymay better trace magnetic activity in ultracool dwarfs than the ratio of theluminosity to the bolometric luminosity. Deeper radio observations (and atshort frequencies) are required to determine if and when the cut-off in radioactivity occurs in L and T dwarfs, and what kind of emission mechanism takesplace in ultracool dwarfs.

A Chandra X-ray detection of the L dwarf binary Kelu-1: Simultaneous Chandra and Very Large Array observations