Context : 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 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 } \times 10 ^ { 25 } ergs s ^ { -1 } , while it remained undetected in the radio down to a 3 \sigma limit of L _ { R } \leq 1.4 \times 10 ^ { 13 } ergs s ^ { -1 } Hz ^ { -1 } .
We argue that , whereas the X-ray and H \alpha 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 .