  Radio halos are faint radio sources usually located at the center of merging
clusters of galaxies. These diffuse radio sources are rare, having so far been
found only in about 30 clusters of galaxies, suggesting that particular
conditions are needed to form and maintain them. It is interesting to
investigate the presence of radio halos in close pairs of interacting clusters
in order to possibly clarify their origin in relation to the evolutionary state
of the merger. In this work, we study the case of the close pair of galaxy
clusters A399 and A401. A401 is already known to contain a faint radio halo,
while a hint of diffuse emission in A399 has been suggested based on the NVSS.
To confirm this possibility, we analyzed deeper Very Large Array observations
at 1.4 GHz of this cluster. We find that the central region of A399 is
permeated by a diffuse low-surface brightness radio emission that we classify
as a radio halo with a linear size of about 570 kpc and a central brightness of
0.3 micro-Jy/arcsec^2. Indeed, given their comparatively small projected
distance of about 3 Mpc, the pair of galaxy clusters A401 and A399 can be
considered as the first example of double radio halo system. The discovery of
this double halo is extraordinary given the rarity of these radio sources in
general and given that current X-ray data seem to suggest that the two clusters
are still in a pre-merger state. Therefore, the origin of the double radio halo
is likely to be attributed to the individual merging histories of each cluster
separately, rather than to the result of a close encounter between the two
systems.
