  Supernova remnants interacting with molecular/atomic clouds are interesting
X-ray sources to study broadband nonthermal emission. X-ray line emission in
these systems can be produced by different processes, e.g. low energy cosmic
rays interacting with the cloud and fast ejecta fragments moving in the cloud.
The paper aims at studying the origin of the non-thermal X-ray emission of the
southwestern limb of SN 1006 beyond the main shock, in order to distinguish if
the emission is due to low energy cosmic rays diffusing in the cloud or to
ejecta knots moving into the cloud. We analyzed the X-ray emission of the
southwestern limb of SN 1006, where the remnant interacts with an atomic cloud,
with three different X-ray telescopes ({NuSTAR, Chandra and XMM-Newton) and
performed a combined spectro-imaging analysis of this region. The analysis of
the non thermal X-ray emission of the southwestern limb of SN 1006, interacting
with an atomic cloud, has shown the detection of an extended X-ray source in
the atomic cloud, approximately $2$ pc upstream of the shock front. The source
is characterized by a hard continuum (described by a power law with photon
index $\Gamma\sim1.4$) and by Ne, Si and Fe emission lines. The observed flux
suggests that the origin of the X-ray emission is not associated with low
energy cosmic rays interacting with the cloud. On the other hand, the spectral
properties of the source, together with the detection of an IR counterpart
visible with \textit{Spitzer}-MIPS at 24 $\mu$m are in good agreement with
expectations for a fast ejecta fragment moving within the atomic cloud. We
detected X-ray and IR emission from a possible ejecta fragment, with radius
approximately 1$\times10^{17}$ cm, and mass approximately $10^{-3}M_\odot$ at
about 2 pc out of the shell of SN 1006, in the interaction region between the
southwestern limb of the remnant and the atomic cloud.
