  The theory of Non-Linear Diffusive Shock Acceleration (NLDSA) predicts the
formation of a precursor upstream of the shock, where accelerated particles
diffuse and induce magnetic field amplification through streaming instability.
The non detection of this precursor in X-rays in {\it Chandra} observations of
the north-eastern region of SN 1006 (G329.6+14.6) led to impose an upper limit
to the X-ray emission generated by accelerated electrons diffusing in this
precursor, at an emissivity level of <1.5 per cent of the emission from the
downstream region (Long et al. 2003). This has been used as an argument against
Fermi acceleration at this shock. Here we calculate the spectrum and spatial
distribution of accelerated particles in SN 1006 and show that Chandra results
(including more recent data) are in perfect agreement with the predictions of
NLDSA suggesting efficient particle acceleration and magnetic field
amplification upstream of the shock by a factor ~10.
