XMM-Newton evidence of shocked ISM in SN 1006: indications of hadronic acceleration

Context. Shock fronts in young supernova remnants are the best candidates for being sites of cosmic ray acceleration up to a few PeV, though conclusive experimental evidence is still lacking. Aims: Hadron acceleration is expected to increase the shock compression ratio, providing higher postshock densities, but X-ray emission from shocked ambient medium has not firmly been detected yet in remnants where particle acceleration is at work. We exploited the deep observations of the XMM-Newton large program on SN 1006 to verify this prediction. Methods: We performed spatially resolved spectral analysis of a set of regions covering the southeastern rim of SN 1006. We studied the spatial distribution of the thermodynamic properties of the ambient medium and carefully verified the robustness of the result with respect to the analysis method. Results: We detected the contribution of the shocked ambient medium. We also found that the postshock density of the interstellar medium significantly increases in regions where particle acceleration is efficient. Under the assumption of uniform preshock density, we found that the shock compression ratio reaches a value of ~6 in regions near the nonthermal limbs. Conclusions: Our results support the predictions of shock modification theory and indicate that effects of acceleration of cosmic ray hadrons on the postshock plasma can be observed in supernova remnants.