The Slow Temperature Equilibration behind the Shock Front of SN 1006

We report on the observation of O VII Doppler line broadening in a compact knot at the edge of SN 1006 detected with the reflective grating spectrometer on board XMM-Newton. The observed line width of σ=3.4+/-0.5 eV at a line energy of 574 eV indicates an oxygen temperature of kT=528+/-150 keV. Combined with the observed electron temperature of ~1.5 keV, the observed broadening is direct evidence for temperature nonequilibration in high Mach number shocks and slow subsequent equilibration. The O VII line emission allows an accurate determination of the ionization state of the plasma, which is characterized by a relatively high forbidden line contribution, indicating logn_et~=9.2.