Geometry of the non-thermal emission in SN 1006. Azimuthal variations of cosmic-ray acceleration

Abstract

SN 1006 is the prototype of shell supernova remnants, in which non-thermal synchrotron emission dominates the X-ray spectrum. The non-thermal emission is due to the cosmic-ray electrons accelerated behind the blast wave. The X-ray synchrotron emission is due to the highest energy electrons, and is thus a tracer of the maximum energy electrons may reach behind a shock. We have put together all XMM-Newton observations to build a full map of SN 1006. The very low brightness above 2 keV in the interior indicates that the bright non-thermal limbs are polar caps rather than an equator. This implies that the ambient magnetic field runs southwest to northeast, along the Galactic plane. We used a combined VLA/Parkes radio map to anchor the spectrum at low energy, and model the spectra with synchrotron emission from a cut-off power-law electron distribution, plus a thermal component. We present radial and azimuthal profiles of the cut-off frequency. The cut-off frequency decreases steeply with radius towards the center and with position angle away from the maximum emission. The maximum energy reached by accelerated particles, as well as their number, must be higher at the bright limbs than elsewhere. This implies interesting constraints for acceleration at perpendicular shocks. Overall the XMM-Newton data is consistent with the model in which the magnetic field is amplified where acceleration is efficient.