What is accelerating particles in SNR G5.7-0.1?

Currently pulsar wind nebulae (PWNe) and supernova remnants (SNRs) are believed to be the dominant source of Galactic cosmic rays. The observed radio, X-ray, and TeV emission from SNR G5.7-0.1 suggests it is a source of cosmic rays, but its non-thermal emission is incompatible with either a SNR or a PWN. Instead this source requires that 1) either electrons are currently escaping a PWN, or 2) the SNR shell is a rare site of highly efficient electron and/or proton acceleration, and/or 3) the unlikely spatial coincidence of a luminous diffuse non-thermal X-ray source and a thermal X-ray SNR. With the requested 30 ks XMM observation, we will determine what is accelerating particles in this enigmatic object.