  Electrons can be accelerated to ultrarelativistic energies at strong
(high-Mach number) collisionless shock waves that form when stellar debris
rapidly expands after a supernova. Collisionless shock waves also form in the
flow of particles from the Sun (the solar wind), and extensive spacecraft
observations have established that electron acceleration at these shocks is
effectively absent whenever the upstream magnetic field is roughly parallel to
the shock surface normal (quasi-parallel conditions). However, it is unclear
whether this magnetic dependence of electron acceleration also applies to the
far stronger shocks around young supernova remnants, where local magnetic
conditions are poorly understood. Here we present Cassini spacecraft
observations of an unusually strong solar system shock wave (Saturn's bow
shock) where significant local electron acceleration has been confirmed under
quasi-parallel magnetic conditions for the first time, contradicting the
established magnetic dependence of electron acceleration at solar system
shocks. Furthermore, the acceleration led to electrons at relativistic energies
(~MeV), comparable to the highest energies ever attributed to
shock-acceleration in the solar wind. These observations demonstrate that at
high-Mach numbers, like those of young supernova remnant shocks, quasi-parallel
shocks become considerably more effective electron accelerators.
