  Supernova remnants (SNRs) have long been considered as one of the most
promising sources of Galactic cosmic rays. In the SNR paradigm,
petaelectronvolt (PeV) proton acceleration may only be feasible at the early
evolution stage, lasting a few hundred years, when the SNR shock speed is high.
While evidence supporting the acceleration of PeV protons in young SNRs has yet
to be discovered, X-ray synchrotron emission is an important indicator of fast
shock. We here report the first discovery of X-ray synchrotron emission from
the possibly middle-aged SNR G106.3+2.7, implying that this SNR is still an
energetic particle accelerator despite its age. This discovery, along with the
ambient environmental information, multiwavelength observation, and theoretical
arguments, supports SNR G106.3+2.7 as a likely powerful PeV proton accelerator.
