A Possible Gamma-Ray Enhancement Event in Tycho's Supernova Remnant

We report a possible $\gamma$-ray enhancement event detected from Tycho's supernova remnant (SNR), the outcome of a type Ia supernova explosion that occurred in year 1572. The event lasted for 1.5 years and showed a factor of 3.6 flux increase mainly in the energy range of 4--100 GeV, while notably accompanied with two 478\,GeV photons. Several young SNRs (including Tycho's SNR) were previously found to show peculiar X-ray structures with flux variations in one- or several-year timescales, such an event at $\gamma$-ray energies is for the first time seen. The year-long timescale of the event suggests a synchrotron radiation process, but the hard $\gamma$-ray emission requires extreme conditions of either ultra-high energies for the electrons upto $\sim$10 PeV (well above the cosmic-ray "knee" energy) or high inhomogeneity of the magnetic field in the SNR. This event in Tycho's SNR is likely analogous to the $\gamma$-ray flares observed in the Crab nebula, the comparably short timescales of them both requiring a synchrotron process, and similar magnetohydrodynamic processes such as magnetic reconnection would be at work as well in the SNR to accelerate particles to ultra-relativistic energies. The event, if confirmed, helps reveal the more complicated side of the physical processes that can occur in young SNRs.