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 the year 1572. The event lasted for 1.5 yr 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 ultrahigh energies for the electrons up to similar to 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 ultrarelativistic energies. The event, if confirmed, helps reveal the more complicated side of the physical processes that can occur in young SNRs.