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Study on the escape timescale of high-energy particles from supernova remnants through thermal X-ray properties

In this decade, GeV/TeV gamma-ray observations of several supernova remnants (SNRs) have implied that accelerated particles are escaping from their acceleration sites. However, when and how they escape from the SNR vicinities are yet to be understood. Recent studies have suggested that the particle escape might develop with thermal plasma ages of the SNRs. In this paper, we present a systematic study on time evolution of particle escape using thermal X-ray properties and gamma-ray spectra. We used 38 SNRs which associate with GeV/TeV gamma-ray emissions. We conducted spectral fittings on the gamma-ray spectra using exponential cutoff power law and broken power law models to estimate the exponential cutoff or the break energies, both of which are indicators of particle escape. The plots of the gamma-ray cutoff/break energies over the plasma ages show similar tendencies to those predicted by simple theories of the particle escape under conditions in which a shock is interacting with thin interstellar medium or clouds. The particle escape timescale is estimated as $\sim$100 kyr from decreasing trends of the total energy of the confined protons with the plasma age. The large dispersions of the cutoff/break energies of the data may suggest an intrinsic variety of particle escape environments. This might be the cause of the complicated Galactic cosmic-ray spectral shape measured on Earth.