Cosmic ray (CR) protons that escaped from supernova remnants (SNRs) illuminate surrounding molecular clouds (MCs) and then emit hadronic gamma rays. Since it takes the diffusion time to reach MCs after escaping from SNRs, the gamma-ray spectrum reflects the past distribution of accelerated particles in the SNR. Therefore, such emissions are called ``delayed'' gamma rays. We analyzed 12-yr Fermi-LAT data around SNR HB9 and detected gamma-ray emissions not only at the SNR shell, as found in previous studies, but also at MCs. The energy spectra at the cloud regions were fitted with a simple power-law function ($dN/dE \propto E^{-\Gamma}$) in an energy range of 1--500~GeV, resulting in a flatter spectral index ($\Gamma \sim 1.8$) than that of the SNR shell ($\Gamma = 2.55 \pm 0.10$). Spectral modeling results show that the spectra of molecular clouds can be reproduced with emissions of protons that escaped in the past from the SNR, the so-called delayed gamma rays. By comparing the energy spectra at the molecular cloud regions and the SNR shell, we investigated a time evolution of the maximum energy of CRs accelerated in the SNR. We then found evidence that this SNR accelerated CRs up to higher energies in the past than the present shell. The obtained lower limit on the maximum energy is $> 10$~TeV, which will be determined better with TeV gamma-ray observations.
We also found that the diffusion coefficient around HB9 is equivalent to the Galactic mean, suggesting that CR-self confinement is inefficient in this region.
These results have been presented in the publication [TO & WI, 2022, PASJ, 74, 625], to which we refer readers for details.