We present an X-ray study of the GeV gamma-ray supernova remnant (SNR) HB 21 with Suzaku. HB 21 is interacting with molecular clouds and the faintest in the GeV band among known GeV SNRs. We discovered strong radiative recombination continua of Si and S from the center of the remnant, which provide the direct evidence of a recombining plasma (RP). The total emission can be explained with the RP and ionizing plasma components. The electron temperature and recombination timescale of the RP component were estimated as 0.17 (0.15-0.18) keV and 3.2 (2.0-4.8) $times$ 10$^{11}$ s cm$^{-3}$, respectively. The estimated age of the RP (RP age; $sim$ 170 kyr) is the longest among known recombining GeV SNRs, because of very low density of electrons ($sim$ 0.05 cm$^{-3}$). We have examined dependencies of GeV spectral indices on each of RP ages and SNR diameters for nine recombining GeV SNRs. Both showed possible positive correlations, indicating that both the parameters can be good indicators of properties of accelerated protons, for instance, degree of escape from the SNR shocks. A possible scenario for a process of proton escape is introduced; interaction with molecular clouds makes weaker magnetic turbulence and cosmic-ray protons escape, simultaneously cooling down the thermal electrons and generate an RP.
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