  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.
