Speaker
Description
A number of middle-aged SNRs with molecular clouds (MCs) emit GeV gamma-rays originating from accelerated protons. About half of GeV SNRs also have recombining plasma (RP; the plasma with ionization temperatures that are higher than electron temperatures), implying rapid cooling of the electrons. These facts indicate thermal plasma can be the key to understand the escaping process of accelerated protons from SNR shocks.
We have made thermal X-ray study of the GeV SNR HB 21. HB 21 is interacting with MCs and the faintest in GeV band among GeV SNRs. We discovered a strong radiative recombination continuum of Si from the center of the remnant, which is the direct evidence of RP (electron temperature of $0.16\pm0.01$ keV and recombination timescale of $(2.8\pm0.5)\times10^{11}$ s cm$^{-3}$).
The estimated RP age ($\sim100$ kyr) is the longest among GeV SNRs with RP.
Systematic study of GeV SNRs shows a strong correlation between RP age and photon index in the GeV band.
It supports the following scenario; interaction with MCs makes a magnetic field partially weaker and protons escape, simultaneously cooling down the SNR plasma to be recombining.
