Discovery of recombining plasma inside the extended gamma-ray supernova remnant HB9

Abstract

We present the results from the Suzaku X-ray Imaging Spectrometer observation of the mixed-morphology supernova remnant (SNR) HB9 (G160.9+2.6). We discovered recombining plasma (RP) in the western Suzaku observation region and the spectra here are well described by a model having collisional ionization equilibrium (CIE) and RP components. On the other hand, the X-ray spectra from the eastern Suzaku observation region are best reproduced by the CIE and non-equilibrium ionization model. We discuss possible scenarios to explain the origin of the RP emission based on the observational properties and concluded that the rarefaction scenario is a possible explanation for the existence of RP. In addition, the gamma-ray emission morphology and spectrum within the energy range of 0.2$-$300 GeV are investigated using $\sim$10 years of data from the Fermi Large Area Telescope (LAT). The gamma-ray morphology of HB9 is best described by the spatial template of radio continuum emission. The spectrum is well-fit to a log-parabola function and its detection significance was found to be $\sim$25$\sigma$. Moreover, a new gamma-ray point source located just outside the south-east region of the SNR's shell was detected with a significance of $\sim$6$\sigma$. We also investigated the archival HI and CO data and detected an expanding shell structure in the velocity range of $-10.5$ and $+1.8$ km s$^{-1}$ that is coinciding with a region of gamma-ray enhancement at the southern rim of the HB9 shell.