Diffuse Hard X-ray Sources Discovered with the ASCA Galactic Plane Survey

We found diffuse hard X-ray sources, G11.0+0.0, G25.5+0.0, and G26.6−0.1 in the ASCA Galactic plane survey data. The X-ray spectra are featureless with no emission line, and are fitted with both models of a thin thermal plasma in non-equilibrium ionization and a power-law function. The source distances are estimated to be 1–8 kpc, using the best-fit NH values on the assumption that the mean density in the line of sight is 1 H cm. The source sizes and luminosities are then 4.5–27 pc and (0.8–23)×10 ergs s. Although the source sizes are typical to supernova remnants (SNR) with young to intermediate ages, the Xray luminosity, plasma temperature, and weak emission lines in the spectra are all unusual. This suggests that these objects are either shell-like SNRs dominated by X-ray synchrotron emission, like SN 1006, or, alternatively, plerionic SNRs. The total number of these classes of SNRs in our Galaxy is also estimated. Subject headings: acceleration of particles — supernova remnants: individual (G11.0+0.0, G25.5+0.0, G26.6−0.1) — X-rays: ISM