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 nonequilibrium ionization and a power-law function. The source distances are estimated to be 1-8 kpc, using the best-fit N-H values under the assumption that the mean density in the line of sight is 1 H cm(-3). The source sizes and luminosities are then 4.5-27 pc and (0.8-23) x 10(33) ergs s(-1). Although the source sizes are typical for supernova remnants (SNR) with young-to-intermediate ages, the X-ray 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.