We show that interactions of fast particles with the boundary shell of the local hot bubble could make an important contribution to the 0.5-1 keV diffuse X-ray background observed with ROSAT. The bulk of these nonthermal X-rays are due to line emission from fast O ions of energies around 1 MeV nucleon-1. The unequivocal signature of lines from de-excitations in fast ions is their large width (δE/E0.1 for O lines), which distinguishes them from X-ray lines produced in a hot plasma. If the 0.5-1 keV nonthermal X-rays are produced by accelerated particles with a sufficiently hard energy spectrum, the gamma-ray line emission produced in the boundary shell of the local hot bubble could account for the recently reported COMPTEL observations of nuclear gamma-ray lines from a broad region toward the Galactic center.
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