Transition radiation from interstellar dust grains

The spectrum of transition radiation emitted by interstellar dust grains subject to bombardment by relativistic electrons is calculated in detail for grains for realistic sizes. The results of the calculation are valid for frequencies w, grain sizes a, and electron energies E such that wa/c 1, y = E/ 1, and w/w 1, where Wp is the plasma frequency of the material in the grain. Our results do not depend on the assumption used by previous authors, that the grains are thick relative to the length of the formation zone, a > y2c/w. Because of the small size of interstellar grains, the transition radiation is strongly suppressed at X-ray frequencies. We conclude that transition radiation is not a significant source of diffuse galactic X-rays, and that it can be important in astrophysical phenomena only under very special conditions. Subject headings: cosmic rays - interstellar matter - X-rays