Discovery of Soft X-Ray Emission from Io, Europa, and the Io Plasma Torus.
R. F. Elsner, G. R. Gladstone, J. H. Waite, F. J. Crary, R. R. Howell, R. E. Johnson, P. G. Ford, A. E. Metzger, K. C. Hurley, E. D. Feigelson, G. P. Garmire, A. Bhardwaj, D. C. Grodent, T. Majeed, A. F. Tennant, and M. C Weisskopf
Astrophysical Journal In Press
We report the discovery of soft (0.25 - 2 keV) x-ray emission from the Galilean satellites Io and Europa, probably Ganymede, and from the Io Plasma Torus (IPT). Bombardment by energetic (> 10 keV) H, O, and S ions from the region of the IPT seems the likely source of the x-ray emission from the Galilean satellites. According to our estimates, fluorescent x-ray emission excited by solar x-rays, even during flares from the active Sun, charge-exchange processes, previously invoked to explain Jupiter's x-ray aurora and cometary x-ray emission, and ion stripping by dust grains fail to account for the observed emission. On the other hand, bremsstrahlung emission of soft X-rays from non-thermal electrons in the few hundred to few thousand eV range may account for a substantial fraction of the observed x-ray flux from the IPT.