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X

X-band

A band of radio frequencies extending from 5200 to 10 900 MHz

XEUS mission

Future global X-ray astronomy mission. See http://sci.esa.int/xeus/

XMM-Newton (X-ray Multi-Mirror) mission

ESA's X-ray space observatory mission, with its X-ray Multi-Module design using three telescopes each with 58 nested X-ray mirrors. Named also in honour of Sir Isaac Newton. See http://sci.esa.int/xmm-newton/

X-ray

Electromagnetic radiation with wavelengths between those of ultraviolet and gamma rays, approximately 0.01-10 nm. At these short wavelengths, it is more common to talk in terms of photon energies. These energies range from 0.1-100 keV.

X-ray (hard)

Higher-energy part of the X-ray spectrum ranging from approximately 5 keV to 100 keV.

X-ray (soft)

Band of low energy X-rays, between 0.1 keV and approximately 5 keV.

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