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The letter z has many definitions in X-ray astronomy. The intended one must be deduced from the context in which it appears:

• The coordinate along the optical axis of Chandra. A standard NASA convention used in many missions.

• The height above the plane of a galaxy, accretion disk or other flattened distribution.

• The fractional abundance of "metals" (i.e. elements heavier than helium).

• The redshift. When due to expansion of the Universe, generally used to give distance.

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