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In X-ray astronomy, a hardness ratio is the equivalent of a photometric color index, and is calculated as a normalized difference of the exposure corrected counts in two energy bands A (high energy), B (low energy). A typical definition is HR = ( A - B ) / ( A + B ) but other schemes are also used.

The Chandra Source Catalog uses 3 wide energy bands and uses the sum of all three in the denomintor of the above equation.

For a discussion of different definitions of hardness ratio and their biases in low counts regimes, users can consult this paper by Park et al.

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