...(Vikhlinin et al. 1998)

http://hea-www.harvard.edu/saord/zhtools/

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... or

We adopted the DIRBE bandwidths from http://lambda.gsfc.nasa.gov/product/cobe/about_dirbe.cfm.

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... depth

Although counts of extragalactic sources exhibit small field-to-field variations due to the large-scale structure of the Universe (see e.g. Cappelluti et al. 2007; Voss & Gilfanov 2006).

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