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   NED object is ABELL 2721; Reference is  1999ApJ...511...65J

   FIG. 2.Isointensity contour maps of 220 clusters observed with the Imaging
 Proportional Counter (IPC) on the are superposed on optical sky survey
 photographs. (For southern clusters, the photographs are from the European
 Southern Observatory survey). These show the X-ray emission from the 0.5-4.5
 keV energy band, binned in 16 arcsec X 16 arcsec cells and smoothed with a 32
 arcsec Gaussian profile. As described more fully in the text, the contour
 levels are determined in {sigma}levels above the background. Table 2 gives the
 surface brightness values in IPC counts s**(-1) arcmin**(-2), as well as
 {sigma}levels, for the contours in each cluster image.

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