...(Haberl et al. 2006)

http://www.mpe.mpg.de/xray/research/normal_galaxies/index.php

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... earlier

http://cfa-www.harvard.edu/iau/CBAT_M31.html#2006-11a

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... CCD images

http://cfa-www.harvard.edu/iau/CBAT_M31.html#2006-11a

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... type

http://mintaka.sdsu.edu/faculty/shafter/extragalactic_novae/HET/index.html

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...Williams et al. (2006)

WSR = (WM-WS)/(WM+WS) and WHR = (WH-WS)/(WH+WS), where WS, WM and WH are the source counts in the 0.3-1.0, 1.0-2.0 and 2.0-7.0 keV band, respectively. Note that these ratios are different from the hardness ratios defined in Sect. 3.1

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