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3.1.5. X-ray

Like the radio, the X-ray luminosity is but a small fraction of the total bolometric luminosity of all starbursts and most AGN, albeit usually a few orders of magnitude in nuLnu larger than for the radio, but still several orders of magnitude less than the infrared luminosity of ULIGs. It is not clear that the observed X-ray spectrum can directly be related to the bolometric luminosity of ULIGs. However, by analogy with the mean X-ray properties of QSOs or starbursts, it may at least be possible to say whether a QSO-like X-ray nucleus is present.

The 2-10 keV luminosity of all five ULIGs in Table 2 is weak relative to the strong far-infrared emission (e.g. Nakagawa et al. 1999); however, the X-ray luminosity of ULIGs is not weak in relation to RQQSOs when comparing the X-ray luminosity to the luminosity in the optical (e.g. Ogasaka et al. 1997; Iwasawa 1999; Turner 1999). Strong absorption appears to affect both the optical/UV and soft X-rays in ULIGs, as might have been expected.

Absorption effects are minimized by considering only the hard X-ray emission, LHX ident L(5-10 keV), detected with ASCA. Three of the five ULIGs were detected in hard X-rays, all at a level consistent with the mean hard X-ray luminosity observed for RQQSOs [Note: Mrk 231 is compared here to the mean properties of BALQSOs which are somewhat weaker in their observed LHX than other RQQSOs (Turner 1999).]