We report on Chandra observations of the three quasars SDSSp J083643+005453, SDSSp J103027+052455, and SDSSp J130608+035626 at redshifts 5.82, 6.28, and 5.99, respectively. All three sources are clearly detected in the X-ray band, up to rest-frame energies of ~55 keV. These observations demonstrate the unprecedented sensitivity of Chandra to detect faint sources in relatively short exposure times (5.7-8.2 ks). The broadband X-ray properties of these highest redshift quasars do not appear to be any different from their lower redshift cousins, implying little or no evolution for these few sources out to z ~ 6, at redshifts where the optical space density is falling. Spectra of the sources could not be determined with only a few counts detected. Observations with XMM-Newton will be able to constrain the spectral shapes, if they are simple. Determination of complex spectra in a reasonable amount of time, however, will have to await the next generation of X-ray missions.
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Mathur et al. (2002) studied this question.