tokens: counts and fluxes from the eregionanalyse task. They have less than 35 counts in the full band; for three of them, we even only get upper limits. The EPIC net source counts and fluxes in the full (0.2–10 keV) energy range (0.2–0.5, 0.5–2.0, 2.0–5.0, and 5.0–10.0 keV energy ranges obtained for these quasars) are summarized in Table 3 . The full-band XMM 100 arcsec EPIC mosaics of these six quasars are shown in Fig. 1 (rows 2 and 3). Table 3. X-ray net counts and fluxes for the new z > 6.4 X-ray quasars. Note . The upper limits quoted in this table correspond to the 3σ upper limits and are used only if the net counts or fluxes are smaller than their 3σ error. But only the quasars with a given value in the ‘Det.’ column have an X-ray detection. For the two X-ray detected quasars in this work, the source counts and fluxes presented in Tables 2 and 3 are from the edetect_chain task. 4 DISCUSSION For the full sample of nine quasars, we derived a set of optical and X-ray properties (see Table 4 ): The absolute magnitude at the rest-frame wavelength of λ = 1450 Å ( M 1450 ) was computed by extrapolating the J -band magnitude to the monochromatic magnitude at ( m 1450 ) assuming a power-law fit of the continuum in the UV–optical with α ν = −0.3 (Selsing et al. 2016 ). The monochromatic flux at the rest-frame wavelength of () was derived from m 1450 assuming also a power-law slope of α ν = −0.3. The X-ray luminosity in the rest-frame 2–10 keV band was computed from the count rate obtained by the eregionanalyse task on the 2–10 keV image (as done in Section 2 for the other energy ranges) and then K -correction was applied assuming a photon index Γ = 1.9. For the two detected quasars VDES J0224−4711 and PSO J159.2257−02.5438, the flux was obtained directly from their X-ray spectrum. Upper limits are used if the luminosity is smaller than its 3σ error (which is the case for all the non-detected sources) and are at the 3σ level. The hardness ratio is defined as HR = ( H − S )/( H + S ), where S and H are the number of counts in the soft (0.5–2.0 keV) and hard (2.0–5.0 keV) bands, respectively. The optical-to-X-ray slope (α ox ) is defined as where is the monochromatic flux at the rest-frame wavelength of as defined above, and the rest-frame monochromatic flux at 2 keV () was inferred from the flux in the 0.2–10 keV range assuming a photon index Γ = 1.9. Upper limits for α ox are used if the flux at 2 keV is smaller than its 3σ error. Table 4. Summary of X-ray and optical properties of the DES and new z > 6.4 X-ray quasars. Note . a Reminder: VDES J0020−3653 and PSO J036+03 have a measurement of α ox with errors because their 2 keV flux (from the 0.2–10 keV flux) is at a 3σ significance, but they are not significantly (>4σ) detected as VDES J0224−4711 and PSO J159−02. We used the compilation of X-ray observations of quasars at z > 5.5 by Nanni et al. ( 2017 ) plus the new analysis by Nanni et al. ( 2018 ) for SDSS J1030+0524 ( z = 6.28) and the observation
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