tokens: between the two populations is clear: RQ AGNs occupy only the top-right region of the plot, meaning that they have a weak and steep X-ray emission. On the other hand, most blazars have a stronger and flat emission and they are located in the bottom left-hand region. Based on this distinction, we set two thresholds to differentiate blazars and non-blazar AGNs. In particular, as shown in Fig. 5 , we adopt the values = 1.8 and = 1.355 as thresholds to separate the two populations. These limits include all the confirmed high-z blazars. We then apply these criteria to the high-z AGN in the CLASS sample (Fig. 6 ). In particular, we plot here the 14 objects with a reasonable estimate of the photon index (error 0.4). The remaining 10 will be discussed further below. The sources of our sample which have been already confirmed as blazars in the literature are highlighted by a black circle. In addition, in the plot it is also included the classification as blazar/non-blazars (flat/peaked) based on the radio spectrum and discussed in C19 (blue points = peaked, red squares = flat, and purple diamonds = uncertain). Figure 5. Photon index as a function of the index for the comparison samples. In orange we report the BZCAT blazars observed by Swift-XRT , with red squares the few confirmed blazars at high-z and with blue points the RQ AGNs at high-z. The red star represent the z = 5 blazar DESJ014132.4-542749.9, Belladitta et al. ( 2019 ). The continuous black line represents the dependence of the photon index with a variation of the X-ray intensity () of the jet due to beaming effects considering a coronal emission with = 1.55 and = 2, while for the jet emission we assume = 1.6. The black cross represents the critical angle assuming that the jet seen at θ = 0 is 50 times more intense than the corona at 10 keV. The plot is divided in four areas by a vertical line at = 1.355 and a horizontal one at = 1.8, which correspond to the thresholds assumed for the classification. Figure 6. Photon index as a function of the for the high-z objects in the CLASS sample with an error on the photon index lower than 0.4. The objects are plotted with different colours and markers depending on the radio spectral classification reported in C19 (‘flat’ i.e. good blazar candidates, ‘peaked’ i.e. probably non-blazars and ‘uncertain’) and the black circles represent the few confirmed blazars at z > 4 in the literature. As in the previous figure, the plot is divided in four areas by a vertical line at = 1.355 and a horizontal one at = 1.8. Considering the objects plotted in Fig. 6 , there is only one source whose and are not consistent with a blazar nature (GB6J012126+034646), meaning that its X-ray spectrum is too steep and too weak to be produced by an oriented jet (grey region). On the other hand, as expected, the majority of the sources (13) are in the bottom left-hand region (green), meaning that they are strong and flat enough to suggest that they are bona fide blazars.
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