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We report the X‐ray analysis of a sample of candidate high‐redshift ( 1 &lt;  z  &lt; 2 ) Fanaroff–Riley type I (FR I) sources from the sample of Chiaberge et al., observed in the  Chandra  COSMOS field (C‐COSMOS). Our main goals are to study their nuclear properties by means of unresolved X‐ray emission, and to constrain the presence of clusters surrounding the FR I sources from the diffuse X‐ray emission by the associated hot plasma. Among 19 FR I candidates, six have an X‐ray‐unresolved counterpart in the C‐COSMOS catalogue. Two additional sources are not present in the C‐COSMOS catalogue but are clearly detected in the  Chandra  images. X‐ray spectral analysis, when possible, or hardness ratio of the stacked emission from X‐ray‐detected sources, suggests that some of them have significant intrinsic absorption (  N  H ≳ 10 22   cm  −2  ), and high X‐ray luminosities with respect to local FR Is. From the stacking analysis of the 11 non‐detected sources, however, we find evidence for unresolved soft X‐ray emission and no detected hard emission, suggesting an unabsorbed spectrum. Therefore, the X‐ray properties vary significantly from source to source among these FR I candidates. From the analysis of the stacked images of all 19 FR I candidates we can rule out the presence of virialized haloes with temperatures larger than 2–3 keV; however, the upper limit on the average extended emission is still consistent with the presence of  ∼ 1–2 keV hot gas.

The X‐ray properties of high‐ z Fanaroff–Riley type I candidates in the COSMOS field