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X-rays offer a reliable method to identify Active Galactic Nuclei (AGN).However, in the high-redshift Universe X-ray AGN are poorly sampled due totheir relatively low space density and the small areas covered by X-raysurveys. In addition to wide-area X-ray surveys, it is important to have deepoptical data in order to locate the optical counterparts and determine theirredshifts. In this work, we build a high-redshift ($z\geqslant3.5$) X-rayselected AGN sample in the XMM-XXL northern field using the most updated [0.5-2keV] catalogue along with a plethora of new spectroscopic and multi-wavelengthcatalogues, including the deep optical Subaru Hyper Suprime-Cam (HSC) datareaching magnitude limits i$\sim$26 mag. We select all the spectroscopicallyconfirmed AGN and complement this sample with high-redshift candidates that areHSC g- and r-band dropouts. To confirm the dropouts, we derive theirphotometric redshifts using spectral energy distribution techniques. We end upwith a sample of 54 high-z sources (28 with spec-z), the largest in this fieldso far (almost three times larger than in previous studies), and we estimatethe possible contamination and completeness. We calculate the number counts(logN-logS) in different redshift bins and compare our results with previousstudies and models. We provide the strongest high-redshift AGN constraints yetat bright fluxes ($\rm f_{0.5-2~keV} > 10^{-15} erg~s^{-1}~cm^{-2}$). Thesamples of $z\geqslant3.5$, $z\geqslant4$ and $z\geqslant5$ are in agreementwith an exponential decline model similar to that witnessed at opticalwavelengths. Our work emphasizes the importance of using wide area X-raysurveys with deep optical data to uncover high redshift AGN.

astronomy and astrophysics

XXL-HSC: An updated catalogue of high-redshift ($z\geqslant3.5$) X-ray  AGN in the XMM-XXL northern field. Constraints on the bright end of the soft  logN-logS