  The 500ks Chandra ACIS-I observation of the field around the $z=6.31$ quasar
SDSS J1030+0524 is currently the 5th deepest extragalactic X-ray survey. The
rich multi-band coverage of the field allowed for an effective identification
and redshift determination of the X-ray source counterparts: to date a catalog
of 243 extragalactic X-ray sources with either a spectroscopic or photometric
redshift estimate in the range $z\approx0-6$ is available over a 355 arcmin$^2$
area. Given its depth and the multi-band information, this catalog is an
excellent resource to investigate X-ray spectral properties of distant Active
Galactic Nuclei (AGN) and derive the redshift evolution of their obscuration.
We performed a thorough X-ray spectral analysis for each object in the sample,
measuring its nuclear column density $N_{\rm H}$ and intrinsic (de-absorbed)
2-10 keV rest-frame luminosity, $L_{2-10}$. Whenever possible, we also used the
presence of the Fe K$_\alpha$ emission line to improve the photometric redshift
estimates. We measured the fractions of AGN hidden by column densities in
excess of $10^{22}$ and $10^{23}$cm$^{-2}$ ($f_{22}$ and $f_{23}$,
respectively) as a function of $L_{2-10}$ and redshift, and corrected for
selection effects to recover the intrinsic obscured fractions. At $z\sim 1.2$,
we found $f_{22}\sim0.7-0.8$ and $f_{23}\sim0.5-0.6$, respectively, in broad
agreement with the results from other X-ray surveys. No significant variations
with X-ray luminosity were found within the limited luminosity range probed by
our sample (log$L_{2-10}\sim 42.8-44.3$). When focusing on luminous AGN with
log$L_{2-10}\sim44$ to maximize the sample completeness up to large
cosmological distances, we did not observe any significant change in $f_{22}$
or $f_{23}$ over the redshift range $z\sim0.8-3$. Nonetheless, the obscured
fractions we measure are significantly higher than ...
