6 Conclusions

As expected, the majority (25 out of 29 or 86%) of the X-ray sources identified in this sample are AGN. These are divided into 25% Narrow-line AGN and 75% Broad-Line AGN. Narrow-line AGNs have some hints of photoelectric absorption with respect to the BLAGN population, but the effect is less than $\sim$$2\sigma$ significant in our small sample. Inferred columns for narrow-line AGNs at $z\sim 0.4$would be around $\sim$ $5\times 10^{22}\, {\rm cm}^{-2}$.

Perhaps the most remarkable finding is that 2 out of 24 AGN are BAL QSOs, i.e., $\sim$$10\%$. This is similar to the fraction of BAL QSOs in optically selected samples. The X-ray spectrum of these BAL QSOs does not show evidence for photoelectric absorption, and the limits we derive for the intrinsic absorption column (< $10^{22}\, {\rm cm}^{-2}$) tell us that these are unusual objects among the BAL QSO population in their X-ray spectra.

The AXIS high-galactic latitude medium survey is now progressing with $\sim$250 X-ray sources spectroscopically identified. Large numbers of sources are needed, not only to study in detail the overall properties of the dominant source population (AGN in this case), but also to find and characterize the rarer populations, e.g., BAL QSOs or optically dull galaxies (Mushotzky et al. 2000; Baldi et al. 2001).

Acknowledgements

We thank Alberto Fernández-Soto for help with the photometric redshift techniques and the referee Ioannis Georgantopoluos for interesting suggestions. We are grateful to the CCI of the observatories of the Canary Islands for a generous allocation of telescope time, through the International Time Programme scheme. The INT/WHT and TNG telescopes are operated on the island of La Palma by the Isaac Newton Group and the Centro Galileo Galilei respectively in the Spanish Observatorio del Roque de Los Muchachos of the Instituto de Astrofísica de Canarias. XB, FJC, MTC and SM acknowledge financial support by the Spanish MCYT under project AYA2000-1690. RDC and TM acknowledge partial financial support by the Italian Space Agency (ASI) and by the MURST (Cofin00-32-36). This project was supported by the DLR under grants 50 OR 9908 0 (GPS) and 50 OX 9801 3 (HB).