Involved people at OAB: Comastri, Ciliegi, Mignoli, Vignali.
Bologna Observatory collaborates with the observatories of Arcetri
(R. Maiolino), Rome (F. Fiore), the Third University of Rome (G. Matt,
F. La Franca) and with C. Vignali (University of Bologna) in the analysis of a large sample
of hard X-ray selected sources
discovered by BeppoSAX in the 5--10 keV energy range. The survey
(dubbed ``High Energy Llarge Area Survey'', HELLAS) has now been completed.
The final sample consist of 147 sources detected down to a flux limit
of 5 
erg cm
s
where about 20--30 % of the hard
X--ray background is resolved. A sizeable fraction of the HELLAS sources
are characterized by hard spectra, as can be judged from the hardness ratio
X--ray analysis, suggesting the presence of intrinsic absorption.
The number counts and absorption distribution are in substantial
agreement with AGN synthesis model predictions (Comastri et al. 1999).
In order to better understand the nature of the hard X--ray sources
several multiwavelength (radio, sub--mm, optical and near infrared)
follow--up programs have been or are being carried out.
The most interesting observational results indicate that the optical and X--ray properties of X--ray obscured AGN are different from what expected (i.e. narrow line type--2 AGN). Even though the presence of an obscured AGN is well established by X--ray observations, the optical morphology, brightness profiles and photometric colors in various bands are similar to those of non-active galaxies (Vignali et al. 2000; Maiolino et al. 2000). Moreover the optical counterparts of a few hard absorbed sources have been identified with broad--line ``blue" quasars. As a result the hard X-ray population contributing to the XRB appears to be characterized by a wider range of X-ray and optical properties than previously thought, and that other parameters, besides the nuclear absorption, are required for a complete description of its properties. The Chandra discovery of faint hard X--ray sources associated with optically faint and non-active galaxies (Mushotzky et al. 2000) has confirmed and extended the scenario above described.
A 5 GHz survey has been obtained for the HELLAS sources with declination lower than -40 deg in June 1999 using the Australia Telescope Compact Array (ATCA). The results obtained with the ATCA observations show that a very high fraction of the HELLAS sources show radio emission (8 of the 20 HELLAS sources have a radio counterpart). Vice-versa, radio follow-up of soft (0.5-2 keV) X-ray selected samples has shown different percentages of radio/X-ray associations (about 10 per cents). In order to check if this high percentage of radio/X-ray associations will be confirmed also for large sample of hard X-ray sources and for very faint sources selected with the Chandra and XMM satellite, VLA radio observations of about 100 HELLAS sources northern than DEC -40 deg and radio (with ATCA) follow-up of two Chandra deep fields will be carried out during the year 2000. If the nature of the hard X-ray selected sources is really different from that of soft X-ray selected AGN (as supposed by different authors), the radio observations will provide a useful tool to better understand the nature of these objects and than to solve the hard X-ray background problem.