MG acknowledges the receipt of an ESA Research Fellowship. The authors acknowledge discussions with F.Bocchino. This research has made use of the NASA/IPAC Extragalactic Database, which is operated by the Jet Propulsion Laboratory under contract with NASA, and of data obtained through the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA/Goddard Space Flight Center.
© European Southern Observatory (ESO) 2000
Online publication: March 17, 2000