This research has made use of SAXDAS linearized and cleaned event files (rev0) produced at the BeppoSAX Science Data Center. F. Fiore is warmly thanked for enlightenments concerning SAX instrumental properties and data analysis. Stimulating discussions with F. Brighenti, L. Ciotti, A. D'Ercole and G. Trinchieri are acknowledged. The referee, D. Buote, is also thanked for comments that improved the paper.
© European Southern Observatory (ESO) 1999
Online publication: March 1, 1999