We thank the referees for several helpful comments and suggestions. We are grateful to Nail Inogamov and Nail Sibgatullin for useful discussions. This research has made use of data obtained through the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA/Goddard Space Flight Center. W. Forman and C. Jones acknowledge support from NASA contract NAS8-39073.
© European Southern Observatory (ESO) 2000
Online publication: April 17, 2000