We thank J. Timmer for his assistance concerning the LSSM analysis method and C. Gantert for writing the LSSM code which has been kindly provided from the "Freiburger Zentrum für Datenanalyse und Modellbildung". We thank M. Nowak and J. Dove for useful discussions as well as S. Kitamoto, the referee, for helpful comments. This research has made use of data obtained through the HEASARC Online Service, provided by NASA-GSFC.
© European Southern Observatory (ESO) 1998
Online publication: May 12, 1998