BeppoSAX Observations of Markarian 501 in June 1999
  We present the preliminary results of a long BeppoSAX observation of the BL Lac object Mkn501 carried out in June 1999. The source was fainter than found during the BeppoSAX pointings of 1997 and 1998, but is still detected with a good signal-to-noise ratio up to ~40 keV. The X-ray spectrum in the energy range 0.1-40 keV, produced through synchrotron radiation, is steeper than in the previous years, it is clearly curved, and peaks (in nu*F_nu) at ~0.5 keV. This energy is much lower than those at which the synchrotron component was found to peak in 1997 and 1998. Some intraday variability suggests that activity of the source on small time scales accompanies the large long time scale changes of brightness and spectrum. 
E. Pian, L. Chiappetti, P. Giommi, F. Tavecchio, L. Maraschi, E.   Palazzi, F. Aharonian, M. Catanese, A. Celotti, B. Degran...