Abstract. The XMM-Newton Observatory is a cornerstone mission of the European Space Agency's Horizon 2000 programme, and is the largest scientific satellite it has launched to date. This paper summarises the principal characteristics of the Observatory which are pertinent to scientific operations. The scientific results appearing in this issue have been enabled by the unprecedentedly large effective area of the three mirror modules, which are briefly described. The in-orbit performance and preliminary calibrations of the observatory are briefly summarised. The observations from the XMM-Newton calibration and performance verification phase, which are public and from which most papers in this issue have been derived, are listed. The flow of data from the spacecraft, through the ground segment, to the production of preliminary science products supplied to users is also discussed.

The Whipple Observatory 10 m reflector, operating as a 37 pixel camera, has been used to observe the Crab Nebula in TeV gamma rays. By selecting gamma-ray images based on their predicted properties, more than 98% of the background is rejected; a detection is reported at the 9.0 a level, corresponding to a flux of 1.8 x 10 11 photons cm 2 s 1 above 0.7 TeV (with a factor of 1.5 uncertainty in both flux and energy). Less than 25% of the observed flux is pulsed at the period of PSR 0531. There is no evidence for variability on time scales from months to years. Although continuum emission from the pulsar cannot be ruled out, it seems more likely that the observed flux comes from the hard Compton synchrotron spectrum of the nebula.

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