Why we need AXAF - the new X-ray astronomy observatory

X-ray astronomy using space-based instruments is the major source of data on violent, energetic events underlying the evolution of the universe. X-ray observations permit studying supernova remnants, quasars and ultracompact objects (neutron stars and black holes). Einstein Observatory data permitted measuring the energy emitted by quasars and calculations of the mass of clusters of galaxies, the latter results being strong evidence that 90 percent of the mass needed for the clusters to be gravitationally stable is not being seen. Identification of the characteristics of the dark matter, and the consequent implications for a closed or open universe, is in large part dependent on the launch and operation of the Advanced X-ray Astrophysics Facility by NASA, to be serviced from the Space Station over a 15 yr operational lifetime