[1] Univ Southampton, Sch Phys & Astron, Southampton SO17 1BJ, Hants, England
[2] Harvard Smithsonian Ctr Astrophys, Cambridge, MA 02138 USA
[3] Univ Space Res Assoc, Xray Astrophys Lab, NASA, Goddard Space Flight Ctr, Greenbelt, MD 20771 USA
[4] Univ Warsaw Observ, PL-00478 Warsaw, Poland
[5] S African Astron Observ, ZA-7935 Cape Town, South Africa
[6] So African Large Telescope Fdn, ZA-7935 Cape Town, South Africa