Discovery of a 283-second periodic variation in the X-ray source 3U 0900--40

A 283 s periodic pulsation in the X-ray system 3U 0900--40 has been discovered during observations by the SAS-3 X-ray observatory. Pulse profiles of the 283 s periodicity are presented in five energy intervals covering the range 1--30 keV for the period 1975 July 19.4--23.9. The averaged profile is relatively simple at higher energies and is markedly more complex at lower energies. The peak 1--40 keV intensity observed for the source is 1.2 x 10-8 ergs cm-2 s-1, which corresponds to a luminosity of 2.1 x 1036 ergs s-1 at a distance of 1.2 kpc. A search for soft X-ray emission (E<1 keV) yielded upper limits of 2 x 10-11 and 5 x 10-11 ergs cm-2 s-1 in the energy intervals 0.16--0.28 keV and 0.5--0.7 keV, respectively