Transient quasi-periodic oscillations in in the X-ray flux of Cygnus X-3

Abstract

Cygnus X-3, a bright X-ray source at a distance¹ of more than ∼12 kpc beyond the edge of the galactic plane, has peculiar properties including very strong radio outbursts², possibly showing jet-like features³, and emission in various γ-ray bands^4–6 (but see ref. 7). The discovery of its 4.8-h X-ray period⁸ showed that the source is probably a compact (∼10¹¹ cm) binary system, but its nature and that of its companion, the mode of energy generation (accretion or Crab-like pulsar) and also the cause of the X-ray modulation are still uncertain. We describe here X-ray data on Cyg X-3, obtained with EXOSAT, showing transient quasi-periodic oscillations with amplitudes between 5 and 20% of the 1–10-keV flux and periods in the range 50–1,500 s. The oscillations persisted typically for 5–40 (quasi-oscillation) cycles, occurred exclusively in the phase interval 0.0–0.75 (rising branch and top of the X-ray light curve), were seen during the high state and probably also during the low state of the source, and, at least once, were observed with two different periods simultaneously. We suggest that the quasi-periodic flux variations are caused by quasi-periodic phenomena in an accretion disk which partially occults the X-ray source. This tentative interpretation is in accordance with the idea⁹ that the 4.8-h modulation of the X-ray flux in Cyg X-3 arises by partial occultations of the X-ray source by an accretion disk, like in low-mass X-ray binaries such as 4U1822–37.