Quasi-periodic oscillations in the X-ray flux of GX 3+1 (4U 1744-26)

The X-ray source GX 3+1 was observed with EXOSAT in 1985 for 8.5 hr on March 24 to 25, and for 16 hr on September 4. During both observations the flux levels vary by 20%. The mean flux in September is 8% lower than in March. Throughout the March 24 to 25 observations, low-frequency noise (LFN) is observed below 10 Hz, but no quasi-periodic oscillations (QPO) are detected. During the September 4 observations, LFN is detected throughout, but QPO are also detected (with a mean frequency of 8 Hz) during a 3.3 hr period. The strength of the QPO is 3%. The LFN strength increases from 2% to 6% for photon energies increasing from 2 to 10 keV. There appears to be no correlation between the occurrence of QPO and source intensity, however, QPO are only observed (but not always) when the spectral hardness is below a certain value. The QPO are not observed in the horizontal spectral branch.