Quasi-periodic oscillations in the X-ray flux of GX 3+1 (4U 1744-26).
van Paradijs, J.; van der Klis, M.; Lewin, W. H. G.; Hasinger, G.; Jansen, F.; Sztajno, M.; Langmeier, A.; Trumper, J.; Penninx, W. H.; Basinska, E. M.
United States, Netherlands, Germany
Abstract
GX 3+1, one of the brightest X-ray sources, was observed with EXOSAT in 1985 for 8.5 hr on March 24 - 25, and for 16 hr on September 4. During both observations the flux levels varied by about 20 per cent. The mean flux in September was about 8 per cent lower than in March. Throughout the March 24 - 25 observations, low-frequency noise (LFN) was observed below roughly 10 Hz, but no quasi-periodic oscillations (QPO) were detected. During the September 4 observations, LFN was again detected throughout, but this time QPO were also detected (with a mean frequency of about 8 Hz) during an approximately 3.3-hr period. QPO were only observed (but not always) when the spectral hardness was below a certain value. QPO were not observed in the so-called 'horizontal spectral branch'. It is possible that these QPO in GX 3+1 are similar in nature to the intensity-dependent 5.6-Hz QPO observed in Cyg X-2 which are also not observed in the horizontal branch.