Quasi-periodic oscillations in the bright galactic bulge X-ray source GX 340+0

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Abstract
GX 340 + 0 was observed with EXOSAT for 13 hr in 1985 and for 12 hr in 1986. During the 1985 observations the spectral state (in the spectral hardness–intensity diagram) varied between the horizontal branch and the upper end of the normal branch; no QPO were observed (however, the time resolution was only 31 ms). During the 1986 observations (the time resolution was 1 ms) the spectral suite corresponded to the normal branch and 5.6 Hz QPO were observed (the equivalent sinusoidal rms variation was about 1.8 per cent), but only when the spectral hardness was within a certain range which covered approximately the middle one-third of the normal branch. Very low frequency noise (VLFN) below 0.1 Hz was observed during both observations in all spectral states. The VLFN in the power density spectra could be well fitted by a power law (the slopes were different for the two observations). Red noise (above 0.5 Hz) was exclusively observed during the 1985 observations; its strength increased from ≲1.5 to 3 per cent as the spectral state changed from the normal to the horizontal branch. This behaviour is similar to that in Cyg X-2 and GX5 – 1 which also show approximately 5−6Hz QPO in the normal branch, red noise above 0.5 Hz (with associated high-frequency 20−50Hz intensity-dependent QPO) in the horizontal branch, and VLFN in both branches. It thus appears that GX340 + 0 is similar in its QPO behaviour to Cyg X-2 and GX5 − 1 (possibly also to GX 17 + 2), and we suspect that high-frequency QPO was present during our 1985 observations when the source was in the horizontal branch, but that our time resolution of 31 ms was insufficient to detect it. Future observations will show whether high-frequency QPO is present during the horizontal branch state as we suspect.

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