Quasi-periodic oscillations in the Z source GX 340 + 0

Abstract

We have made X-ray observations of the Z source GX 340 + 0 with the Ginga Observatory and found the source in three spectral states: the flaring branch (FB), the normal branch (NB) and the horizontal branch (HB). In accordance with previous observations, quasi-periodic oscillations (QPO) with frequencies ~ 32–50 Hz were observed in the HB and near 6 Hz in a portion of the NB. In the HB the power spectra also showed low-frequency noise (LFN). The fractional rms variation of both types of QPO and of the LFN were strongly dependent on the X-ray energy band. Rough values for all three were ~ 2 per cent at ~ 3 keV and ~ 8 per cent at ~ 12 keV. We observed no QPO or LFN in the FB, and no LFN or high-frequency QPO in the NB. Very low-frequency noise (below ~ 1 Hz) was present in all three spectral branches; it increased in strength as the source moved ‘up’ the flaring branch. Our results show that GX 340 + 0 is a clear example of a Z-type source. The X-ray flux of GX 340 + 0 above 14 keV decreases as one moves ‘up’ the flaring branch. This has not previously been reported for Z sources. From a mutual comparison of overall properties of the six Z sources we confirm that GX 340 + 0 is most similar to Cyg X-2.