Discovery of Kilohertz Quasi-periodic Oscillations in the Z source GX 340+0

Open Access
Publisher Website
Google Scholar
Abstract
We have discovered two simultaneous kHz quasi-periodic oscillations (QPOs) in the Z source GX 340+0 with the Rossi X-Ray Timing Explorer. The X-ray hardness-intensity and color-color diagrams each show a full Z track, with an extra limb branching off the flaring branch of the Z. Both peaks moved to higher frequencies when the mass accretion rate increased. The two peaks moved from 247±6 and 567±39 Hz at the left end of the horizontal branch to 625±18 and 820±19 Hz at its right end. The higher frequency peak's rms amplitude (5-60 keV) and FWHM decreased from ~5% and 383±135 Hz to ~2% and 145±62 Hz, respectively. The rms amplitude and FWHM of the lower peak were consistent with being constant near 2.5% and 100 Hz. The kHz QPO separation was consistent with being constant at 325±10 Hz. Simultaneous with the kHz QPOs, we detected the horizontal-branch oscillation (HBO) and its second harmonic, at frequencies between 20 and 50 Hz, and 38 and 69 Hz, respectively. The normal-branch oscillations were only detected on the upper and middle normal branch and became undetectable on the lower normal branch. The HBO frequencies do not fall within the range predicted for Lense-Thirring precession, unless either the ratio of the neutron star moment of inertia to neutron star mass is at least 4, 1045g cm2 M, the frequencies of the HBOs are in fact the second harmonic oscillations, or the observed kHz peak difference is half the spin frequency and not the spin frequency. During a 1.2 day gap between two observations, the Z track in the hardness-intensity diagram moved to higher count rates by about 3.5%. Comparing data before and after this shift, we find that the HBO properties are determined by position on the Z track and not directly by count rate or X-ray colors.
All Related Versions