Revealing the nature of the QPO and its harmonic in GX 339-4 using frequency-resolved spectroscopy

We use frequency-resolved spectroscopy to examine the energy spectra of the prominent low-frequency quasi-periodic oscillation (QPO) and its harmonic in GX 339-4. We track the evolution of these spectra as the source makes a transition from a bright low/hard to hard intermediate state. In the hard/intermediate states, the QPO and time-averaged spectra are similar and the harmonic is either undetected or similar to the QPO. By contrast, in the softer states, the harmonic is dramatically softer than the QPO spectrum and the time-averaged spectrum, and the QPO spectrum is dramatically harder than the time-averaged spectrum. Clearly, the existence of these very different spectral shaped components mean that the time-averaged spectra are complex, as also seen by the fact that the softer spectra cannot be well described by a disc, Comptonization and its reflection. We use the frequency-resolved spectra to better constrain the model components, and find that the data are consistent with a time-averaged spectrum which has an additional low-temperature, optically thick Comptonization component. The harmonic can be described by this additional component alone, while the QPO spectrum is similar to that of the hard Comptonization and its reflection. Neither QPO nor harmonic shows signs of the disc component even when it is strong in the time-averaged spectrum. This adds to the growing evidence for inhomogeneous Comptonization in black hole binaries. While the similarity between the harmonic and QPO spectra in the intermediate state can be produced from the angular dependence of Compton scattering in a single region, this cannot explain the dramatic differences seen in the soft state. Instead, we propose that the soft Compton region is located predominantly above the disc while the hard Compton is from the hotter inner flow. Our results therefore point to multiple possible mechanisms for producing harmonic features in the power spectrum. The dominant mechanism in a given observation is likely a function of both inclination angle and inner disc radius.