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

Abstract

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 existance 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, Comptonisation 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 Comptonisation component. The harmonic can be described by this additional component alone, while the QPO spectrum is similar to that of the hard Comptonisation and its reflection. Neither QPO nor harmonic show signs of the disc component even when it is strong in the time averaged spectrum. This adds to the growing evidence for inhomogeneous Comptonisation 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.

X-Ray Quasi-periodic Oscillations in the Lense–Thirring Precession Model. II. Variability of the Relativistic Iron Kα Line

“…In each panel, the blue arrow at the center represents the projection of the corona axis J C on the disk plane. Animated versions of these plots can be viewed at and downloaded from http://202.127.29.4/AGN/beiyou/QPO/index_QPO.html for the observed harmonic in the power spectrum of BHXRB (Axelsson et al 2014;Axelsson & Done 2016;Huang et al2018;de Ruiter et al 2019;Xiao et al 2019;Stevens & Uttley 2016;Stevens et al 2018), which will be further studied in the next paper.…”

A Study of Low-mass X-Ray Binaries in the Low-luminosity Regime

“…In a recent study, Axelsson & Done (2016), using frequency-resolved spectra suggest further evidence for two major Comptonization regions in BH binaries i.e., a soft one, the origin of which is associated with the disk, and a hard one which is apparently independent of the disk. They further suggest that both are likely to be inhomogeneous.…”

A review of quasi-periodic oscillations from black hole X-ray binaries: Observation and theory

“…The second harmonic, in contrast, is significantly softer than the time-averaged spectrum. A similar analysis of GX 339-4 [12] additionally revealed that the rms spectrum of the second harmonic can be described by a low-temperature, optically thick Comptonized component that is not required to be present in the rms spectrum of the fundamental, which instead includes a higher temperature Comptonized component. The authors suggested that these results could be explained by precession of a stratified inner accretion flow, which is hotter closer to the BH.…”

“…We can understand that these two equations have wave-like solutions forl, therefore explaining the name 'wave-like regime', by exploring the invicid (α = 0), Keplerian (Ω φ = Ω r = Ω z ) limit. In this case, equations (11) and (12) can be combined to find…”