Phase-Resolved Spectroscopy of Type B Quasi-Periodic Oscillations in GX 339-4

TL;DR

This study introduces a phase-resolved spectral-timing method to analyze Type B QPOs in GX 339-4, revealing spectral shape changes and phase relationships that support a geometric model involving a precessing Comptonising region.

Contribution

The paper presents a novel phase-resolved spectroscopy technique applied to black hole QPOs, demonstrating spectral component variations and phase lags that support a geometric interpretation.

Findings

Both blackbody and power-law components vary during the QPO.

Blackbody variation leads the power-law by ~0.3 in phase.

Results support a precessing Comptonising region model.

Abstract

We present a new spectral-timing technique for phase-resolved spectroscopy and apply it to the low-frequency Type B quasi-periodic oscillation (QPO) from the black hole X-ray binary GX 339-4. We show that on the QPO time-scale the spectrum changes not only in normalisation, but also in spectral shape. Using several different spectral models which parameterise the blackbody and power-law components seen in the time-averaged spectrum, we find that both components are required to vary, although the fractional rms amplitude of blackbody emission is small, ~1.4 per cent compared to ~25 per cent for the power-law emission. However the blackbody variation leads the power-law variation by ~0.3 in relative phase (~110 degrees), giving a significant break in the Fourier lag-energy spectrum that our phase-resolved spectral models are able to reproduce. Our results support a geometric interpretation for the QPO variations where the blackbody variation and its phase relation to the power-law are explained by quasi-periodic heating of the approaching and receding sides of the disc by a precessing Comptonising region. The small amplitude of blackbody variations suggests that the Comptonising region producing the QPO has a relatively large scale-height, and may be linked to the base of the jet, as has previously been suggested to explain the binary orbit inclination-dependence of Type B QPO amplitudes.