Phase-resolved spectroscopy of Type B quasi-periodic oscillations in GX 339-4

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 normalization, but also in spectral shape. Using several different spectral models which parametrize 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°), 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 Comptonizing region. The small amplitude of blackbody variations suggests that the Comptonizing region producing the QPO has a relatively large scaleheight, 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.