Phase-resolved spectroscopy of the lower kHz QPO in 4U1608-52

Kilohertz quasi-periodic oscillations (kHz QPOs) are the most rapid (quasi-)coherent kind of variability that have been detected in the light curves of accreting neutron star X-ray binaries. Previous spectral-timing work using the rms spectrum revealed that the QPO emission is a Comptonized blackbody, consistent with that expected from the boundary layer between the accretion flow and neutron star surface. Furthermore, the lag-energy spectra and covariance spectra indicate that the lower kHz QPO arises from a more complex energy-dependent variability than just an overall modulation in the flux normalization. To better interpret the spectral variability, we present phase-resolved spectroscopy of a kHz QPO for the first time, using a method based on the energy-dependent cross-correlation function (CCF). The best-fitting spectral parameterisation requires the power-law index, high-energy cut-off temperature and seed blackbody temperature of the Comptonized emission to vary with QPO phase. Additionally, the variations in these three parameters show small but non-zero phase differences, which together can explain the previously observed lag-energy dependence. We suggest that these spectral variations could be explained by a "breathing" oscillation in the Comptonizing boundary layer; this possibility can be explored in greater detail with current and future X-ray missions such as AstroSat, NICER, eXTP, and STROBE-X.