Phase lags of quasi-periodic oscillations across source states in the low-mass X-ray binary 4U 1636-53

TL;DR

This study investigates how phase lags of quasi-periodic oscillations in the X-ray binary 4U 1636-53 depend on energy and frequency across different source states, providing insights into the physical conditions of the emitting medium.

Contribution

First comprehensive analysis of energy and frequency dependence of QPO phase lags in 4U 1636-53 across all detected frequencies and states.

Findings

Phase lags vary with energy and frequency.

Results support up-scattering Comptonization models.

Phase lag dependencies reveal medium size scales and physical conditions.

Abstract

While there are many dynamical mechanisms and models that try to explain the origin and phenomenology of the quasi-periodic oscillations (QPOs) seen in the X-ray light curves of low-mass X-ray binaries, few of them address how the radiative processes occurring in these extreme environments give rise to the rich set of variability features actually observed in these light curves. A step towards this end comes from the study of the energy and frequency dependence of the phase lags of these QPOs. Here we used a methodology that allowed us to study, for the first time, the dependence of the phase lags of all QPOs in the range of 1 Hz to 1300 Hz detected in the low-mass X-ray binary 4U 1636-53 upon energy and frequency as the source changes its states as it moves through the colour-colour diagram. Our results suggest that within the context of models of up-scattering Comptonization, the phase lags dependencies upon frequency and energy can be used to extract size scales and physical conditions of the medium that produces the lags.