The Comptonising medium of the neutron star in 4U 1636-53 through its lower kilohertz Quasi-Periodic Oscillations

TL;DR

This paper models the Comptonising medium around neutron stars in LMXBs to explain the energy dependence of kHz QPOs, revealing its significant role in their radiative properties and providing a method to probe its geometry and feedback mechanisms.

Contribution

The study introduces a new model predicting the energy dependence of kHz QPOs and identifies the medium's extent, feedback effects, and dominant influence of electron temperature on variability.

Findings

A 1-8 km Comptonising medium is necessary to fit the data.

Predicted time delay between NS temperature and electron temperature oscillations.

Variability is mainly driven by oscillating electron temperature.

Abstract

Inverse Compton scattering dominates the high energy part of the spectra of neutron star (NS) low mass X-ray binaries (LMXBs). It has been proposed that inverse Compton scattering also drives the radiative properties of kilohertz quasi periodic oscillations (kHz QPOs). In this work, we construct a model that predicts the energy dependence of the rms amplitude and time lag of the kHz QPOs. Using this model, we fit the rms amplitude and time lag energy spectra of the lower kHz QPO in the NS LMXB 4U 1636-53 over 11 frequency intervals of the QPO and report three important findings: (i) A medium that extends 1-8 km above the NS surface is required to fit the data; this medium can be sustained by the balance between gravity and radiation pressure, without forcing any equilibrium condition. (ii) We predict a time delay between the oscillating NS temperature, due to feedback, and the oscillating electron temperature of the medium which, with the help of phase resolved spectroscopy, can be used as a probe of the geometry and the feedback mechanism. (iii) We show that the observed variability as a function of QPO frequency is mainly driven by the oscillating electron temperature of the medium. This provides strong evidence that the Comptonising medium in LMXBs significantly affects, if not completely drives, the radiative properties of the lower kHz QPOs regardless of the nature of the dynamical mechanism that produces the QPO frequencies.