New Results on the Time lags of the Quasi-Periodic Oscillations in the Low-mass X-ray Binary 4U 1636--53

TL;DR

This paper analyzes energy and frequency-dependent Fourier time lags of various QPOs in the neutron-star binary 4U 1636-53, revealing complex lag behaviors and discussing potential physical origins such as reflection and scattering.

Contribution

It provides new detailed measurements of time lags across different QPO frequencies in 4U 1636-53, enhancing understanding of their physical mechanisms.

Findings

Time lags vary with QPO frequency, showing positive, negative, and zero values.

Lag behavior differs between break frequency QPOs and hectoHertz QPOs.

Results suggest reflection and scattering processes influence observed lags.

Abstract

We present an analysis of the energy and frequency dependence of the Fourier time lags of the hectoHertz quasi-periodic oscillations (QPOs) and of the QPOs at the frequency at which the power density spectrum shows a break in the neutron-star low-mass X-ray binary 4U 1636-53, using a large data set obtained with the Rossi X-ray Timing Explorer. We found that: (i) For the break frequency QPO: for low frequencies, in general the time lag is positive, but it is decreasing with increasing frequency, reaching zero lag at 20 Hz. Between 20 and 35 Hz there is a small fluctuation around zero, from where the time lags become positive again and increase slightly above zero up to 65 Hz. (ii) For the hHz QPO: we see that when the frequency is 100 Hz the time lag is negative, but it increases to zero already at 110 Hz, being consistent with this value up to 130 Hz from where it increases to 0.5 msec at around 140 Hz. From 140 Hz the time lag decreases sharply, being strongly negative for hHz greater than 220 Hz. We compare these lags with our previous results for the lags of the kiloHertz QPOs in this same source and discuss possible scenarios for producing the lags in this system in the context of reflection off the accretion disc or (up-)down-scattering in a hot medium close to the neutron star.