Long-term quasi-periodicity of 4U 1636-536 resulting from accretion disc instability

TL;DR

This study analyzes the long-term quasi-periodic behavior of the low-mass X-ray binary 4U 1636-536, linking observed X-ray variability to accretion disc instabilities through temporal data analysis and modeling.

Contribution

It demonstrates the connection between observed quasi-periodicity and hydrogen thermal-ionization instability in the accretion disc, using a time-dependent model to explain the phenomenon.

Findings

Confirmed ~45-day quasi-periodicity during 2004-2006

Observed quasi-periodicity is transient and drifts at other times

Model reproduces observed period and amplitude based on disc instability

Abstract

We present the results of a study of the low-mass X-ray binary 4U 1636-536. We have performed temporal analysis of all available RXTE/ASM, Swift/BAT and MAXI data. We have confirmed the previously discovered quasi-periodicity of ~45 d present during ~2004, however we found it continued to 2006. At other epochs, the quasi-periodicity is only transient, and the quasi-period, if present, drifts. We have then applied a time-dependent accretion disc model to the interval with the significant X-ray quasi-periodicity. For our best model, the period and the amplitude of the theoretical light curve agree well with that observed. The modelled quasi-periodicity is due to the hydrogen thermal-ionization instability occurring in outer regions of the accretion disc. The model parameters are the average mass accretion rate (estimated from the light curves), and the accretion disc viscosity parameters, for the hot and cold phases. Our best model gives relatively low values of viscosity parameter for cold phase 0.01 and for hot phase 0.03.