Spectral and timing analysis of the mHz QPOs in the neutron-star low-mass X-ray binary 4U 1636-53

TL;DR

This study analyzes the spectral and timing characteristics of mHz QPOs in the neutron-star low-mass X-ray binary 4U 1636-53, revealing their evolution, relation to X-ray bursts, and challenging existing theoretical models.

Contribution

It provides the first observation of a full mHz QPO lifetime and explores the connection between QPOs and type I X-ray bursts, offering new insights into neutron star accretion processes.

Findings

mHz QPOs show significant frequency variation and disappear before X-ray bursts

no correlation between QPO frequency and neutron-star surface temperature

observed QPO lifetime of 19 ks and a frequency drift timescale of 15 ks

Abstract

We investigate the spectral and timing properties of the millihertz quasi-periodic oscillations (mHz QPOs) in neutron-star low-mass X-ray binary 4U 1636-53 using XMM-Newton and Rossi X-ray Timing Explorer (RXTE) observations. The mHz QPOs in the XMM-Newton/RXTE observations show significant frequency variation and disappear right before type I X-ray bursts. We find no significant correlation between the mHz QPO frequency and the temperature of the neutron-star surface, which is different from theoretical predictions. For the first time we observed the full lifetime of a mHz QPO lasting 19 ks. Besides, we also measure a frequency drift timescale around 15 ks, we speculate that this is the cooling timescale of a layer deeper than the burning depth, possibly heated by the previous burst. Moreover, the analysis of all X-ray bursts in this source shows that all type I X-ray bursts associated with the mHz QPOs are short, bright and energetic, suggesting a potential connection between mHz QPOs and He-rich X-ray bursts.