X-ray time variability across the atoll source states of 4U 1636--53

TL;DR

This study analyzes X-ray time variability in 4U 1636--53 across different states, revealing universal correlation patterns, distinct mechanisms for hectohertz QPOs, and challenging previous harmonic mode switching interpretations.

Contribution

First RXTE analysis of 4U 1636--53 in the island state, comparing variability components with other sources and testing existing models.

Findings

4U 1636--53 follows universal correlation schemes of atoll sources.

Hectohertz QPO mechanism differs from other variability components.

Low-frequency QPO frequencies are not related to spin, angular momentum, or luminosity.

Abstract

We have studied the rapid X-ray time variability in 149 pointed observations with the \textit{Rossi X-ray Timing Explorer} (RXTE)'s Proportional Counter Array of the atoll source 4U~1636--53 in the banana state and, for the first time with RXTE, in the island state. We compare the frequencies of the variability components of 4U~1636--53 with those in other atoll and Z-sources and find that 4U~1636--53 follows the universal scheme of correlations previously found for other atoll sources at (sometimes much) lower luminosities. Our results on the hectohertz QPO suggest that the mechanism that sets its frequency differs from that for the other components, while the amplitude setting mechanism is common. A previously proposed interpretation of the narrow low-frequency QPO frequencies in different sources in terms of harmonic mode switching is not supported by our data, nor by some previous data on other sources and the frequency range that this QPO covers is found not to be related to spin, angular momentum or luminosity.