Swift X-ray Telescope study of the Black Hole Binary MAXI J1659-152: Variability from a two component accretion flow

TL;DR

This study analyzes the energy-dependent X-ray variability of MAXI J1659-152 during its 2010 outburst, revealing distinct behaviors of variability components linked to different accretion flow regions and proposing models for QPO origins.

Contribution

First to report an rms spectrum of variability components in the 0.5-30 keV range for this black hole binary, linking variability to accretion disk and hot flow regions.

Findings

Low-frequency variability decreases with energy.

High-frequency variability increases with energy.

QPO coherence varies with energy and intensity.

Abstract

We present an energy dependent X-ray variability study of the 2010 outburst of the black hole X-ray binary MAXI J1659-152 with the Swift X-ray Telescope (XRT). The broad-band noise components and the quasi periodic oscillations (QPO) observed in the power spectra show a strong and varied energy dependence. Combining Swift XRT data with data from the Rossi X-ray Timing Explorer, we report, for the first time, an rms spectrum (fractional rms amplitude as a function of energy) of these components in the 0.5-30 keV energy range. We find that the strength of the low-frequency component (< 0.1 Hz) decreases with energy, contrary to the higher frequency components (> 0.1 Hz) whose strengths increase with energy. In the context of the propagating fluctuations model for X-ray variability, we suggest that the low-frequency component originates in the accretion disk (which dominates emission below ~ 2 keV) and the higher frequency components are formed in the hot flow (which dominates emission above ~ 2 keV). As the properties of the QPO suggest that it may have a different driving mechanism, we investigate the Lense-Thirring precession of the hot flow as a candidate model. We also report on the QPO coherence evolution for the first time in the energy band below 2 keV. While there are strong indications that the QPO is less coherent at energies below 2 keV than above 2 keV, the coherence increases with intensity similar to what is observed at energies above 2 keV in other black-hole X-ray binaries.