Energy dependent power spectral states and origin of aperiodic variability in black hole binaries

TL;DR

This study investigates the energy-dependent variability in black hole binary MAXI J1659-152, revealing distinct spectral states and associating specific variability components with different accretion disk regions.

Contribution

It demonstrates the energy-dependent nature of power spectral states and links variability components to specific physical regions in the accretion flow.

Findings

PLN linked to the optically thick disk across states

BLN and QPOs originate from the innermost hot flow

Energy cut-offs follow seed photon temperature

Abstract

We found the black hole candidate MAXI J1659-152 showed distinct power spectra, i.e., a power-law noise (PLN) vs. band-limited noise (BLN) plus quasi-periodic oscillations (QPOs), below and above about 2 keV respectively, in the observations with the Swift and the RXTE during the 2010 outburst, indicating a high energy cut-off of the PLN and a low energy cut-off of the BLN and the QPOs around 2 keV. The emergence of the PLN and the fading of the BLN and the QPOs initially took place from below 2 keV when the source entered the hard intermediate state and finally settled in the soft state three weeks later. The evolution was accompanied by the emergence of the disk spectral component and decreases in the amplitudes of variability in the soft X-ray and the hard X-ray bands. Our results indicate that the PLN is associated with the optically thick disk in both hard and intermediate states, and power spectral state is independent of the X-ray energy spectral state in a broadband view. We suggest that in the hard and the intermediate state, the BLN and the QPOs emerge from the innermost hot flow subjected to Comptonization, while the PLN originates from the optically thick disk further out. The energy cut-offs of the PLN and the BLN or QPOs then follow the temperature of the seed photons from the inner edge of the optically thick disk, while the high frequency cut-off of the PLN follows the orbital frequency at the inner edge of the optically thick disk as well.