Breaking the spectral degeneracies in black hole binaries with fast timing data

TL;DR

This paper demonstrates that fast timing analysis of black hole binary data can resolve spectral degeneracies by revealing distinct variability properties of different Comptonisation regions, improving understanding of accretion flows.

Contribution

It introduces a method to break spectral degeneracies in black hole binaries using variability power spectra, revealing the nature of Comptonisation components.

Findings

Fast variability spectra differ significantly at short timescales.

Inner hot flow's seed photons originate from a cooler Comptonisation region.

Spectral degeneracies can be resolved with timing analysis.

Abstract

The spectra of black hole binaries in the low/hard state are complex, with evidence for multiple different Comptonisation regions contributing to the hard X-rays in addition to a cool disc component. We show this explicitly for some of the best RXTE data from Cyg X-1, where the spectrum strongly requires (at least) two different Comptonisation components in order to fit the continuum above 3 keV, where the disc does not contribute. However, it is difficult to constrain the shapes of these Comptonisation components uniquely using spectral data alone. Instead, we show that additional information from fast variability can break this degeneracy. Specifically, we use the observed variability power spectra in each energy channel to reconstruct the energy spectra of the variability on timescales of ~10s, 1s and 0.1s. The two longer timescale spectra have similar shapes, but the fastest component is dramatically harder, and has strong curvature indicating that its seed photons are not from the cool disc. We interpret this in the context of propagating fluctuations through a hot flow, where the outer regions are cooler and optically thick, so that they shield the inner region from the disc. The seed photons for the hot inner region are then from the cooler Comptonisation region rather than the disc itself.