Stronger Reflection from Black Hole Accretion Disks in Soft X-ray States

TL;DR

This study analyzes thousands of X-ray spectra from black hole candidates to show that reflection features are significantly stronger in soft states, likely due to differences in corona properties affecting scattering and geometry.

Contribution

It provides the first large-scale comparison of reflection strength across spectral states using a consistent method focusing on the Compton power law.

Findings

Reflection is ~3 times stronger in soft states.

Reflection strength varies with spectral state, influenced by corona properties.

Dilution of Fe line by Compton scattering explains the observed differences.

Abstract

We analyze 15,000 spectra of 29 stellar-mass black hole candidates collected over the 16-year mission lifetime of RXTE using a simple phenomenological model. As these black holes vary widely in luminosity and progress through a sequence of spectral states, which we broadly refer to as hard and soft, we focus on two spectral components: The Compton power law and the reflection spectrum it generates by illuminating the accretion disk. Our proxy for the strength of reflection is the equivalent width of the Fe-K line as measured with respect to the power law. A key distinction of our work is that for all states we estimate the continuum under the line by excluding the thermal disk component and using only the component that is responsible for fluorescing the Fe-K line, namely the Compton power law. We find that reflection is several times more pronounced (~3) in soft compared to hard spectral states. This is most readily caused by the dilution of the Fe line amplitude from Compton scattering in the corona, which has a higher optical depth in hard states. Alternatively, this could be explained by a more compact corona in soft (compared to hard) states, which would result in a higher reflection fraction.