Black hole spin of Cyg X-1 determined from the softest state ever observed

TL;DR

This study analyzes the softest spectrum of Cyg X-1 observed with Suzaku, revealing that a multi-zone Comptonisation model suggests a lower black hole spin than previous estimates based on simpler models.

Contribution

The paper introduces a multi-zone Comptonisation model to better interpret the spectrum and timing data, leading to a revised, lower estimate of the black hole's spin.

Findings

The softest spectrum of Cyg X-1 was observed with Suzaku in 2013.

A multi-zone Comptonisation model fits the data better than a single non-thermal model.

The revised model indicates a significantly lower black hole spin.

Abstract

We show the softest ever spectrum from Cyg~X-1, detected in 2013 with Suzaku. This has the weakest high energy Compton tail ever seen from this object, so should give the cleanest view of the underlying disk spectrum, and hence the best determination of black hole spin from disk continuum fitting. Using the standard model of a disk with simple non-thermal Comptonisation to produce the weak high energy tail gives a high spin black hole. However, we get a significantly better fit by including an additional, low temperature thermal Comptonisation component, which allows a much lower black hole spin. Corroboration of the existence of an additional Compton component comes from the frequency dependent hard lags seen in the rapid variability in archival high/soft state data. These can not be explained if the continuum is a single non-thermal Comptonisation component, but are instead consistent with a radially stratified, multi zone Comptonisation spectrum, where the spectrum is softer further from the black hole. A complex multi-zone Comptonisation continuum is required to explain both spectra and timing together, and this has an impact on the derived black hole spin.