Testing wind as an explanation for the spin problem in the continuum-fitting method

TL;DR

This paper investigates whether disk outflows (winds) can explain the observed decrease in black hole spin measurements with increasing luminosity in X-ray binaries, suggesting winds as a potential solution.

Contribution

The study demonstrates that models with accretion rate-dependent winds can replicate observed spectral features and address the spin decrease artifact, offering a new perspective on disk modeling.

Findings

Wind models with accretion rate dependence match observed spectra.

Non-dependent wind models cannot reproduce the spin decrease.

Alternative explanation with truncated inner disk also viable.

Abstract

The continuum-fitting method is one of the two most advanced methods of determining the black hole spin in accreting X-ray binary systems. There are, however, still some unresolved issues with the underlying disk models. One of them manifests as an apparent decrease in spin for increasing source luminosity. Here, we perform a few simple tests to establish whether outflows from the disk close to the inner radius can address this problem. We employ four different parametric models to describe the wind and compare these to the apparent decrease in spin with luminosity measured in the sources LMC~X-3 and GRS~1915+105. Wind models in which parameters do not explicitly depend on the accretion rate cannot reproduce the spin measurements. Models with mass accretion rate dependent outflows, however, have spectra that emulate the observed ones. The assumption of a wind thus effectively removes the artifact of spin decrease. This solution is not unique; the same conclusion can be obtained with a truncated inner disk model. To distinguish among valid models, high resolution X-ray data and a realistic description of the Comptonization in the wind will be needed.