Relativistic accretion-disc models of Cyg X-1 in the soft state: a case study
Abstract
The thermal continuum of Cygnus X-1 in the soft state is often modelled with a multicolour disc in a Newtonian spacetime approximation. That choice is convenient for estimating the inner radius, but it neglects relativistic effects near the black hole; these are usually restored through colour-correction and boundary-condition factors. Using Suzaku data from one exemplary observation, we compare this Newtonian description with some fully relativistic disc models, keeping the same treatment of absorption, ionised winds, Comptonization and reflection. With the mass, distance and inclination fixed to the best current dynamical values, the Newtonian inner radius is consistent with a high-spin Kerr innermost stable circular orbit. The relativistic models do not provide a uniformly better description of the spectrum: some worsen the fit or leave spin, inclination and hardening unconstrained, while the formally best model requires an inclination much larger than the dynamical one or a smaller spin. Apparent tensions with the models arise from data degeneracies and from the limited energy band, not from a failure of general relativity.
Related
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BibTeX
@misc{ye2026relativistic,
title = {{Relativistic accretion-disc models of Cyg X-1 in the soft state: a case study}},
author = {Luciano Ye and Luigi Foschini},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.27557},
eprint = {2609.27557},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.27557},
}
RIS
TY - UNPB
TI - Relativistic accretion-disc models of Cyg X-1 in the soft state: a case study
AU - Luciano Ye
AU - Luigi Foschini
PY - 2026
DA - 2026/09/23
PB - arXiv
DO - 10.48550/arxiv.2609.27557
UR - https://arxiv.org/abs/2609.27557
AB - The thermal continuum of Cygnus X-1 in the soft state is often modelled with a multicolour disc in a Newtonian spacetime approximation. That choice is convenient for estimating the inner radius, but it neglects relativistic effects near the black hole; these are usually restored through colour-correction and boundary-condition factors. Using Suzaku data from one exemplary observation, we compare this Newtonian description with some fully relativistic disc models, keeping the same treatment of absorption, ionised winds, Comptonization and reflection. With the mass, distance and inclination fixed to the best current dynamical values, the Newtonian inner radius is consistent with a high-spin Kerr innermost stable circular orbit. The relativistic models do not provide a uniformly better description of the spectrum: some worsen the fit or leave spin, inclination and hardening unconstrained, while the formally best model requires an inclination much larger than the dynamical one or a smaller spin. Apparent tensions with the models arise from data degeneracies and from the limited energy band, not from a failure of general relativity.
ER -