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The Extreme Spin of the Black Hole in Cygnus X-1

Lijun Gou, Jeffrey E. McClintock, Mark J. Reid, Jerome A. Orosz, James F. Steiner, Ramesh Narayan, Jingen Xiang, Ronald A. Remillard, Keith A. Arnaud, Shane W. Davis

TL;DR

Cygnus X-1's black hole spin is measured by fitting the thermal continuum with a fully relativistic thin-disk model, leveraging new dynamical measurements of distance, mass, and inclination. Using three broadband spectra and a self-consistent continuum plus reflection framework (kerrbb2-based), the authors obtain a near-extreme spin a* > 0.95 (3σ) for the favored asynchronous dynamical model, with a similar high value under a synchronous alternative. They extensively test robustness against Fe Kα modeling, high-energy bandwidth, reflection schemes, viscosity and metallicity, and alternative dynamical configurations, finding the extreme spin remains robust and dominated by uncertainties in D, M, i, and flux calibration. The study also discusses the implications for natal spin origin and energy extraction, and contrasts with spin estimates from Fe Kα and QPO methods, arguing the extreme spin interpretation is reliable within the continuum-fitting framework.

Abstract

The compact primary in the X-ray binary Cygnus X-1 was the first black hole to be established via dynamical observations. We have recently determined accurate values for its mass and distance, and for the orbital inclination angle of the binary. Building on these results, which are based on our favored (asynchronous) dynamical model, we have measured the radius of the inner edge of the black hole's accretion disk by fitting its thermal continuum spectrum to a fully relativistic model of a thin accretion disk. Assuming that the spin axis of the black hole is aligned with the orbital angular momentum vector, we have determined that Cygnus X-1 contains a near-extreme Kerr black hole with a spin parameter a/M>0.95 (3σ). For a less probable (synchronous) dynamical model, we find a/M>0.92 (3σ). In our analysis, we include the uncertainties in black hole mass, orbital inclination angle and distance, and we also include the uncertainty in the calibration of the absolute flux via the Crab. These four sources of uncertainty totally dominate the error budget. The uncertainties introduced by the thin-disk model we employ are particularly small in this case given the extreme spin of the black hole and the disk's low luminosity.

The Extreme Spin of the Black Hole in Cygnus X-1

TL;DR

Cygnus X-1's black hole spin is measured by fitting the thermal continuum with a fully relativistic thin-disk model, leveraging new dynamical measurements of distance, mass, and inclination. Using three broadband spectra and a self-consistent continuum plus reflection framework (kerrbb2-based), the authors obtain a near-extreme spin a* > 0.95 (3σ) for the favored asynchronous dynamical model, with a similar high value under a synchronous alternative. They extensively test robustness against Fe Kα modeling, high-energy bandwidth, reflection schemes, viscosity and metallicity, and alternative dynamical configurations, finding the extreme spin remains robust and dominated by uncertainties in D, M, i, and flux calibration. The study also discusses the implications for natal spin origin and energy extraction, and contrasts with spin estimates from Fe Kα and QPO methods, arguing the extreme spin interpretation is reliable within the continuum-fitting framework.

Abstract

The compact primary in the X-ray binary Cygnus X-1 was the first black hole to be established via dynamical observations. We have recently determined accurate values for its mass and distance, and for the orbital inclination angle of the binary. Building on these results, which are based on our favored (asynchronous) dynamical model, we have measured the radius of the inner edge of the black hole's accretion disk by fitting its thermal continuum spectrum to a fully relativistic model of a thin accretion disk. Assuming that the spin axis of the black hole is aligned with the orbital angular momentum vector, we have determined that Cygnus X-1 contains a near-extreme Kerr black hole with a spin parameter a/M>0.95 (3σ). For a less probable (synchronous) dynamical model, we find a/M>0.92 (3σ). In our analysis, we include the uncertainties in black hole mass, orbital inclination angle and distance, and we also include the uncertainty in the calibration of the absolute flux via the Crab. These four sources of uncertainty totally dominate the error budget. The uncertainties introduced by the thin-disk model we employ are particularly small in this case given the extreme spin of the black hole and the disk's low luminosity.

Paper Structure

This paper contains 24 sections, 1 equation, 5 figures.

Table of Contents

  1. Introduction
  2. Data Selection, Observations and Data Reduction
  3. Data Analysis
  4. Seven Preliminary Models
  5. Our Adopted Model
  6. Results
  7. Robustness of Spin Estimates
  8. Effect of iron line and edges
  9. Effect of extending the bandwidth to 150 keV
  10. Effect of using a different reflection model
  11. Effect of Varying the Viscosity Parameter and Metallicity
  12. Relaxing the Spin Parameter
  13. Comprehensive Error Analysis
  14. discussion
  15. Measurement of Spin via the Fe K$\alpha$ Method
  16. ...and 9 more sections

Figures (5)

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