A NuSTAR Reflection-Spectroscopy Survey of Cygnus X-1

Abstract

Relativistic-reflection measurements of Cygnus X-1 disagree on the extent of disk truncation and commonly infer supersolar iron abundances. We analyze a selected sample of 26 archival NuSTAR observations obtained between 2012 and 2024 using two configurations from one reflection-model family, with posterior modes sampled by preconditioned sequential Monte Carlo. In the baseline recovered modes, disk-surface ionization increases with photon index (Pearson r = +0.59), with state medians rising from log xi approximately 3.3 in the hard state to approximately 3.9 in the soft state. The corresponding free-emissivity fits give median inner radii of 6.6, 4.4, and 4.0 R_ISCO in the hard, intermediate, and soft states. Fixing q = 3 moves three of eight soft-state observations to the ISCO and one to 2.4 R_ISCO; four fixed-q fits have lower chi^2 than the sampled free-q solutions, showing that those runs missed higher-likelihood regions. The inferred radii are therefore model- and mode-dependent, and the spectra neither require nor exclude an ISCO disk or R_in greater than or approximately 20 R_ISCO. Baseline fitted abundances span A_Fe = 1.6-8.6, with a median of 4.9. Two observations separated by 7.2 hr yield A_Fe = 1.9 +/- 0.2 and 4.5 +/- 1.0, indicating that fitted abundance is not a direct composition measurement. Fixing A_Fe = 1.6 drives some densities toward the grid boundary and worsens the fits relative to A_Fe = 4.5. The wind parameters remain sensitive to the continuum, abundance, and orbital-phase sampling.