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Discovery of Strong Energy-Dependent X-ray Polarization in the Intermediate State of GS 1354-64

Swati Ravi, Lorenzo Marra, James F. Steiner, Guglielmo Mastroserio, Mason Ng, Joey Neilsen, Herman L. Marshall, Fiamma Capitanio, Sudeb Ranjan Datta, Elise Egron, Javier A. Garcia, Adam Ingram, Philip Kaaret, Ole Koenig, Honghui Liu, Romana Mikusincova, Edward J. R. Nathan, P. -O. Petrucci, Jakub Podgorny, Chiara Salvaggio, Jiri Svoboda, Alexandra Veledina, Yuexin Zhang

Abstract

We report the discovery of significant X-ray polarization from the dynamically confirmed black hole X-ray binary (BHXB) GS 1354-64 during its 2025-2026 outburst, obtained with the Imaging X-ray Polarimetry Explorer (IXPE). The observation, obtained shortly after a bright X-ray flare, captures the source in an intermediate state following a stalled (failed) state transition. We discover significant 2-8 keV polarization at the ~4% level with high statistical support--14-sigma significance from frequentist analysis and log Bayes Factor 283+/-1 in a Bayesian framework--measuring PD 4.0+/-0.2% and PA-1+/-2 degrees (90% credible interval). The PD exhibits a statistically significant increasing trend with energy--the strongest such increase yet observed by IXPE in a BHXB--going from 2.1+/-0.3% in the 2-3 keV band to 11+/-3% in the 6.5-7 keV band, while the PA appears stable across both energy and time to within statistical uncertainties. Timing analysis of the IXPE data reveals a ~5 Hz Type-C quasi-periodic oscillation. IXPE + NuSTAR spectropolarimetric modeling suggests that the data can be described by polarized thermal disk and Comptonized components with PAs differing by ~90 degrees, or by a dominant Comptonized polarized component whose effective PD increases across the IXPE bandpass--the inferred component-level polarization levels are therefore model-dependent. In either picture, GS 1354-64 retains a strong coronal component during the transitional period observed by IXPE. These results illustrate how X-ray polarimetry can provide a sensitive diagnostic of the accretion state and geometry in black hole X-ray binary accretion flows, exploring a liminal phase at the cusp of state transition.

Discovery of Strong Energy-Dependent X-ray Polarization in the Intermediate State of GS 1354-64

Abstract

We report the discovery of significant X-ray polarization from the dynamically confirmed black hole X-ray binary (BHXB) GS 1354-64 during its 2025-2026 outburst, obtained with the Imaging X-ray Polarimetry Explorer (IXPE). The observation, obtained shortly after a bright X-ray flare, captures the source in an intermediate state following a stalled (failed) state transition. We discover significant 2-8 keV polarization at the ~4% level with high statistical support--14-sigma significance from frequentist analysis and log Bayes Factor 283+/-1 in a Bayesian framework--measuring PD 4.0+/-0.2% and PA-1+/-2 degrees (90% credible interval). The PD exhibits a statistically significant increasing trend with energy--the strongest such increase yet observed by IXPE in a BHXB--going from 2.1+/-0.3% in the 2-3 keV band to 11+/-3% in the 6.5-7 keV band, while the PA appears stable across both energy and time to within statistical uncertainties. Timing analysis of the IXPE data reveals a ~5 Hz Type-C quasi-periodic oscillation. IXPE + NuSTAR spectropolarimetric modeling suggests that the data can be described by polarized thermal disk and Comptonized components with PAs differing by ~90 degrees, or by a dominant Comptonized polarized component whose effective PD increases across the IXPE bandpass--the inferred component-level polarization levels are therefore model-dependent. In either picture, GS 1354-64 retains a strong coronal component during the transitional period observed by IXPE. These results illustrate how X-ray polarimetry can provide a sensitive diagnostic of the accretion state and geometry in black hole X-ray binary accretion flows, exploring a liminal phase at the cusp of state transition.
Paper Structure (15 sections, 5 figures)

This paper contains 15 sections, 5 figures.

Table of Contents

  1. Introduction
  2. Observations and Data Reduction
  3. Observations
  4. MAXI Monitoring
  5. IXPE
  6. NuSTAR
  7. Data Reduction
  8. Model-Independent Polarimetric Analyses
  9. IXPE Timing Analysis
  10. Results
  11. The IXPE Observation
  12. Polarimetry
  13. Timing
  14. Spectropolarimetry
  15. Discussion

Figures (5)

  • Figure 1: MAXI/GSC light curve of GS 1354$-$64 during its 2025-2026 outburst (MJD 61038--61090). The boxed region in the 2--20 keV light curve depicts the IXPE observation and the gray shading denotes the simultaneous NuSTAR observation. The IXPE 2--8 keV light curve is shown in detail in the top panel, from the combined DU1+DU3 event list and binned at 50 s.
  • Figure 2: MAXI/GSC HID of GS 1354$-$64 from MJD 55053 to 61090, using 12-hour intervals. Colored points illustrate the ongoing 2025--2026 outburst, with colors corresponding to outburst phase; the IXPE observation interval is marked with star symbols.
  • Figure 3: Polarimetric summary of the GS 1354$-$64 IXPE observation. Left: Constant-PA (scout_const) QUEEN-BEE polar plot. Contours represent $68\%, 95\%,~\mathrm{and}~99.7\%$ credible intervals. Right: Energy-dependent PD and PA behavior obtained from binned PCUBE analysis. Error bars denote $1\sigma$ uncertainties. Points in grey denote bins with $\textless3\sigma$ significance. The corresponding disk fraction---defined as the ratio of disk flux to total (disk + Comptonized) flux---is overplotted in pink.
  • Figure 4: Timing properties of GS 1354$-$64 from IXPE. Top: Noise-subtracted PDS plotted as $\nu(P(\nu)-P_{\rm noise})$ (fractional rms$^2$), with the shaded region indicating the QPO band. Middle: The rms-normalized power spectrum $P(\nu)$ (fractional rms$^2$ Hz$^{-1}$) with the best-fit model of a constant and two Lorentzians overplotted. Bottom: Fit residuals.
  • Figure 5: Joint IXPE + NuSTAR spectral fitting for GS 1354$-$64. Left: IXPE I + NuSTAR spectra and best-fitting model with fit residuals. The model components are shown separately, but the minimal laor contribution falls below the scale of the plot. Right: IXPE Q + IXPE U spectra and best-fitting "Compton-increasing" model with fit residuals. The baseline "Compton-increasing" model assumes an unpolarized disk and a Comptonizing component linearly trending PD. The fit residuals for the formally preferred "orthogonal components" model are included in the bottommost panel.