The First X-ray Polarimetry of GRS 1739--278 Reveals Its Rapidly Spinning Black Hole
Qing-Chang Zhao, Michal Dovciak, Han-Cheng Li, Lian Tao, Hua Feng, Federico Vincentelli, Giorgio Matt, Philip Kaaret, Shuang-Nan Zhang
TL;DR
This study reports the first X-ray polarization detection of the black hole X-ray binary GRS 1739–278 during a 2025 soft-state outburst using IXPE and NuSTAR. Model-independent and spectro-polarimetric analyses show an energy-dependent polarization, with the PD rising toward higher energies, consistent with a substantial contribution from returning radiation rather than direct disk emission alone. Self-consistent modeling with kynbbrr yields an extreme spin of $a=0.994^{+0.004}_{-0.003}$ and an inclination of $i=54^{+8}_{-4}$ degrees, with the polarization dominated by returning radiation that aligns with the projected system axis ($\psi=58\pm4^\circ$). This work demonstrates that X-ray polarimetry can directly probe relativistic disk geometry and spin in stellar-mass black holes and motivates future time-resolved polarization studies with upcoming missions.
Abstract
We present a joint spectro-polarimetric analysis of the black hole X-ray binary GRS~1739--278 during its 2025 mini-outburst, using simultaneous observations from \ixpe\ and \nustar. The \ixpe\ data show a polarization degree of ${\rm PD} = (2.3 \pm 0.4)\%$ and a polarization angle of ${\rm PA} = 62^\circ \pm 5^\circ$ in the 2--8~keV range. The model-independent analysis reveals that the PD increases from $\sim 2\%$ at 2~keV to $\sim 10\%$ in the 6--8~keV band, while the PA remains stable across the \ixpe\ band within statistical uncertainties. Broadband spectral modeling of the combined \ixpe\ and \nustar\ datasets shows that hard Comptonization contributes negligibly in this soft-state observation, while a substantial reflected component is required in addition to the thermal disk emission. We then model the \ixpe\ Stokes spectra using the \texttt{kynbbrr} model. The best-fitting results indicate that high-spin configurations enhance the contribution of reflected returning radiation, which dominates the observed polarization properties. From the \texttt{kynbbrr} modeling, we infer an extreme black hole spin of $a = 0.994^{+0.004}_{-0.003}$ and a system inclination of $i = 54^\circ{}^{+8^\circ}_{-4^\circ}$. Owing to the large contribution from returning radiation, the observed polarization direction is nearly parallel to the projected system axis, the position angle of which is predicted to be $58^\circ \pm 4^\circ$. Our results demonstrate that X-ray polarimetry, combined with broadband spectroscopy, directly probes the geometry and relativistic effects in accretion disks around stellar-mass black holes.