The First X-Ray Polarimetry of GRS 1739–278 Reveals Its Rapidly Spinning Black Hole

We present a joint spectropolarimetric 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 (PD) of PD = (2.3% +/- 0.4%) and polarization angle (PA) of PA = 62 degrees +/- 5 degrees in the 2-8 keV range. The model-independent analysis reveals that PD increases from similar to 2% at 2 keV to similar to 10% in the 6-8 keV band, while 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 kynbbrr model. The best fits indicate that high-spin configurations enhance the contribution of the reflected returning radiation, which dominates the observed polarization properties. From the kynbbrr modeling, we infer an extreme black hole spin of a=0.994-0.003+0.004 and a system inclination of i=54 degrees-4 degrees+8 degrees . Owing to the large contribution from the returning radiation, the observed polarization direction is nearly parallel to the projected system axis, the position angle of which is predicted at 58 degrees +/- 4 degrees. 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.