Speaker
Description
I will present an analysis of the recent IXPE observation of the black hole X-ray binary GRS 1739–278 during its current mini-outburst. The 2–8 keV spectrum is dominated by thermal disc emission with a contribution of thermal reflection component interpreted as caused by the disc self-irradiation. The hard Comptonized contribution is found to be negligible. The IXPE data show an averaged polarization degree of PD=(2.3±0.4)% and polarization angle of PA=62°±5° in the 2–8 keV range. The model-independent PCUBE analysis reveals that PD increases from ∼2% at 2 keV to ∼10% in the highest 6–8 keV bin, while PA remains constant across the IXPE band within statistical uncertainties. Modelling with Novikov-Thorne standard disc including the disc self-radiation indicates that high-spin configurations enhance the contribution of the reflected returning thermal radiation, which dominates the observed polarization properties. We infer an extreme black hole spin of a≃0.994 and a system inclination of θ≃54°. Owing to the large contribution from the reflected returning radiation, the observed polarization direction is nearly parallel to the projected system axis, the position angle of which is predicted to be at ∼58° east of north.