Unveiling the X-ray polarimetric properties of LMC X−3 with IXPE, NICER, and Swift/XRT

Abstract

Abstract The incoming Imaging X-ray Polarimetry Explorer (IXPE) observations of X-ray binaries provide a new tool to investigate the underlying accretion geometry. Here we report the first measurements of X-ray polarization of the extra-galactic black-hole X-ray binary LMC X−3. We find a polarization fraction of $\sim 3~{{\%}}$ at a polarization angle of ∼135○ in the 2 − 8 keV energy band with statistical significance at the 7σ level. This polarization measurement significantly exceeds the minimum detectable polarization threshold of 1.2 % for the source, ascertained at a 99 % confidence level within the 2 − 8 keV energy band. The simultaneous spectro-polarimetric fitting of NICER, Swift/XRT, and IXPE revealed the presence of a disc with a temperature of ∼1 keV and a Comptonized component with a power-law index of ∼ 2.4, confirming the soft nature of the source. The polarization degree increases with energy from ∼3 % in the 2 − 5.7 keV band to ∼9 % in the 5.7 − 8 keV band, while the polarization angle is energy independent. The observed energy dependence and the sudden jump of polarization fraction above 5 keV supports the idea of a static slab coronal geometry for the Comptonizing medium of LMC X−3.