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

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 extragalactic black-hole X-ray binary LMC X-3. We find a polarization fraction of similar to 3 at a polarization angle of similar to 135 degrees in the 2-8 keV energy band with statistical significance at the 7 sigma level. This polarization measurement significantly exceeds the minimum detectable polarization threshold of 1.2 per cent for the source, ascertained at a 99 per cent confidence level within the 2-8 keV energy band. The simultaneous spectro-polarimetric fitting of Neutron Star Interior Composition Explorer, Swift/X-Ray Telescope (XRT), and IXPE revealed the presence of a disc with a temperature of similar to 1 keV and a Comptonized component with a power-law index of similar to 2.4, confirming the soft nature of the source. The polarization degree increases with energy from similar to 3 per cent in the 2-5.7 keV band to similar to 9 per cent 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.