First X-ray polarization measurement confirms the low black-hole spin in LMC X-3

TL;DR

This study presents the first X-ray polarization measurement of LMC X-3, confirming its low black-hole spin and demonstrating polarization as a tool for understanding accretion geometry.

Contribution

It provides the first detection of X-ray polarization in LMC X-3 and demonstrates that polarization measurements can independently confirm black-hole spin estimates.

Findings

Detected 3.2% polarization degree with stable angle

Confirmed low black-hole spin with an upper limit of a < 0.7

Observed a possible increase in polarization with energy

Abstract

X-ray polarization is a powerful tool to investigate the geometry of accreting material around black holes, allowing independent measurements of the black hole spin and orientation of the innermost parts of the accretion disk. We perform the X-ray spectro-polarimetric analysis of an X-ray binary system in the Large Magellanic Cloud, LMC X-3, that hosts a stellar-mass black hole, known to be persistently accreting since its discovery. We report the first detection of the X-ray polarization in LMC X-3 with the Imaging X-ray Polarimetry Explorer, and find the average polarization degree of 3.2% +- 0.6% and a constant polarization angle -42 deg +- 6 deg over the 2-8 keV range. Using accompanying spectroscopic observations by NICER, NuSTAR, and the Neil Gehrels Swift observatories, we confirm previous measurements of the black hole spin via the X-ray continuum method, a ~ 0.2. From polarization analysis only, we found consistent results with low black-hole spin, with an upper limit of a < 0.7 at a 90% confidence level. A slight increase of the polarization degree with energy, similar to other black-hole X-ray binaries in the soft state, is suggested from the data but with a low statistical significance.