The very high X-ray polarization of accreting black hole IGR J17091−3624 in the hard state

Abstract

We report the first detection of the X-ray polarization of the transient black hole X-ray binary IGR J17091-3624 taken with the Imaging X-ray polarimetry Explorer (IXPE) in 2025 March, and present the results of an X-ray spectropolarimetric analysis. The polarization was measured in the 2-8 keV band with 5.2 sigma statistical confidence. We report a polarization degree (PD) of 9.1 +/- 1.6 per cent and a polarization angle of 83 degrees +/- 5 degrees (errors are 1 sigma confidence). There is a hint of a positive correlation of PD with energy that is not statistically significant. We report that the source is in the corona-dominated hard state, which is confirmed by a hard power-law-dominated spectrum with weak reflection features and the presence of a Type-C quasi-periodic oscillation at similar to 0.2 Hz. The orientation of the emitted radio jet is not known, and so we are unable to compare it with the direction of X-ray polarization, but we predict the two to be parallel if the geometry is similar to that in Cygnus X-1 and Swift J1727.8-1613, the two hard state black hole binaries previously observed by IXPE. In the Comptonization scenario, the high observed PD requires a very favourable geometry of the corona, a high inclination angle (supported by the presence of a dip in the light curve) and possibly a mildly relativistic outflow and/or scattering in an optically thick wind.