2025/03/28 by Ewing, Melissa, Parra, Maxime, Mastroserio, Guglielmo +26 · 4 citations
#FOS: Physical sciences #High Energy Astrophysical Phenomena (astro-ph.HE)
paper · doi:10.48550/arxiv.2503.22665
We report the first detection of the X-ray polarisation of the transient black hole X-ray binary IGRJ17091-3624 taken with the Imaging X-ray polarimetry Explorer (IXPE) in March 2025, and present the results of an X-ray spectro-polarimetric analysis. The polarisation was measured in the 2--8 keV band with 5.2σ statistical confidence. We report a polarisation degree (PD) of 9.1±1.6 per cent and a polarisation angle of 83∘ ± 5∘ (errors are 1σ 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 ∼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 Comptonisation 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.