The very high X-ray polarisation of accreting black hole IGRJ17091-3624 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 σ statistical confidence. We report a polarization degree (PD) of
9.1 ± 1.6 per cent and a polarization angle of 83◦ ± 5◦ (errors are 1 σ 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 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

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Identifiers

ORCID for Poshak Gandhi: orcid.org/0000-0003-3105-2615

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