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 5.2s statistical confidence. We report a polarization degree (PD) of 9.1 +- 1.6 per cent and a polarization angle of 83º +- 5º (errors are 1s 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.

ME and AI acknowledge support from the Royal Society. POP acknowledges financial support from the French Space National Agency (CNES) and the National Center of Scientifc Research (CNRS) via its ‘Action Thématique’ PEM. TDR and MCB are INAF IAF research fellows. GM and SB acknowledge financial support by the Italian Space Agency (Agenzia Spaziale Italiana, ASI) through the contract ASI-INAF-2022-19-HH.0. BDM acknowledges support via a Ramón y Cajal Fellowship (RYC2018-025950-I), the Spanish MINECO grants PID2023-148661NB-I00, PID2022-136828NB-C44, and the AGAUR/Generalitat de Catalunya grant SGR-386/2021. MD and MG thank GACR project 21–06825X for the support and institutional support from RVO:67985815. The work of GM and LM is partially supported by the PRIN 2022–2022LWPEXW – ‘An X-ray view of compact objects in polarized light’, CUP C53D23001180006. AV acknowledges support from the Academy of Finland grant 355672. Nordita is supported in part by NordForsk.This research has made use of data and/or software provided by the High Energy Astrophysics Science Archive Research Center (HEASARC), which is a service of the Astrophysics Science Division at NASA/GSFC and the High Energy Astrophysics Division of the Smithsonian Astrophysical Observatory.

Peer Reviewed

Melissa Ewing, Maxime Parra, Guglielmo Mastroserio, Alexandra Veledina, Adam Ingram, Michal Dovčiak, Javier A García, Thomas D Russell, Maria C Baglio, Juri Poutanen, Oluwashina Adegoke, Stefano Bianchi, Fiamma Capitanio, Riley Connors, Melania Del Santo, Barbara De Marco, María Díaz Trigo, Poshak Gandhi, Maitrayee Gupta, Chulsoo Kang, Elias Kammoun, Vladislav Loktev, Lorenzo Marra, Giorgio Matt, Edward Nathan, Pierre-Olivier Petrucci, Megumi Shidatsu, James F Steiner, Francesco Tombesi, Federico M Vincentelli

Postprint (published version)