Dramatic Drop in the X-Ray Polarization of Swift J1727.8–1613 in the Soft Spectral State

Svoboda, Jiří; Dovčiak, Michal; Steiner, James F.; Kaaret, Philip; Podgorný, Jakub; Poutanen, Juri; Veledina, Alexandra; Muleri, Fabio; Taverna, Roberto; Krawczynski, Henric; Brigitte, Maïmouna; Datta, Sudeb Ranjan; Bianchi, Stefano; Muñoz-Darias, Teo; Negro, Michela; Rodriguez Cavero, Nicole; Castro Segura, Noel; Bollemeijer, Niek; García, Javier A.; Ingram, Adam; Matt, Giorgio; Nathan, Edward; Weisskopf, Martin C.; Altamirano, Diego; Baldini, Luca; Capitanio, Fiamma; Egron, Elise; Emami, Razieh; Hu, Kun; Marra, Lorenzo; Mastroserio, Guglielmo; Petrucci, Pierre-Olivier; Ratheesh, Ajay; Soffitta, Paolo; Tombesi, Francesco; Yang, Yi-Jung; Zhang, Yuexin
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The Astrophysical Journal

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Black hole X-ray binaries exhibit different spectral and timing properties in different accretion states. The X-ray outburst of a recently discovered and extraordinarily bright source, Swift J1727.8–1613, has enabled the first investigation of how the X-ray polarization properties of a source evolve with spectral state. The 2–8 keV polarization degree was previously measured by the Imaging X-ray Polarimetry Explorer (IXPE) to be ≈4% in the hard and hard intermediate states. Here we present new IXPE results taken in the soft state, with the X-ray flux dominated by the thermal accretion disk emission. We find that the polarization degree has dropped dramatically to ≲1%. This result indicates that the measured X-ray polarization is largely sensitive to the accretion state and the polarization fraction is significantly higher in the hard state when the X-ray emission is dominated by upscattered radiation in the X-ray corona. The combined polarization measurements in the soft and hard states disfavor a very high or low inclination of the system.