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

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.

Dovčiak, Michal (author)
Steiner, James F. (author)
Kaaret, Philip (author)
Podgorný, Jakub (author)
Poutanen, Juri (author)
Veledina, Alexandra,1987-Stockholms universitet,Nordiska institutet för teoretisk fysik (Nordita),University of Turku, Finland(Swepub:su)avele (author)
Muleri, Fabio (author)
Taverna, Roberto (author)
Krawczynski, Henric (author)
Brigitte, Maïmouna (author)
Datta, Sudeb Ranjan (author)
Bianchi, Stefano (author)
Muñoz-Darias, Teo (author)
Negro, Michela (author)
Cavero, Nicole Rodriguez (author)
Segura, Noel Castro (author)
Bollemeijer, Niek (author)
García, Javier A. (author)
Ingram, Adam (author)
Matt, Giorgio (author)
Nathan, Edward (author)
Weisskopf, Martin C. (author)
Altamirano, Diego (author)
Baldini, Luca (author)
Capitanio, Fiamma (author)
Egron, Elise (author)
Emami, Razieh (author)
Hu, Kun (author)
Marra, Lorenzo (author)
Mastroserio, Guglielmo (author)
Petrucci, Pierre-Olivier (author)
Ratheesh, Ajay (author)
Soffitta, Paolo (author)
Tombesi, Francesco (author)
Yang, Yi-Jung (author)
Zhang, Yuexin (author)
Stockholms universitetNordiska institutet för teoretisk fysik (Nordita) (creator_code:org_t)

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NATURAL SCIENCES: NATURAL SCIENCES; and Physical Science ...; and Astronomy Astrop ...

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