Svoboda, JiříJiříSvobodaDovčiak, MichalMichalDovčiakSteiner, James F.James F.SteinerKaaret, PhilipPhilipKaaretPodgorný, JakubJakubPodgornýPoutanen, JuriJuriPoutanenVeledina, AlexandraAlexandraVeledinaMULERI, FABIOFABIOMULERITaverna, RobertoRobertoTavernaKrawczynski, HenricHenricKrawczynskiBrigitte, MaïmounaMaïmounaBrigitteDatta, Sudeb RanjanSudeb RanjanDattaBianchi, StefanoStefanoBianchiMuñoz-Darias, TeoTeoMuñoz-DariasNegro, MichelaMichelaNegroRodriguez Cavero, NicoleNicoleRodriguez CaveroCastro Segura, NoelNoelCastro SeguraBollemeijer, NiekNiekBollemeijerGarcía, Javier A.Javier A.GarcíaIngram, AdamAdamIngramMatt, GiorgioGiorgioMattNathan, EdwardEdwardNathanWeisskopf, Martin C.Martin C.WeisskopfAltamirano, DiegoDiegoAltamiranoBaldini, LucaLucaBaldiniCAPITANIO, FIAMMAFIAMMACAPITANIOEGRON, Elise Marie JeanneElise Marie JeanneEGRONEmami, RaziehRaziehEmamiHu, KunKunHuMARRA, LorenzoLorenzoMARRAMASTROSERIO, GuglielmoGuglielmoMASTROSERIOPetrucci, Pierre-OlivierPierre-OlivierPetrucciRATHEESH, AJAYAJAYRATHEESHSOFFITTA, PAOLOPAOLOSOFFITTATOMBESI, FrancescoFrancescoTOMBESIYang, Yi-JungYi-JungYangZhang, YuexinYuexinZhang2025-03-072025-03-0720242041-8205http://hdl.handle.net/20.500.12386/36483Black 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.STAMPAenArticle10.3847/2041-8213/ad402e2-s2.0-85193022270https://api.elsevier.com/content/abstract/scopus_id/85193022270https://iopscience.iop.org/article/10.3847/2041-8213/ad402e2024ApJ...966L..35SFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation