A1 Refereed original research article in a scientific journal
An IXPE-led X-Ray Spectropolarimetric Campaign on the Soft State of Cygnus X-1: X-Ray Polarimetric Evidence for Strong Gravitational Lensing
Authors: Steiner, James F.; Nathan, Edward; Hu, Kun; Krawczynski, Henric; Dovčiak, Michal; Veledina, Alexandra; Muleri, Fabio; Svoboda, Jiri; Alabarta, Kevin; Parra, Maxime; Bhargava, Yash; Matt, Giorgio; Poutanen, Juri; Petrucci, Pierre-Olivier; Tennant, Allyn F.; Baglio, M. Cristina; Baldini, Luca; Barnier, Samuel; Bhattacharyya, Sudip; Bianchi, Stefano; Brigitte, Maimouna; Cabezas, Mauricio; Cangemi, Floriane; Capitanio, Fiamma; Casey, Jacob; Rodriguez Cavero, Nicole; Castellano, Simone; Cavazzuti, Elisabetta; Chun, Sohee; Churazov, Eugene; Costa, Enrico; Di Lalla, Niccolò; Di Marco, Alessandro; Egron, Elise; Ewing, Melissa; Fabiani, Sergio; García, Javier A.; Green, David A.; Grinberg, Victoria; Hadrava, Petr; Ingram, Adam; Kaaret, Philip; Kislat, Fabian; Kitaguchi, Takao; Kravtsov, Vadim; Kubátová, Brankica; La Monaca, Fabio; Latronico, Luca; Loktev, Vladislav; Malacaria, Christian; Marin, Frédéric; Marinucci, Andrea; Maryeva, Olga; Mastroserio, Guglielmo; Mizuno, Tsunefumi; Negro, Michela; Omodei, Nicola; Podgorný, Jakub; Rankin, John; Ratheesh, Ajay; Rhodes, Lauren; Russell, David M.; Šlechta, Miroslav; Soffitta, Paolo; Spooner, Sean; Suleimanov, Valery; Tombesi, Francesco; Trushkin, Sergei A.; Weisskopf, Martin C.; Zane, Silvia; Zdziarski, Andrzej A.; Zhang, Sixuan; Zhang, Wenda; Zhou, Menglei; Agudo, Iván; Antonelli, Lucio A.; Bachetti, Matteo; Baumgartner, Wayne H.; Bellazzini, Ronaldo; Bongiorno, Stephen D.; Bonino, Raffaella; Brez, Alessandro; Bucciantini, Niccolò; Chen, Chien-Ting; Ciprini, Stefano; De Rosa, Alessandra; Del Monte, Ettore; Di Gesu, Laura; Donnarumma, Immacolata; Doroshenko, Victor; Ehlert, Steven R.; Enoto, Teruaki; Evangelista, Yuri; Ferrazzoli, Riccardo; Gunji, Shuichi; Hayashida, Kiyoshi; Heyl, Jeremy; Iwakiri, Wataru; Jorstad, Svetlana G.; Karas, Vladimir; Kolodziejczak, Jeffery J.; Liodakis, Ioannis; Maldera, Simone; Manfreda, Alberto; Marscher, Alan P.; Marshall, Herman L.; Massaro, Francesco; Mitsuishi, Ikuyuki; Ng, Chi-Yung; O’Dell, Stephen L.; Oppedisano, Chiara; Papitto, Alessandro; Pavlov, George G.; Peirson, Abel L.; Perri, Matteo; Pesce-Rollins, Melissa; Pilia, Maura; Possenti, Andrea; Puccetti, Simonetta; Ramsey, Brian D.; Roberts, Oliver J.; Romani, Roger W.; Sgrò, Carmelo; Slane, Patrick; Spandre, Gloria; Swartz, Douglas A.; Tamagawa, Toru; Tavecchio, Fabrizio; Taverna, Roberto; Tawara, Yuzuru; Thomas, Nicholas E.; Trois, Alessio; Tsygankov, Sergey S.; Turolla, Roberto; Vink, Jacco; Wu, Kinwah; Xie, Fei
Publisher: Institute of Physics Publishing
Publication year: 2024
Journal: Astrophysical Journal Letters
Journal name in source: Astrophysical Journal Letters
Article number: L30
Volume: 969
Issue: 2
ISSN: 2041-8205
eISSN: 2041-8213
DOI: https://doi.org/10.3847/2041-8213/ad58e4
Web address : https://iopscience.iop.org/article/10.3847/2041-8213/ad58e4
Self-archived copy’s web address: https://research.utu.fi/converis/portal/detail/Publication/457307169
We present the first X-ray spectropolarimetric results for Cygnus X-1 in its soft state from a campaign of five IXPE observations conducted during 2023 May-June. Companion multiwavelength data during the campaign are likewise shown. The 2-8 keV X-rays exhibit a net polarization degree PD = 1.99% ± 0.13% (68% confidence). The polarization signal is found to increase with energy across the Imaging X-ray Polarimetry Explorer’s (IXPE) 2-8 keV bandpass. The polarized X-rays exhibit an energy-independent polarization angle of PA = −25.°7 ± 1.°8 east of north (68% confidence). This is consistent with being aligned to Cyg X-1’s au-scale compact radio jet and its parsec-scale radio lobes. In comparison to earlier hard-state observations, the soft state exhibits a factor of 2 lower polarization degree but a similar trend with energy and a similar (also energy-independent) position angle. When scaling by the natural unit of the disk temperature, we find the appearance of a consistent trend line in the polarization degree between the soft and hard states. Our favored polarimetric model indicates that Cyg X-1’s spin is likely high (a * ≳ 0.96). The substantial X-ray polarization in Cyg X-1's soft state is most readily explained as resulting from a large portion of X-rays emitted from the disk returning and reflecting off the disk surface, generating a high polarization degree and a polarization direction parallel to the black hole spin axis and radio jet. In IXPE’s bandpass, the polarization signal is dominated by the returning reflection emission. This constitutes polarimetric evidence for strong gravitational lensing of X-rays close to the black hole.
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Funding information in the publication:
J.F.S. acknowledges support from NASA contract NAS8-03060 and NuSTAR General Observer Program 80NSSC23K1660. E.N. acknowledges support from NASA theory grant 80NSSC20K0540. M.D., J.S., J.Pod., and V.Kar. thank GACR project 21-06825X for the support and institutional support from RVO:67985815. A.V. thanks the Academy of Finland grant 355672 for support. A.A.Z. acknowledges support from the Polish National Science Center grant 2019/35/B/ST9/03944. P.-O.P. and M.P. acknowledge support from the High Energy National Programme (PNHE) of Centre national de la recherche scientifique (CNRS) and from the French space agency (CNES). I.L. was supported by the NASA Postdoctoral Program at the Marshall Space Flight Center, administered by Oak Ridge Associated Universities under contract with NASA. F.Mu., E.Co., A.D.M., R.F., P.So., S.F., and F.L.M. are partially supported by MAECI with grant CN24GR08 "GRBAXP: Guangxi-Rome Bilateral Agreement for X-ray Polarimetry in Astrophysics."
