X-ray polarimetry and spectroscopy of the neutron star low-mass X-ray binary GX 9+9: An in-depth study with IXPE and NuSTAR

F. Ursini, R. Farinelli, A. Gnarini, J. Poutanen, S. Bianchi, F. Capitanio, A. Di Marco, S. Fabiani, F. La Monaca, C. Malacaria, G. Matt, R. Mikušincová, M. Cocchi, P. Kaaret, J. J.E. Kajava, M. Pilia, W. Zhang, I. Agudo, L. A. Antonelli, M. BachettiL. Baldini, W. H. Baumgartner, R. Bellazzini, S. D. Bongiorno, R. Bonino, A. Brez, N. Bucciantini, S. Castellano, E. Cavazzuti, C. T. Chen, S. Ciprini, E. Costa, A. De Rosa, E. Del Monte, L. Di Gesu, N. Di Lalla, I. Donnarumma, V. Doroshenko, M. Dovčiak, S. R. Ehlert, T. Enoto, Y. Evangelista, R. Ferrazzoli, J. A. Garcia, S. Gunji, K. Hayashida, J. Heyl, W. Iwakiri, S. G. Jorstad, V. Karas, F. Kislat, T. Kitaguchi, J. J. Kolodziejczak, H. Krawczynski, L. Latronico, I. Liodakis, S. Maldera, A. Manfreda, F. Marin, A. Marinucci, A. P. Marscher, H. L. Marshall, F. Massaro, I. Mitsuishi, T. Mizuno, F. Muleri, M. Negro, C. Y. Ng, S. L. O'Dell, N. Omodei, C. Oppedisano, A. Papitto, G. G. Pavlov, A. L. Peirson, M. Perri, M. Pesce-Rollins, P. O. Petrucci, M. Pilia, A. Posenti, S. Puccetti, B. D. Ramsey, J. Rankin, A. Ratheesh, O. J. Roberts, R. W. Romani, C. Sgrò, P. Slane, P. Soffitta, G. Spandre, D. A. Swartz, T. Tamagawa, F. Tavecchio, R. Taverna, Y. Tawara, A. F. Tennant, N. E. Thomas, F. Tombesi, A. Trois, S. S. Tsygankov, R. Turolla, J. Vink, M. C. Weisskopf, K. Wu, F. Xie, S. Zane

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Abstract

We report on a comprehensive analysis of simultaneous X-ray polarimetric and spectral data of the bright atoll source GX 9+9 with the Imaging X-ray Polarimetry Explorer (IXPE) and NuSTAR. The source is significantly polarized in the 4- 8 keV band, with a degree of 2.2% ± 0.5% (uncertainty at the 68% confidence level). The NuSTAR broad-band spectrum clearly shows an iron line, and is well described by a model including thermal disc emission, a Comptonized component, and reflection. From a spectro-polarimetric fit, we obtain an upper limit to the polarization degree of the disc of 4% (at the 99% confidence level), while the contribution of Comptonized and reflected radiation cannot be conclusively separated. However, the polarization is consistent with resulting from a combination of Comptonization in a boundary or spreading layer, plus reflection off the disc, which significantly contributes in any realistic scenario.

Original languageEnglish (US)
Article numberA20
JournalAstronomy and Astrophysics
Volume676
DOIs
StatePublished - Aug 1 2023

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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