X-ray polarimetry and spectroscopy of the neutron star low-mass X-ray binary GX 9+9: An in-depth study with IXPE and <i>NuSTAR</i>
F. Ursini, R. Farinelli, Andrea Gnarini, Juri Poutanen, S. Bianchi, Fiamma Capitanio, Alessandro Di Marco, Sergio Fabiani, Fabio La Monaca, Christian Malacaria, G. Matt, Romana Mikušincová, M. Cocchi, P. Kaaret, J. J. E. Kajava, M. Pilia, Wenda Zhang, I. Agudo, L. A. Antonelli, M. Bachetti, L. Baldini, W. H. Baumgartner, R. Bellazzini, S. D. Bongiorno, R. Bonino, A. Brez, N. Bucciantini, S. Castellano, E. Cavazzuti, Chien‐Ting Chen, S. Ciprini, E. Costa, Alessandra De Rosa, E. Del Monte, L. Di Gesu, N. Di Lalla, I. Donnarumma, Victor Doroshenko, Michal Dovčiak, S. R. Ehlert, Teruaki Enoto, Y. Evangelista, R. Ferrazzoli, J. A. Garcia, Shuichi Gunji, K. Hayashida, J. Heyl, W. Iwakiri, Svetlana G. Jorstad, V. Karas, Fabian Kislat, T. Kitaguchi, J. J. Kolodziejczak, H. Krawczynski, L. Latronico, Ioannis Liodakis, S. Maldera, A. Manfreda, Frédéric Marin, Andrea Marinucci, A. P. Marscher, H. L. Marshall, F. Massaro, Ikuyuki Mitsuishi, Tsunefumi Mizuno, F. Muleri, Michela Negro, C.‐Y. Ng, Stephen L. O’Dell, N. Omodei, C. Oppedisano, A. Papitto, G. G. Pavlov, Abel L. Peirson, M. Perri, M. Pesce-Rollins, P.-O. Petrucci, M. Pilia, Andrea Possenti, S. Puccetti, B. D. Ramsey, J. Rankin, A. Ratheesh, O. J. Roberts, R. W. Romani, C. Sgrò, Patrick Slane, P. Soffitta, G. Spandre, Douglas A. Swartz, T. Tamagawa, F. Tavecchio, Roberto Taverna, Y. Tawara, A. F. Tennant, N. E. Thomas, Francesco Tombesi, A. Trois, Sergey S. Tsygankov, R. Turolla
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.