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
Creators: name: Ursini, F.
name: Farinelli, R.
name: Gnarini, A.
name: Poutanen, J.
name: Bianchi, S.
name: Capitanio, F.
name: Di Marco, A.
name: Fabiani, S.
name: La Monaca, F.
name: Malacaria, C.
name: Matt, G.
name: Mikušincová, R.
name: Cocchi, M.
name: Kaaret, P.
name: Kajava, J. J. E.
name: Pilia, M.
name: Zhang, W.
name: Agudo, I.
name: Antonelli, L. A.
name: Bachetti, M.
name: Baldini, L.
name: Baumgartner, W. H.
name: Bellazzini, R.
name: Bongiorno, S. D.
name: Bonino, R.
name: Brez, A.
name: Bucciantini, N.
name: Castellano, S.
name: Cavazzuti, E.
name: Chen, C. -T.
name: Ciprini, S.
name: Costa, E.
name: De Rosa, A.
name: Del Monte, E.
name: Di Gesu, L.
name: Di Lalla, N.
name: Donnarumma, I.
name: Doroshenko, V.
name: Dovčiak, M.
name: Ehlert, S. R.
name: Enoto, T.
name: Evangelista, Y.
name: Ferrazzoli, R.
name: Garcia, J. A.
name: Gunji, S.
name: Hayashida, K.
name: Heyl, J.
name: Iwakiri, W.
name: Jorstad, S. G.
name: Karas, V.
name: Kislat, F.
name: Kitaguchi, T.
name: Kolodziejczak, J. J.
name: Krawczynski, H.
name: Latronico, L.
name: Liodakis, I.
name: Maldera, S.
name: Manfreda, A.
name: Marin, F.
name: Marinucci, A.
name: Marscher, A. P.
name: Marshall, H. L.
name: Massaro, F.
name: Mitsuishi, I.
name: Mizuno, T.
name: Muleri, F.
name: Negro, M.
name: Ng, C. -Y.
name: O'Dell, S. L.
name: Omodei, N.
name: Oppedisano, C.
name: Papitto, A.
name: Pavlov, G. G.
name: Peirson, A. L.
name: Perri, M.
name: Pesce-Rollins, M.
name: Petrucci, P. -O.
name: Pilia, M.
name: Possenti, A.
name: Puccetti, S.
name: Ramsey, B. D.
name: Rankin, J.
name: Ratheesh, A.
name: Roberts, O. J.
name: Romani, R. W.
name: Sgrò, C.
name: Slane, P.
name: Soffitta, P.
name: Spandre, G.
name: Swartz, D. A.
name: Tamagawa, T.
name: Tavecchio, F.
name: Taverna, R.
name: Tawara, Y.
name: Tennant, A. F.
name: Thomas, N. E.
name: Tombesi, F.
name: Trois, A.
name: Tsygankov, S. S.
name: Turolla, R.
name: Vink, J.
name: Weisskopf, M. C.
name: Wu, K.
name: Xie, F.
name: Zane, S.
Publisher: name: arXiv
Year: 2023
Subjects: subject: High Energy Astrophysical Phenomena (astro-ph.HE)
subject: Solar and Stellar Astrophysics (astro-ph.SR)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: 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 $2.2\% \pm 0.5\%$ (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 disk emission, a Comptonized component, and reflection. From a spectro-polarimetric fit, we obtain an upper limit to the polarization degree of the disk of 4% (at 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 gives a significant contribution in any realistic scenario.
description: 10 pages, accepted for publication in A&amp;A