Accretion geometry of the neutron star low mass X-ray binary Cyg X-2 from X-ray polarization measurements

Creators: Farinelli, R¹; Fabiani, S²; Poutanen, J^{3, 4}; Ursini, F⁵; Ferrigno, C⁶; Bianchi, S⁵; Cocchi, M⁷; Capitanio, F²; De Rosa, A²; Gnarini, A⁵; Kislat, F⁸; Matt, G⁵; Mikusincova, R⁵; Muleri, F²; Agudo, I⁹; Antonelli, L A^{10, 11}; Bachetti, M⁷; Baldini, L^{12, 13}; Baumgartner, W H¹⁴; Bellazzini, R¹²; Bongiorno, S D¹⁴; Bonino, R^{15, 16}; Brez, A¹²; Bucciantini, N^{17, 18, 19}; Castellano, S¹²; Cavazzuti, E¹¹; Ciprini, S^{11, 20}; Costa, E²; Del Monte, E²; Di Gesu, L¹¹; Di Lalla, N²¹; Di Marco, A²; Donnarumma, I¹¹; Doroshenko, V²²; Dovčiak, M²³; Ehlert, S R¹⁴; Enoto, T²⁴; Evangelista, Y²; Ferrazzoli, R²; Garcia, Javier A.²⁵; Gunji, S²⁶; Hayashida, K²⁷; Heyl, J²⁸; Iwakiri, W²⁹; Jorstad, S G^{30, 31}; Karas, V²³; Kitaguchi, T²⁴; Kolodziejczak, J J¹⁴; Krawczynski, H³²; La Monaca, F²; Latronico, L¹⁵; Liodakis, I³; Maldera, S¹⁵; Manfreda, A¹²; Marin, F³³; Marscher, A P³⁰; Marshall, H L³⁴; Mitsuishi, I³⁵; Mizuno, T³⁶; Ng, C-Y³⁷; O'Dell, S L¹⁴; Omodei, N²¹; Oppedisano, C¹⁵; Papitto, A¹⁰; Pavlov, G G³⁸; Peirson, A L²¹; Perri, M^{10, 11}; Pesce-Rollins, M¹²; Petrucci, P O³⁹; Pilia, M⁷; Possenti, A⁷; Puccetti, S¹¹; Ramsey, B D¹⁴; Rankin, J²; Ratheesh, A²; Romani, R W²¹; Sgrò, C¹²; Slane, P⁴⁰; Soffitta, P²; Spandre, G¹²; Tamagawa, T²⁴; Tavecchio, F⁴¹; Taverna, R⁴²; Tawara, Y³⁵; Tennant, A F¹⁴; Thomas, N E¹⁴; Tombesi, F^{20, 43, 44}; Trois, A⁷; Tsygankov, S S^{3, 4}; Turolla, R^{42, 45}; Vink, J⁴⁶; Weisskopf, M C¹⁴; Wu, K⁴⁵; Xie, F^{2, 47}; Zane, S⁴⁵

1. Osservatorio di Astrofisica e Scienza dello Spazio di Bologna
2. National Institute for Astrophysics
3. University of Turku
4. Space Research Institute
5. Roma Tre University
6. University of Geneva
7. Osservatorio Astronomico di Cagliari
8. University of New Hampshire
9. Instituto de Astrofísica de Andalucía
10. Astronomical Observatory of Rome
11. Agenzia Spaziale Italiana
12. INFN Sezione di Pisa
13. University of Pisa
14. Marshall Space Flight Center
15. INFN Sezione di Torino
16. University of Turin
17. Arcetri Astrophysical Observatory
18. University of Florence
19. INFN Sezione di Firenze
20. INFN Sezione di Roma II
21. Stanford University
22. University of Tübingen
23. Astronomical Institute
24. RIKEN
25. California Institute of Technology
26. Yamagata University
27. Osaka University
28. University of British Columbia
29. Chuo University
30. Boston University
31. St Petersburg University
32. Washington University in St. Louis
33. Observatory of Strasbourg
34. Massachusetts Institute of Technology
35. Nagoya University
36. Hiroshima University
37. University of Hong Kong
38. Pennsylvania State University
39. Institut de Planétologie et d'Astrophysique de Grenoble
40. Harvard-Smithsonian Center for Astrophysics
41. Brera Astronomical Observatory
42. University of Padua
43. University of Rome Tor Vergata
44. University of Maryland, College Park
45. University College London
46. University of Amsterdam
47. Guangxi University

Abstract

We report spectro-polarimetric results of an observational campaign of the bright neutron star low-mass X-ray binary Cyg X-2 simultaneously observed by IXPE, NICER, and INTEGRAL. Consistently with previous results, the broad-band spectrum is characterized by a lower-energy component, attributed to the accretion disc with kT_(in) ≈ 1 keV, plus unsaturated Comptonization in thermal plasma with temperature kT_e = 3 keV and optical depth τ ≈ 4, assuming a slab geometry. We measure the polarization degree in the 2–8 keV band P = 1.8 ± 0.3 per cent and polarization angle ϕ = 140° ± 4°, consistent with the previous X-ray polarimetric measurements by OSO-8 as well as with the direction of the radio jet which was earlier observed from the source. While polarization of the disc spectral component is poorly constrained with the IXPE data, the Comptonized emission has a polarization degree P = 4.0 ± 0.7 per cent and a polarization angle aligned with the radio jet. Our results strongly favour a spreading layer at the neutron star surface as the main source of the polarization signal. However, we cannot exclude a significant contribution from reflection off the accretion disc, as indicated by the presence of the iron fluorescence line.

Copyright and License

This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model).

Funding

The Imaging X-ray Polarimetry Explorer (IXPE) is a joint US and Italian mission. The US contribution is supported by the National Aeronautics and Space Administration (NASA) and led and managed by its Marshall Space Flight Center (MSFC), with industry partner Ball Aerospace (contract NNM15AA18C).

The Italian contribution is supported by the Italian Space Agency (Agenzia Spaziale Italiana, ASI) through contract ASI-OHBI-2017-12-I.0, agreements ASI-INAF-2017-12-H0 and ASI-INFN-2017.13-H0, and its Space Science Data Center (SSDC) with agreements ASI-INAF-2022-14-HH.0 and ASI-INFN 2021-43-HH.0, and by the Istituto Nazionale di Astrofisica (INAF) and the Istituto Nazionale di Fisica Nucleare (INFN) in Italy.

This research used data products provided by the IXPE Team (MSFC, SSDC, INAF, and INFN) and distributed with additional software tools by the High-Energy Astrophysics Science Archive Research Center (HEASARC), at NASA Goddard Space Flight Center (GSFC). JP and SST were supported by the Russian Science Foundation grant 20-12-00364 and the Academy of Finland grants 333112, 349144, 349373, and 349906.

Data Availability

The data used in this paper are publicly available in the HEASARC database.

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