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Published August 20, 2022 | Version Published
Journal Article Open

Limits on X-Ray Polarization at the Core of Centaurus A as Observed with the Imaging X-Ray Polarimetry Explorer

Creators

  • 1. ROR icon Marshall Space Flight Center
  • 2. ROR icon National Institute for Astrophysics
  • 3. ROR icon Agenzia Spaziale Italiana
  • 4. ROR icon Massachusetts Institute of Technology
  • 5. ROR icon Astronomical Observatory of Rome
  • 6. ROR icon Institute for Space Astrophysics and Planetology
  • 7. ROR icon Institut de Planétologie et d'Astrophysique de Grenoble
  • 8. ROR icon Observatory of Strasbourg
  • 9. ROR icon Roma Tre University
  • 10. ROR icon University of Turku
  • 11. ROR icon SLAC National Accelerator Laboratory
  • 12. ROR icon Boston University
  • 13. ROR icon Space Research Institute
  • 14. ROR icon University College London
  • 15. ROR icon Instituto de Astrofísica de Andalucía
  • 16. ROR icon Osservatorio Astronomico di Cagliari
  • 17. ROR icon INFN Sezione di Pisa
  • 18. ROR icon University of Pisa
  • 19. ROR icon INFN Sezione di Torino
  • 20. ROR icon University of Turin
  • 21. ROR icon Arcetri Astrophysical Observatory
  • 22. ROR icon University of Florence
  • 23. ROR icon INFN Sezione di Firenze
  • 24. ROR icon INFN Sezione di Roma II
  • 25. ROR icon Stanford University
  • 26. ROR icon University of Tübingen
  • 27. ROR icon Astronomical Institute
  • 28. ROR icon RIKEN
  • 29. ROR icon California Institute of Technology
  • 30. ROR icon Yamagata University
  • 31. ROR icon Osaka University
  • 32. ROR icon University of British Columbia
  • 33. ROR icon Chuo University
  • 34. ROR icon St Petersburg University
  • 35. ROR icon Washington University in St. Louis
  • 36. ROR icon Nagoya University
  • 37. ROR icon Hiroshima University
  • 38. ROR icon University of Maryland, Baltimore County
  • 39. ROR icon Goddard Space Flight Center
  • 40. ROR icon University of Hong Kong
  • 41. ROR icon Pennsylvania State University
  • 42. ROR icon Harvard-Smithsonian Center for Astrophysics
  • 43. ROR icon Brera Astronomical Observatory
  • 44. ROR icon University of Padua
  • 45. Dipartimento di Fisica, Università egli Studi di Roma "Tor Vergata,", Via della Ricerca Scientifica 1, I-00133 Roma, Italy
  • 46. ROR icon University of Maryland, College Park
  • 47. ROR icon University of Amsterdam
  • 48. ROR icon Guangxi University
  • 49. ROR icon Research Institute in Astrophysics and Planetology

Abstract

We present measurements of the polarization of X-rays in the 2–8 keV band from the nucleus of the radio galaxy Centaurus A (Cen A), using a 100 ks observation from the Imaging X-ray Polarimetry Explorer (IXPE). Nearly simultaneous observations of Cen A were also taken with the Swift, NuSTAR, and INTEGRAL observatories. No statistically significant degree of polarization is detected with IXPE. These observations have a minimum detectable polarization at 99% confidence (MDP₉₉) of 6.5% using a weighted, spectral model-independent calculation in the 2–8 keV band. The polarization angle ψ is consequently unconstrained. Spectral fitting across three orders of magnitude in X-ray energy (0.3–400 keV) demonstrates that the SED of Cen A is well described by a simple power law with moderate intrinsic absorption (N_H ∼ 10²³ cm⁻²) and a Fe Kα emission line, although a second unabsorbed power law is required to account for the observed spectrum at energies below 2 keV. This spectrum suggests that the reprocessing material responsible for this emission line is optically thin and distant from the central black hole. Our upper limits on the X-ray polarization are consistent with the predictions of Compton scattering, although the specific seed photon population responsible for the production of the X-rays cannot be identified. The low polarization degree, variability in the core emission, and the relative lack of variability in the Fe Kα emission line support a picture where electrons are accelerated in a region of highly disordered magnetic fields surrounding the innermost jet.

Copyright and License

© 2022. The Author(s). Published by the American Astronomical Society.

Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

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), 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). We acknowledge Dawoon E. Kim for providing the polar plot script. J.R. acknowledges financial support under the INTEGRAL ASI-INAF agreement 2019-35-HH.0 and ASI/INAF No. 2019-35.HH.0. The research leading to these results has received funding from the European Union's Horizon 2020 Programme under the AHEAD2020 project(grant agreement n. 871158). The INTEGRAL SPI project has been completed under the responsibility and leadership of CNES. Part of the French contribution is supported by the Scientific Research National Center (CNRS) and the French spatial agency (CNES). We are grateful to ASI, CEA, CNES, DLR, ESA, INTA, NASA, and OSTC for their support. The research at Boston University was supported in part by the National Science Foundation grant AST-2108622.

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Additional details

Related works

Is new version of
Discussion Paper: arXiv:2207.06625 (arXiv)

Funding

National Aeronautics and Space Administration
NNM15AA18C
Agenzia Spaziale Italiana
ASI-OHBI-2017-12-I.0
Agenzia Spaziale Italiana
ASI-INAF-2017-12-H0
Agenzia Spaziale Italiana
ASI-INFN-2017.13-H0
National Institute for Astrophysics
Istituto Nazionale di Fisica Nucleare
Agenzia Spaziale Italiana
2019-35-HH.0
Agenzia Spaziale Italiana
2019-35.HH.0
European Research Council
871158
Centre National de la Recherche Scientifique
Centre National d'Études Spatiales
National Science Foundation
AST-2108622

Dates

Accepted
2022-07-10
Accepted
Available
2022-08-22
Published

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