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

Creators: Ehlert, Steven R.¹; Ferrazzoli, Riccardo²; Marinucci, Andrea³; Marshall, Herman L.⁴; Middei, Riccardo^{3, 5}; Pacciani, Luigi⁶; Perri, Matteo^{3, 5}; Petrucci, Pierre-Olivier⁷; Puccetti, Simonetta³; Barnouin, Thibault⁸; Bianchi, Stefano⁹; Liodakis, Ioannis¹⁰; Madejski, Grzegorz¹¹; Marin, Frédéric⁸; Marscher, Alan P.¹²; Matt, Giorgio⁹; Poutanen, Juri^{10, 13}; Wu, Kinwah¹⁴; Agudo, Iván¹⁵; Antonelli, Lucio A.^{3, 5}; Bachetti, Matteo¹⁶; Baldini, Luca^{17, 18}; Baumgartner, Wayne H.¹; Bellazzini, Ronaldo¹⁷; Bongiorno, Stephen D.¹; Bonino, Raffaella^{19, 20}; Brez, Alessandro¹⁷; Bucciantini, Niccoló^{21, 22, 23}; Capitanio, Fiamma²; Castellano, Simone¹⁷; Cavazzuti, Elisabetta³; Ciprini, Stefano^{3, 24}; Costa, Enrico²; De Rosa, Alessandra²; Del Monte, Ettore²; Di Gesu, Laura³; Di Lalla, Niccoló²⁵; Di Marco, Alessandro²; Donnarumma, Immacolata³; Doroshenko, Victor^{13, 26}; Dovčiak, Michal²⁷; Enoto, Teruaki²⁸; Evangelista, Yuri²; Fabiani, Sergio²; Garcia, Javier A.²⁹; Gunji, Shuichi³⁰; Hayashida, Kiyoshi³¹; Heyl, Jeremy³²; Iwakiri, Wataru³³; Jorstad, Svetlana G.^{12, 34}; Karas, Vladimir²⁷; Kitaguchi, Takao²⁸; Kolodziejczak, Jeffery J.¹; Krawczynski, Henric³⁵; La Monaca, Fabio²; Latronico, Luca¹⁹; Maldera, Simone¹⁹; Manfreda, Alberto¹⁷; Massaro, Francesco^{19, 20}; Mitsuishi, Ikuyuki³⁶; Mizuno, Tsunefumi³⁷; Muleri, Fabio²; Negro, Michela^{38, 39}; Ng, C.-Y.⁴⁰; O'Dell, Stephen L.¹; Omodei, Nicola²⁵; Oppedisano, Chiara¹⁹; Papitto, Alessandro⁵; Pavlov, George G.⁴¹; Peirson, Abel L.²⁵; Pesce-Rollins, Melissa¹⁷; Pilia, Maura¹⁶; Possenti, Andrea¹⁶; Ramsey, Brian D.¹; Rankin, John²; Ratheesh, Ajay²; Romani, Roger W.²⁵; Sgrò, Carmelo¹⁷; Slane, Patrick⁴²; Soffitta, Paolo²; Spandre, Gloria¹⁷; Tamagawa, Toru²⁸; Tavecchio, Fabrizio⁴³; Taverna, Roberto⁴⁴; Tawara, Yuzuru³⁶; Tennant, Allyn F.¹; Thomas, Nicholas E.¹; Tombesi, Francesco^{24, 45, 46}; Trois, Alessio¹⁶; Tsygankov, Sergey^{10, 13}; Turolla, Roberto^{14, 44}; Vink, Jacco⁴⁷; Weisskopf, Martin C.¹; Xie, Fei⁴⁸; Zane, Silvia¹⁴; (IXPE Collaboration); Rodi, James⁴⁹; Jourdain, Elisabeth⁴⁹; Roques, Jean-Pierre

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

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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