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Resolving acceleration to very high energies along the jet of Centaurus A

A Publisher Correction to this article was published on 24 June 2020

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The nearby radio galaxy Centaurus A belongs to a class of active galaxies that are luminous at radio wavelengths. Most show collimated relativistic outflows known as jets, which extend over hundreds of thousands of parsecs for the most powerful sources. Accretion of matter onto the central supermassive black hole is believed to fuel these jets and power their emission1. Synchrotron radiation from relativistic electrons causes the radio emission, and it has been suggested that the X-ray emission from Centaurus A also originates in electron synchrotron processes2,3,4. Another possible explanation is inverse Compton scattering with cosmic microwave background (CMB) soft photons5,6,7. Synchrotron radiation needs ultrarelativistic electrons (about 50 teraelectronvolts) and, given their short cooling times, requires some continuous re-acceleration mechanism8. Inverse Compton scattering, on the other hand, does not require very energetic electrons, but the jets must stay highly relativistic on large scales (exceeding 1 megaparsec). Some recent evidence disfavours inverse Compton-CMB models9,10,11,12, although other work seems to be compatible with them13,14. In principle, the detection of extended γ-ray emission, which directly probes the presence of ultrarelativistic electrons, could distinguish between these options. At gigaelectronvolt energies there is also an unusual spectral hardening15,16 in Centaurus A that has not yet been explained. Here we report observations of Centaurus A at teraelectronvolt energies that resolve its large-scale jet. We interpret the data as evidence for the acceleration of ultrarelativistic electrons in the jet, and favour the synchrotron explanation for the X-rays. Given that this jet is not exceptional in terms of power, length or speed, it is possible that ultrarelativistic electrons are commonplace in the large-scale jets of radio-loud active galaxies.

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Fig. 1: Multiwavelength image of Centaurus A.
Fig. 2: Spectral energy distribution of Centaurus A.

Data availability statement

The raw H.E.S.S. data and the code used in this study are not public, but belong to the H.E.S.S. collaboration. All derived higher-level data that are shown in the plots will be made available on the H.E.S.S. collaboration’s website on publication of this study.

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  1. Blandford, R., Meier, D. & Readhead, A. Relativistic jets from active galactic nuclei. Annu. Rev. Astron. Astrophys. 57, 467–509 (2019).

    ADS  CAS  Google Scholar 

  2. Feigelson, E. D. et al. The X-ray structure of Centaurus A. Astrophys. J. 251, 31–51 (1981).

    ADS  CAS  Google Scholar 

  3. Kraft, R. P. et al. Chandra observations of the X-ray jet in Centaurus A. Astrophys. J. 569, 54–71 (2002).

    ADS  Google Scholar 

  4. Snios, B. et al. Variability and proper motion of X-ray knots in the jet of Centaurus A. Astrophys. J. 871, 248 (2019).

    ADS  CAS  Google Scholar 

  5. Celotti, A., Ghisellini, G. & Chiaberge, M. Large-scale jets in active galactic nuclei: multiwavelength mapping. Mon. Not. R. Astron. Soc. 321, L1–L5 (2001).

    ADS  Google Scholar 

  6. Harris, D. E. & Krawczynski, H. X-ray emission from extragalactic jets. Annu. Rev. Astron. Astrophys. 44, 463–506 (2006).

    ADS  Google Scholar 

  7. Simionescu, A. et al. Serendipitous discovery of an extended X-ray jet without a radio counterpart in a high-redshift quasar. Astrophys. J. Lett. 816, L15 (2016).

    ADS  Google Scholar 

  8. Liu, R.-Y., Rieger, F. M. & Aharonian, F. A. Particle acceleration in mildly relativistic shearing flows: the interplay of systematic and stochastic effects, and the origin of the extended high-energy emission in AGN jets. Astrophys. J. 842, 39 (2017).

    ADS  Google Scholar 

  9. Georganopoulos, M., Meyer, E. & Perlman, E. Recent progress in understanding the large scale jets of powerful quasars. Galaxies 4, 65 (2016).

    ADS  Google Scholar 

  10. Breiding, P. et al. Fermi non-detections of four X-ray jet sources and implications for the IC/CMB mechanism. Astrophys. J. 849, 95 (2017).

    ADS  Google Scholar 

  11. Sun, X.-N., Yang, R.-Z., Rieger, F. M., Liu, R.-Y. & Aharonian, F. Energy distribution of relativistic electrons in the kiloparsec scale jet of M 87 with Chandra. Astron. Astrophys. 612, A106 (2018).

    Google Scholar 

  12. Marshall, H. L. et al. An X-ray imaging survey of quasar jets: the complete survey. Astrophys. J. 856, 66 (2018).

    ADS  Google Scholar 

  13. Lucchini, M., Tavecchio, F. & Ghisellini, G. Revisiting the EC/CMB model for extragalactic large scale jets. Mon. Not. R. Astron. Soc. 466, 4299–4306 (2017).

    ADS  CAS  Google Scholar 

  14. Meyer, E. T. et al. The origin of the X-ray emission in two well-aligned extragalactic jets: the case for IC/CMB. Astrophys. J. 883, L2 (2019).

    ADS  CAS  Google Scholar 

  15. Sahakyan, N., Yang, R., Aharonian, F. A. & Rieger, F. M. Evidence for a second component in the high-energy core emission from Centaurus A? Astrophys. J. Lett. 770, L6 (2013).

  16. Abdalla, H. et al. The γ-ray spectrum of the core of Centaurus A as observed with H.E.S.S. and Fermi-LAT. Astron. Astrophys. 619, A71 (2018).

    CAS  Google Scholar 

  17. Harris, G. L. H., Rejkuba, M. & Harris, W. E. The distance to NGC 5128 (Centaurus A). Publ. Astron. Soc. Aust. 27, 457–462 (2010).

    ADS  Google Scholar 

  18. Burns, J. O., Feigelson, E. D. & Schreier, E. J. The inner radio structure of Centaurus A — clues to the origin of the jet X-ray emission. Astrophys. J. 273, 128–153 (1983).

    ADS  Google Scholar 

  19. Israel, F. P. Centaurus A – NGC 5128. Astron. Astrophys. Rev. 8, 237–278 (1998).

    ADS  Google Scholar 

  20. Kraft, R. P. et al. A Chandra high-resolution X-ray image of Centaurus A. Astrophys. J. Lett. 531, L9–L12 (2000).

    ADS  CAS  PubMed  Google Scholar 

  21. Aab, A. et al. An indication of anisotropy in arrival directions of ultra-high-energy cosmic rays through comparison to the flux pattern of extragalactic gamma-ray sources. Astrophys. J. 853, L29 (2018).

    ADS  Google Scholar 

  22. Hartman, R. C. et al. The third EGRET catalog of high-energy gamma-ray sources. Astrophys. J. Suppl. Ser. 123, 79–202 (1999).

    ADS  Google Scholar 

  23. Aharonian, F. et al. Discovery of very high energy γ-ray emission from Centaurus A with H.E.S.S. Astrophys. J. Lett. 695, L40–L44 (2009).

    ADS  Google Scholar 

  24. Chiaberge, M., Capetti, A. & Celotti, A. The BL Lac heart of Centaurus A. Mon. Not. R. Astron. Soc. 324, L33–L37 (2001).

    ADS  Google Scholar 

  25. Lenain, J.-P., Boisson, C., Sol, H. & Katarzyński, K. A synchrotron self-Compton scenario for the very high energy γ-ray emission of the radiogalaxy M 87. Unifying the TeV emission of blazars and other AGNs? Astron. Astrophys. 478, 111–120 (2008).

    ADS  CAS  Google Scholar 

  26. Abdo, A. A. et al. Fermi gamma-ray imaging of a radio galaxy. Science 328, 725–729 (2010).

    ADS  CAS  PubMed  Google Scholar 

  27. Aharonian, F. A. et al. Observations of the Crab nebula with HESS. Astron. Astrophys. 457, 899–915 (2006).

    ADS  CAS  Google Scholar 

  28. Holler, M., Chevalier, J., Lenain, J. P., Sanchez, D. & de Naurois, M. Run-wise simulations for imaging atmospheric Cherenkov telescope arrays. In Proc. 35th Int. Cosmic Ray Conf. 755 (Proceedings of Science, 2017).

  29. Freeman, P., Doe, S. & Siemiginowska, A. Astronomical data analysis. Proc. SPIE 4477, 76–87 (2001).

    ADS  Google Scholar 

  30. Hardcastle, M. J. et al. Radio and X-ray observations of the jet in Centaurus A. Astrophys. J. 593, 169–183 (2003).

    ADS  Google Scholar 

  31. Gillesen, S. Sub-Bogenminuten-genaue Positionen von TeV-Quellen mit H.E.S.S. PhD thesis, Ruprecht-Karls-Univ. Heidelberg (2004).

  32. Ma, C. et al. The international celestial reference frame as realized by very long baseline interferometry. Astron. J. 116, 516–546 (1998).

    ADS  Google Scholar 

  33. Hardcastle, M. J. & Croston, J. H. Modelling TeV γ-ray emission from the kiloparsec-scale jets of Centaurus A and M87. Mon. Not. R. Astron. Soc. 415, 133–142 (2011).

    ADS  CAS  Google Scholar 

  34. Bednarek, W. GeV-TeV γ-rays produced by electrons in the kpc-scale jet as a result of Comptonization of the inner jet emission. Mon. Not. R. Astron. Soc. 483, 1003–1007 (2019).

    ADS  CAS  Google Scholar 

  35. Tanada, K., Kataoka, J. & Inoue, Y. Inverse Compton scattering of starlight in the kiloparsec-scale jet in Centaurus A: the origin of excess TeV γ-ray emission. Astrophys. J. 878, 139 (2019).

    ADS  CAS  Google Scholar 

  36. Hardcastle, M. J., Kraft, R. P. & Worrall, D. M. The infrared jet in Centaurus A: multiwavelength constraints on emission mechanisms and particle acceleration. Mon. Not. R. Astron. Soc. Lett. 368, L15–L19 (2006).

    ADS  Google Scholar 

  37. The CTA Consortium. Science with the Cherenkov Telescope Array (World Scientific, 2019).

  38. Tavecchio, F. Gamma rays from blazars. Am. Inst. Phys. Conf. Ser. 1792, 020007 (2017).

    Google Scholar 

  39. Condon, J. J., Helou, G., Sanders, D. B. & Soifer, B. T. A 1.425 GHz atlas of the IRAS bright galaxy sample, part II. Astrophys. J. Suppl. Ser. 103, 81–108 (1996).

    ADS  Google Scholar 

  40. Hahn, J. et al. Impact of aerosols and adverse atmospheric conditions on the data quality for spectral analysis of the H.E.S.S. telescopes. Astropart. Phys. 54, 25–32 (2014).

    ADS  Google Scholar 

  41. de Naurois, M. & Rolland, L. A high performance likelihood reconstruction of γ-rays for imaging atmospheric Cherenkov telescopes. Astropart. Phys. 32, 231–252 (2009).

    ADS  Google Scholar 

  42. Berge, D., Funk, S. & Hinton, J. Background modelling in very-high-energy γ-ray astronomy. Astron. Astrophys. 466, 1219–1229 (2007).

    ADS  Google Scholar 

  43. Cash, W. Parameter estimation in astronomy through application of the likelihood ratio. Astrophys. J. 228, 939–947 (1979).

    ADS  Google Scholar 

  44. Storn, R. & Price, K. Differential evolution: a simple and efficient adaptive scheme for global optimization over continuous spaces. J. Glob. Optim. 11, 341–359 (1997).

    MATH  Google Scholar 

  45. Parsons, R. D. & Hinton, J. A. A Monte Carlo template based analysis for air-Cherenkov arrays. Astropart. Phys. 56, 26–34 (2014).

    ADS  Google Scholar 

  46. Hardcastle, M. J. et al. New results on particle acceleration in the Centaurus A jet and counterjet from a deep Chandra observation. Astrophys. J. Lett. 670, L81–L84 (2007).

    ADS  CAS  Google Scholar 

  47. Yang, R. Z., Sahakyan, N., de Ona Wilhelmi, E., Aharonian, F. & Rieger, F. Deep observation of the giant radio lobes of Centaurus A with the Fermi Large Area Telescope. Astron. Astrophys. 542, A19 (2012).

    Google Scholar 

  48. Sun, X.-n., Yang, R.-z., Mckinley, B. & Aharonian, F. Giant lobes of Centaurus A as seen in radio and γ-ray images obtained with the Fermi-LAT and Planck satellites. Astron. Astrophys. 595, A29 (2016).

    Google Scholar 

  49. Kataoka, J. et al. The X-ray jet in Centaurus A: clues to the jet structure and particle acceleration. Astrophys. J. 641, 158–168 (2006).

    ADS  CAS  Google Scholar 

  50. Struve, C., Oosterloo, T. A., Morganti, R. & Saripalli, L. Centaurus A: morphology and kinematics of the atomic hydrogen. Astron. Astrophys. 515, A67 (2010).

    ADS  Google Scholar 

  51. Israel, F. P., Güsten, R., Meijerink, R., Requena-Torres, M. A. & Stutzki, J. The outflow of gas from the Centaurus A circumnuclear disk. Atomic spectral line maps from Herschel/PACS and APEX. Astron. Astrophys. 599, A53 (2017).

    ADS  Google Scholar 

  52. Weiß, A. et al. LABOCA observations of nearby, active galaxies. Astron. Astrophys. 490, 77–86 (2008).

    ADS  Google Scholar 

  53. Wykes, S. et al. Mass entrainment and turbulence-driven acceleration of ultra-high energy cosmic rays in Centaurus A. Astron. Astrophys. 558, A19 (2013).

    Google Scholar 

  54. van den Bergh, S. The post-eruptive galaxy NGC 5128 = Centaurus A. Astrophys. J. 208, 673–682 (1976).

    ADS  Google Scholar 

  55. Stawarz, Ł. Aharonian, F., Wagner, S. & Ostrowski, M. Absorption of nuclear γ-rays on the starlight radiation in FR I sources: the case of Centaurus A. Mon. Not. R. Astron. Soc. 371, 1705–1716 (2006).

    ADS  CAS  Google Scholar 

  56. Aharonian, F. A. & Atoyan, A. M. Compton scattering of relativistic electrons in compact X-ray sources. Astrophys. Space Sci. 79, 321–336 (1981).

    ADS  MATH  Google Scholar 

  57. Zabalza, V. naima: a Python package for inference of relativistic particle energy distributions from observed nonthermal spectra. In Proc. of Int. Cosmic Ray Conf. 2015 922 (Proceedings of Science, 2015).

  58. Aharonian, F. A., Kelner, S. R. & Prosekin, A. Y. Angular, spectral, and time distributions of highest energy protons and associated secondary gamma rays and neutrinos propagating through extragalactic magnetic and radiation fields. Phys. Rev. D 82, 043002 (2010).

    ADS  Google Scholar 

  59. Khangulyan, D., Aharonian, F. A. & Kelner, S. R. Simple analytical approximations for treatment of inverse Compton scattering of relativistic electrons in the blackbody radiation field. Astrophys. J. 783, 100 (2014).

    ADS  Google Scholar 

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The support of the Namibian authorities and of the University of Namibia in facilitating the construction and operation of H.E.S.S. is gratefully acknowledged, as is the support by the German Ministry for Education and Research (BMBF), the Max Planck Society, the German Research Foundation (DFG), the Helmholtz Association, the Alexander von Humboldt Foundation, the French Ministry of Higher Education, Research and Innovation, the Centre National de la Recherche Scientifique (CNRS/IN2P3 and CNRS/INSU), the Commissariat à l’énergie atomique et aux énergies alternatives (CEA), the UK Science and Technology Facilities Council (STFC), the Knut and Alice Wallenberg Foundation, the National Science Centre, Poland (grant no. 2016/22/M/ST9/00382), the South African Department of Science and Technology, the South African National Research Foundation, the University of Namibia, the National Commission on Research, Science and Technology of Namibia (NCRST), the Austrian Federal Ministry of Education, Science and Research and the Austrian Science Fund (FWF), the Australian Research Council (ARC), the Japan Society for the Promotion of Science and the University of Amsterdam. We appreciate the excellent work of the technical support staff in Berlin, Zeuthen, Heidelberg, Palaiseau, Paris, Saclay, Tübingen and Namibia in the construction and operation of the equipment. This work benefited from services provided by the H.E.S.S. Virtual Organisation, supported by the national resource providers of the EGI Federation.

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Authors and Affiliations



M. Holler, M.d.N. and D.A.S. analysed and interpreted the H.E.S.S. data and prepared the manuscript. F.R. and A.M.T. performed the modelling and prepared the manuscript. The entire H.E.S.S. Collaboration contributed to the publication with involvement at various stages, from the design, construction and operation of the instrument to the development and maintenance of all software for data handling, data reduction and data analysis. All authors reviewed, discussed and commented on the present results and the manuscript.

Corresponding authors

Correspondence to M. Holler, M. de Naurois, F. Rieger, D. A. Sanchez or A. M. Taylor.

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The authors declare no competing interests.

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Peer review information Nature thanks Roopesh Ojha and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 One-dimensional projections of VHE events.

Shown are projections of the VHE γ-ray emission from Centaurus A along the alignment of the semi-major axis obtained from the two-dimensional elliptical morphology fit (left; negative values correspond to φ = 43.4° and positive ones to φ + 180°) and perpendicular to it (right; φ + 90° for negative and φ + 270° for positive distances). The dashed red line shows the projection of the PSF on both sides. The blue line on the left panel corresponds to the PSF-convolved best-fit Gaussian model. Error bars on the ordinate denote statistical uncertainties (±1 s.d.), those along the abscissa illustrate the bin size.

Extended Data Fig. 2 Relevant timescales in the jet.

Characteristic electron cooling timescales (ordinate) in the kiloparsec-scale jet of Centaurus A as a function of electron energy (abscissa; the corresponding electron Lorentz factor is also shown along the top of the graph). Achievable particle energies are essentially limited by synchrotron losses. The solid green line represents the timescale for electron acceleration, and the solid purple line the dynamical or advection timescale. The dashed and dotted lines represent the timescales on which electrons lose energy, that is, via synchrotron radiation (light blue line) or inverse Compton (IC) scattering off ambient photons (dust, yellow line; starlight, orange line; CMB, dark blue line).

Extended Data Fig. 3 γ-ray SED of Centaurus A.

Comparison of the resultant γ-ray SEDs for Centaurus A including an earlier energy cut-off γcmec2 for the electron distribution at γc ≈ 107 (dashed line), everything else being kept the same as for Fig. 2. An extension of the electron distribution to γ = 108 is needed to fully account for the observed VHE spectrum (solid line). Red points refer to Fermi-LAT observations (error bars), usually attributed to emission from the core which is not modelled here. The blue-shaded butterfly represents VHE observations by H.E.S.S.16.

Extended Data Table 1 Modelling parameters

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The H.E.S.S. Collaboration. Resolving acceleration to very high energies along the jet of Centaurus A. Nature 582, 356–359 (2020).

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