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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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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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.
The authors declare no competing interests.
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
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.
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).
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.
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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). https://doi.org/10.1038/s41586-020-2354-1
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