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Progress in unveiling extreme particle acceleration in persistent astrophysical jets


The most powerful persistent accelerators in the Universe are jetted active galaxies. Blazars, galaxies whose jets are directed towards Earth, dominate the extragalactic γ-ray sky. Still, most of the highest-energy particle accelerators probably elude detection. These extreme blazars, whose radiated energy can peak beyond 10 TeV, are ideal targets to study particle acceleration and radiative processes, and may provide links to cosmic rays and astrophysical neutrinos. The growing number of extreme blazars observed at teraelectronvolt energies has been critical for the emergence of γ-ray cosmology, including measurements of the extragalactic background light, tight bounds on the intergalactic magnetic field, and constraints on exotic physics at energies inaccessible with human-made accelerators. Tremendous progress has been achieved over the past decade, which bodes well for the future, particularly with the deployment of the Cherenkov Telescope Array.

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Fig. 1: Prototypical SEDs illustrating the three types of extreme behaviours.
Fig. 2: Radio number counts of extreme-synchrotron blazars and of different samples of BL Lacertae objects.
Fig. 3: Phenomenological models of the observed SED of the extreme blazar 1ES 0229+200.
Fig. 4: Observed TeV spectra of two extreme blazars including different γ-ray propagation models.

Data availability

All data presented in this study are included in this published article and its supplementary information files.


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E.Prandini has received funding from the European Union’s Horizon2020 research and innovation programme under the Marie Sklodowska–Curie grant agreement no. 664931. E.Pueschel acknowledges the Young Investigators Program of the Helmholtz Association. F.T. acknowledges contribution from the grant INAF CTA–SKA ‘Probing particle acceleration and γ-ray propagation with CTA and its precursors’ and the INAF Main Stream project ‘High-energy extragalactic astrophysics: toward the Cherenkov Telescope Array’. This Perspective is the result of several fruitful discussions raised during the meeting eXtreme19 (22–25 January 2019, Padova, Italy). The authors, as chairs of the scientific committee and review speakers, thank all the participants of the meeting: C. Arcaro, B. Balmaverde, U. Barres de Almeida, E. Benítez, D. Bernard, E. Bernardini, M. Boettcher, S. Boula, A. Caccianiga, C. Casadio, I. Christie, A. De Angelis, L. Di Gesu, A. di Matteo, I. Donnarumma, M. Doro, T. Dzhatdoev, V. Fallah Ramazani, R. Ferrazzoli, I. Florou, L. Foffano, L. Foschini, N. I. Fraija, A. Franceschini, G. Galanti, M. Gonzalez, O. Gueta, O. Hervet, S. Kerasioti, F. Krauss, M. Kreter, G. La Mura, R. Lico, R. Lopez Coto, M. Lucchini, M. Mallamaci, M. Manganaro, A. Marinelli, M. Mariotti, K. Nalewajko, E. Nokhrina, F. Oikonomou, L. Olivera-Nieto, S. Paiano, V. Paliya, D. Paneque, Z. Pei, C. Perennes, S. Rainò, P. Romano, A. Sharma, G. Sigl, C. Sinnis, P. Soffitta, A. Spolon, B. Sversut Arsioli, A. Tramacere, S. Vercellone, V. Vittorini and H. Xiao.

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J.B. and E.Prandini coordinated the work and mostly contributed to the introduction and ‘Perspectives’ section. L.C. and P.P. mostly contributed to the ‘Extreme observational properties’ section. M.L., E.R., A.T. and A.Z. mostly contributed to the ‘The challenge of modelling extreme blazars’ section. E.Pueschel and F.T. mostly contributed to the ‘Extreme blazars and tests of γ-ray propagation’ section. All authors discussed the material and contributed to the writing of the manuscript.

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Correspondence to J. Biteau or E. Prandini.

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Biteau, J., Prandini, E., Costamante, L. et al. Progress in unveiling extreme particle acceleration in persistent astrophysical jets. Nat Astron 4, 124–131 (2020).

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