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  • Review Article
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Gamma-ray haloes around pulsars as the key to understanding cosmic-ray transport in the Galaxy

Abstract

Pulsars are factories of relativistic electrons and positrons that propagate away from the pulsar, eventually permeating our Galaxy. The acceleration and propagation of these cosmic particles are a matter of intense debate. In the last few years, we have had the opportunity to directly observe the injection of these particles into the interstellar medium through the discovery of gamma-ray haloes around pulsars. This new type of gamma-ray source is produced by electrons and positrons diffusing out of the pulsar wind nebula and scattering ambient photon fields to produce gamma rays. This correspondingly new field of study comes with a number of observations and constraints at different wavelengths and a variety of theoretical models that can explain the properties of these haloes. We examine the characteristics of the propagation of cosmic rays inferred from the observations of gamma-ray haloes and their local and global implications for particle transport within the Galaxy. We also discuss the prospects for observations of these sources with facilities such as the Large High Altitude Air Shower Observatory, the Cherenkov Telescope Array or the Southern Wide-field Gamma-ray Observatory in the near future.

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Fig. 1: Evolutionary stages in the life of a PWN.
Fig. 2: Map of the sky region around the HAWC detection of Geminga.
Fig. 3: Local all-electron spectrum.
Fig. 4: Local positron spectrum.

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Acknowledgements

This article is the result of fruitful discussions during the First Workshop on Gamma-ray Halos Around Pulsars (https://agenda.infn.it/e/GammaHalos). We first and foremost thank the Scientific Organizing Committee of the Workshop (A. Mitchell, S. Profumo, D. Torres, H. Zhou, R. Zanin), the Local Organizing Committee (M. I. Bernardos, A. de Angelis, A. Spolon) and all the participants (S. Abdollahi, A. Abdulrahman, F. Acero, F. Aharonian, A. Albert, E. Amato, M. Araya, T. Armstrong, V. Baghmanyan, C. Baheeja, A. Baktash, Y. Bao, M. Barnard, U. Barres de Almeida, I. Batkovic, M. Bernardos Martin, P. Blasi, P. Blay, M. Breuhaus, C. Brisbois, D. Burgess, S. Çabuk, F. Calore, T. Capistran Rojas, P. Caraveo, M. Cardillo, A. Carramiñana, M. Carreon Gonzalez, S. Casanova, F. Cassol, S. Chagren, P. Chambery, T. Chand, S. Chen, J.-G. Cheng, S. M. Colak, H. Costantini, R. Crocker, A. De Angelis, E. de la Fuente Acosta, E. de Oña Wilhelmi, A. De Sarkar, J. Devin, M. Di Mauro, B. Dingus, F. Donato, J. Eagle, C. Eckner, K. Egberts, G. Emery, A. Eungwanichayapant, C. Evoli, Y. Eweis, K. L. Fan, K. Fang, L. Fariña, Y. Feng, M. Fiori, H. Fleischhack, O. Fornieri, Y. Gallant, G. Giacinti, M. Gonzalez, E. Gotthelf, J. Goulart Coelho, D. Green, I. Grenier, P. Grespan, M.-H. Grondin, Y. Guo, N. Gupta, A. Hahn, H. Hamed, I. Herzog, J. Hinton, B. Hnatyk, W. Hofmann, B. Hona, D. Huang, Z. Huang, A. Jardin-Blicq, H. Jiachun, H. Jiankun, G. Johannesson, V. Joshi, F. Kamal Youssef, G. Kanbach, D. Khangulyan, B. Khelifi, S. Kisaka, T. Kleiner, J. Knödlseder, D. Kostunin, A. Kundu, M. Kuss, P. C. W. Lai, S. Lalkovski, F. Lavorenti, M. Lemoine-Goumard, F. Leone, M. Linares, T. Linden, R. Liu, S. Lloyd, R. López-Coto, I. Lypova, K. Malone, S. Manconi, V. Marandon, A. Marcowith, J. Martin, P. Martin, F. Massaro, R. Mirzoyan, A. Mitchell, K. Mori, G. Morlino, R. Mukherjee, K. Nakashima, L. Nava, A. Nayerhoda, M. Newbold, M. Nynka, B. Olmi, E. Orlando, Z. Ou, M. Pilia, F. Pintore, I. Plotnikov, T. Porter, R. R. Prado, E. Prandini, G. Principe, S. Profumo, H. Rahman K. K., B. Reville, C. Righi, G. Rodríguez Fernández, L. Romanato, B. Rudak, S. Safi-Harb, T. Saito, A. Sandoval, A. Scherer, P. Sharma, S. Silvestri, A. Sinha, H. Spackman, A. Spolon, G. Stratta, A. Strong, M. Strzys, T. Sudoh, X. Sun, P. H. T. Tam, S. Tanaka, F. Tavecchio, R. Terrier, L. Tibaldo, G. Tingting, D. Torres, R. Torres Escobedo, M. Tsirou, N. Tsuji, A. Tutone, A. J. van Marle, J. van Scherpenberg, G. Verna, J. Vink, E. Vurgun, S. M. Wagner, J. Wang, X. Wang, J. Xia, G. Zaharijas, S. Zane, R. Zanin, D. Zargaryan, D. Zaric, J. Zhang, Y. Zhang, Y. Zhang, H. Zhou).

R.L.-C. acknowledges the financial support of the European Union Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement 754496-FELLINI. R.L.-C. also acknowledges financial support from the State Agency for Research of the Spanish MCIU through the Centre of Excellence Severo Ochoa award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709). E.A. acknowledges support from ASI-INAF under grant 2017-14-H.0) and from INAF under grants PRIN SKA-CTA, INAF Mainstream 2018 and PRIN-INAF 2019.

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R.L.-C. and E.d.O.W. coordinated the manuscript writing. All authors meet the journal’s authorship criteria and have reviewed, discussed and commented on the content of the Review.

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Correspondence to Rubén López-Coto or Emma de Oña Wilhelmi.

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López-Coto, R., de Oña Wilhelmi, E., Aharonian, F. et al. Gamma-ray haloes around pulsars as the key to understanding cosmic-ray transport in the Galaxy. Nat Astron 6, 199–206 (2022). https://doi.org/10.1038/s41550-021-01580-0

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