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HAWC observations of the acceleration of very-high-energy cosmic rays in the Cygnus Cocoon

An Author Correction to this article was published on 06 April 2021

This article has been updated

Abstract

Cosmic rays with energies up to a few PeV are known to be accelerated within the Milky Way1,2. Traditionally, it has been presumed that supernova remnants were the main source of these very-high-energy cosmic rays3,4, but theoretically it is difficult to accelerate protons to PeV energies5,6 and observationally there simply is no evidence of the remnants being sources of hadrons with energies above a few tens of TeV7,8. One possible source of protons with those energies is the Galactic Centre region9. Here, we report observations of 1–100 TeV γ rays coming from the ‘Cygnus Cocoon’10, which is a superbubble that surrounds a region of massive star formation. These γ rays are likely produced by 10–1,000 TeV freshly accelerated cosmic rays that originate from the enclosed star-forming region Cyg OB2. Until now it was not known that such regions could accelerate particles to these energies. The measured flux likely originates from hadronic interactions. The spectral shape and the emission profile of the Cocoon changes from GeV to TeV energies, which reveals the transport of cosmic particles and historical activity in the superbubble.

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Fig. 1: Significance map of the Cocoon region before and after subtraction of the known sources at the region.
Fig. 2: Spectral energy distribution of the γ-ray emission and cosmic ray density at the Cocoon region.

Data availability

The datasets analysed during this study and the scripts used are available at a public data repository (https://github.com/binitahona/Cocoon-paper).

Code availability

The study was carried out by using the Analysis and Event Reconstruction Integrated Environment Likelihood Fitting Framework (AERIE-LiFF), the Multi-Mission Maximum Likelihood (3ML) software and the HAWC Accelerated Likelihood (HAL) framework. The code is open-source and publicly available on Github at https://github.com/rjlauer/aerie-liff, https://github.com/threeML/threeML and https://github.com/threeML/hawc_hal. The software includes instructions on installation and usage.

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Acknowledgements

We acknowledge the support from the United States National Science Foundation; the United States Department of Energy Office of High-Energy Physics; the Laboratory Directed Research and Development program of Los Alamos National Laboratory; Consejo Nacional de Ciencia y Tecnología in Mexico, grant nos. 271051, 232656, 260378, 179588, 254964, 258865, 243290, 132197, A1-S-46288 and A1-S-22784, cátedras 873, 1563, 341 and 323, Red HAWC, Mexico; Dirección General Asuntos del Personal Académico, Universidad Nacional Autónoma de México, grant nos. IG101320, IN111315, IN111716-3, IN111419, IA102019 and IN112218; Vicerrectoría de Investigación y Estudios de Posgrado de la Benemérita Universidad Autónoma de Puebla; Programa Integral de Fortalecimiento Institucional (PIFI) 2012–2013 and Programa de Fortalecimiento de la Calidad Educativa (PROFOCIE) 2014–2015; the University of Wisconsin Alumni Research Foundation; the Institute of Geophysics, Planetary Physics, and Signatures at Los Alamos National Laboratory; the Polish Science Centre, grant no. DEC-2017/27/B/ST9/02272; Coordinación de la Investigación Científica de la Universidad Michoacana; Coordinación General Académica y de Innovación (CGAI-UDG; SEP-PRODEP-UDG-CA-499); the Royal Society, Newton Advanced Fellowship 180385; and Generalitat Valenciana, grant no. CIDEGENT/2018/034. We thank S. Delay, L. Díaz and E. Murrieta for technical support, and thank S. Digel for helpful discussion regarding the source modelling of the Cygnus Cocoon region in the Fermi 4FGL catalogue.

Author information

Affiliations

Authors

Contributions

B.H. analysed the HAWC data and performed the maximum-likelihood fit of the multi-source model, the hadronic model fit and the cosmic ray density study. H.F. and P.H. helped in the development of the multi-source model and in the scientific interpretations of the fit results. K.F. and R.B. helped develop the hadronic emission models and helped in the interpretations of the model fit results. K.F. also developed the leptonic emission model and provided its interpretations. S.C. motivated the Cocoon analysis, helped with the interpretations of the leptonic model and performed the diffusion coefficient suppression study at the Cocoon region. B.H., K.F. and S.C. prepared the manuscript. The full HAWC Collaboration group contributed through the construction, calibration and operation of the detector, the development and maintenance of reconstruction and analysis software, and the vetting of the analysis presented in this manuscript. All authors reviewed, discussed and commented on the results and the manuscript.

Corresponding authors

Correspondence to R. Blandford or S. Casanova or K. Fang or H. Fleischhack or B. Hona or P. Hüntemeyer.

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The authors declare no competing interests.

Additional information

Peer review information Nature Astronomy thanks Songzhan Chen, Emma de Oña Wilhelmi and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Leptonic modelling at the Cocoon region.

Multi-wavelength observations of the Cygnus Cocoon (15; 17; 19) constrain the Synchrotron and Bremsstrahlung radiation of relativistic electrons. The light grey curves correspond to a ‘minimum leptonic model’, where only γ-rays above 1 TeV are explained by electron emission. The electron population is assumed to follow a power-law energy spectrum dN/dE E−2 in a region with magnetic field B= 20 μG and gas density n = 30cm−3 as in the Cocoon (10). The leptonic emission consists of the Synchroton radiation (solid, from radio to hard X-ray), Bremsstrahlung emission (thick dash-dotted), and inverse-Compton scattering of the dust emission in the Cocoon (dashed) and the radiation fields of the two stellar clusters, NGC 6910 (thin dash-dotted) and OB2 (dotted). Observations between 0.1-100 GeV are explained by hadronic interaction (black dashed curve). The red points are the GeV flux points by Fermi-LAT and the blue circles are the HAWC flux points with 1σ statistical errors. The sum of the emission above ~ 0.3 GeV is indicated by the black solid curve. In the inner plot, the blue circles indicate the γ-ray luminosity for the four rings at the Cocoon region and the light grey solid curve is the TeV γ-ray luminosity from the model.

Supplementary information

Supplementary Information

Supplementary Tables 1–3 and Figs. 1–3.

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Abeysekara, A.U., Albert, A., Alfaro, R. et al. HAWC observations of the acceleration of very-high-energy cosmic rays in the Cygnus Cocoon. Nat Astron 5, 465–471 (2021). https://doi.org/10.1038/s41550-021-01318-y

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