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# Ultrahigh-energy photons up to 1.4 petaelectronvolts from 12 γ-ray Galactic sources

## Abstract

The extension of the cosmic-ray spectrum beyond 1 petaelectronvolt (PeV; 1015 electronvolts) indicates the existence of the so-called PeVatrons—cosmic-ray factories that accelerate particles to PeV energies. We need to locate and identify such objects to find the origin of Galactic cosmic rays1. The principal signature of both electron and proton PeVatrons is ultrahigh-energy (exceeding 100 TeV) γ radiation. Evidence of the presence of a proton PeVatron has been found in the Galactic Centre, according to the detection of a hard-spectrum radiation extending to 0.04 PeV (ref. 2). Although γ-rays with energies slightly higher than 0.1 PeV have been reported from a few objects in the Galactic plane3,4,5,6, unbiased identification and in-depth exploration of PeVatrons requires detection of γ-rays with energies well above 0.1 PeV. Here we report the detection of more than 530 photons at energies above 100 teraelectronvolts and up to 1.4 PeV from 12 ultrahigh-energy γ-ray sources with a statistical significance greater than seven standard deviations. Despite having several potential counterparts in their proximity, including pulsar wind nebulae, supernova remnants and star-forming regions, the PeVatrons responsible for the ultrahigh-energy γ-rays have not yet been firmly localized and identified (except for the Crab Nebula), leaving open the origin of these extreme accelerators.

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## Data availability

The data supporting the conclusions of this paper are available through the LHAASO web page (http://english.ihep.cas.cn/lhaaso/index.html) in the section ‘Public Data’. All data are in ASCII, and the code used to produce the figures is publicly accessible and listed in the Public Data section.

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## Acknowledgements

This work is supported in China by National Key R&D programme of China under grants 2018YFA0404201, 2018YFA0404202, 2018YFA0404203 and 2018YFA0404204 and by NSFC (grant numbers 12022502, 11905227, U1931112, 11635011, 11761141001 and U2031105), and in Thailand by RTA6280002 from Thailand Science Research and Innovation. We thank all staff members who work year-round at the LHAASO site at 4,400 m above sea level to maintain the detector and keep the electricity power supply and other components of the experiment operating smoothly. We are grateful to Chengdu Management Committee of Tianfu New Area for constant financial support to research with LHAASO data.

## Author information

Authors

### Contributions

This work is the result of the contributions and efforts of all members and institutes of the LHAASO Collaboration under the leadership of Zhen Cao, who is the spokesperson of the LHAASO collaboration. Most of the listed 32 institutes participate in the construction of the LHAASO detectors. In particular, H.H.H. leads the design and construction of KM2A, whose scintillator detector array is constructed by teams led by X.D.S. and muon detector array is constructed by teams led by G.X. KM2A is operated by teams led by J.L. and X. Zuo. The calibration of LHAASO detectors is led by H.H.H. with the participation of groups from IHEP, led by H.K.L. and X. Zuo; from Shandong University, led by C.F.F.; from the University of Science and Technology of China, led by C. Li; and from Southwest Jiaotong University, led by H.Y.J. The CR event reconstruction is led by S.Z.C. with the participation of groups from IHEP, Shandong University and Southwest Jiaotong University. The data analysis by groups from IHEP and Shandong University is led by S.Z.C.; from Sun Yat-sen University by P.H.T.T.; and from the University of Science and Technology of China by R.Z.Y. The team at Purple Mt Observatory, led by Yi Zhang, performs an independent cross-check of the analysis. R.Z.Y. and R.Y.L. lead the interpretation of the results, with participation of the groups from Nanjing University, led by Y.C. and X.Y.W.; from Yunan University, led by L. Zhang; from Purple Mt Observatory, led by Q.Y. Zhen Cao, F.A.A. and B.D.P. supervised the data analysis and interpretation and also lead the paper writing. S.M.L. and D.d.V. contribute as co-chairs of the editorial board of LHAASO. Many groups participated in the analysis procedures, including the groups from Mahidol University, led by D.R.; from Shanghai Jiaotong University, led by H. Zhou; from Shanghai Observatory, led by Z.X.W.; from Peking University, led by Zhuo Li; from Guangxi University, led by E.W.L.; from National Observatory, led by W.W.T.; from Wuhan University, led by P. H. T. Tam; and from Yunnan Observatory, led by J.C.W. All other groups—from the University of Chinese Academy of Science and Tianfu Cosmic ray Research Center, led by Zhen Cao; from State Key Laboratory of Particle Detection and Electronics, led by Q.A.; from Tsinghua University, led by Y.N.L.; from the National Space Science Center, led by Z.B.S.; from Guangzhou University, led by J.H.F.; from Hebei Normal University, led by S.W.C.; from Zhengzhou University, led by H.D.L.; from Sichuan University, led by C.W.Y.; from Tibet University, led by T.L.C.; and from the Institute for Nuclear Research and Moscow Institute of Physics and Technology, led by Yu.V.S.—contributed to detector construction and to reviewing the final version before submission.

### Corresponding authors

Correspondence to Zhen Cao, F. A. Aharonian, S. Z. Chen, R. Y. Liu or R. Z. Yang.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature thanks Razmik Mirzoyan and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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

## Extended data figures and tables

### Extended Data Fig. 1 Schematic drawing of the LHAASO layout7.

Small red dots indicate the 5,195 scintillator counters, with a spacing of 15 m in the central area of 1 km2 and of 30 m in the skirt area of 0.3 km2 of KM2A. Big blue dots indicate the 1,188 muon detectors distributed in the central area with a spacing of 30 m. The three light-blue rectangles in the centre indicate WCDA, of 78,000 m2 in total. Small black rectangles near WCDA indicate 18 telescopes of WFCTA.

### Extended Data Fig. 2 γ-ray energy distributions in SED bins and corresponding bin purity.

a, Distributions of thrown-in energies Etrue in the simulation of events that are in the bins, defined by reconstructed energies Erec above 10 TeV. The fraction on the vertical axis is defined as (dN/dEtrue)/Ni, where Ni is the total number of events in the ith bin, defined by the reconstructed energy Erec. A bin width of Δ(logErec) = 0.2 is selected, according to the energy resolution of 14% above 100 TeV (ref. 8). A power-law spectrum proportional to E−3.09 is assumed here. b, Bin purity, defined as the fraction of events with Etrue in the bin, as a function of Erec in the range from 10 TeV to 2.5 PeV.

### Extended Data Fig. 3 Distributions (dots) of events in the reconstructed energy bins from 10 TeV to 10 PeV.

The input (solid grey line) events are generated using the power-law SED determined from the measured SED of the Crab Nebula8. For each case, the input SED has an artificial cutoff (dashed lines) at the Ecut values listed in the key. The distributions demonstrate a clear spillover effect by the events in the bins above Ecut. The effect becomes weaker at higher energies. There is no indication of pollution of the bins above 1 PeV by events at input energies lower than 0.3 PeV.

### Extended Data Fig. 4 LHAASO sky map at energies above 100 TeV.

The circles indicate the positions of known very-high-energy γ-ray sources.

### Extended Data Fig. 5 Phenomenological fits to the γ−ray observations of LHAASO J1908+0621, and previous observations of potential counterparts.

The inset shows the KM2A significance map, indicating the potential counterparts of the UHE γ-ray source. The colour bar shows the significance ($$\sqrt{{\rm{TS}}}$$). The green circle indicates the PSF of LHAASO. The Fermi LAT points for LHAASO J1908+0621 analysed in this work, as well as ARGO48, HESS49 and HAWC4 data, are shown together with the LHAASO measurements. The dotted curve shows the leptonic model of radiation, assuming an injection of electron/positron pairs according to the pulsar’s spin-down behaviour, with a breaking index of 2 and an initial rotation period of 0.04 s. A fraction of 6% of the current spin-down power of the pulsar PSR J1907+0602 at a distance of 2.4 kpc is assumed to be converted to e± pairs to support the γ-ray emission. The injection spectrum of electrons is assumed to be $$N(E)\propto {E}_{e}^{-1.75}\exp \{-{[{E}_{e}/(800{\rm{TeV}})]}^{2}\}$$. The solid curves correspond to the hadronic model of radiation. Two types of energy distributions are assumed for the parent proton population: (i) a single power-law spectrum of parent protons, N(E) ≈ E−1.85exp[−E/(380 TeV)] (thin solid curve); (ii) a broken power-law spectrum with an exponential cutoff of parent protons, with indices 1.2 and 2.7 below and above 25 TeV, respectively, and a cutoff energy of 1.3 PeV (thick solid curve). In the inset sky map, the black diamond shows the position of PSR J1907+0602, the black contours correspond to the location of supernova remnant SNR G40.5-0.5 and the white circle is the position and size of HESS J1908+063. The cyan regions are the dense clumps described in Methods. The average density in the whole γ-ray emission region is estimated to be about 10 cm−3. γ-ray absorption due to photon–photon pair production (see Methods) is taken into account in the theoretical curve.

### Extended Data Fig. 6 γ-ray opacity of LHAASO J2226+6057, J1908+0621, J1825-1326 and Crab Nebula.

The absorption due to both ISRFs and CMB is taken into account.

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Cao, Z., Aharonian, F.A., An, Q. et al. Ultrahigh-energy photons up to 1.4 petaelectronvolts from 12 γ-ray Galactic sources. Nature 594, 33–36 (2021). https://doi.org/10.1038/s41586-021-03498-z

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• DOI: https://doi.org/10.1038/s41586-021-03498-z

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