Evidence for particle acceleration approaching PeV energies in the W51 complex

Theγ-ray emission from the W51 complex is widely acknowledged to be attributed to the interaction between the cosmic rays(CRs)accelerated by the shock of supernova remnant(SNR)W51C and the dense molecular clouds in the adjacent star-forming region,W51B.However,the maximum acceleration capability of W51C for CRs remains elusive.Based on observations conducted with the Large High Altitude Air Shower Observatory(LHAASO),we report a significant detection ofγrays emanating from the W51 complex,with energies from 2 to 200 TeV.The LHAASO measurements,for the first time,extend theγ-ray emission from the W51 complex beyond 100 TeV and reveal a significant spectrum bending at tens of TeV.By combining the"π^(0)-decay bump"featured data from Fermi-LAT,the broadbandγ-ray spectrum of the W51 region can be well-characterized by a simple pp-collision model.The observed spectral bending feature suggests an exponential cutoff at~400 TeV or a power-law break at~200 TeV in the CR proton spectrum,most likely providing the first evidence of SNRs serving as CR accelerators approaching the PeV regime.Additionally,two young star clusters within W51B could also be theoretically viable to produce the most energeticγrays observed by LHAASO.Our findings strongly support the presence of extreme CR accelerators within the W51 complex and provide new insights into the origin of Galactic CRs.

Optimization of performance of the KM2A full array using the Crab Nebula

The full array of the Large High Altitude Air Shower Observatory(LHAASO)has been in operation since July 2021.For its kilometer-square array(KM2A),we optimized the selection criteria for very high and ultrahigh energyγ-rays using data collected from August 2021 to August 2022,resulting in an improvement in significance of the detection in the Crab Nebula of approximately 15%,compared with that of previous cuts.With the implementation of these new selection criteria,the angular resolution was also significantly improved by approximately 10%at tens of TeV.Other aspects of the full KM2A array performance,such as the pointing error,were also calibrated using the Crab Nebula.The resulting energy spectrum of the Crab Nebula in the energy range of 10-1000 TeV are well fitted by a log-parabola model,which is consistent with the previous results from LHAASO and other experiments.