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.

Effects of MgO modified β-TCP nanoparticles on the microstructure and properties of β-TCP/Mg-Zn-Zr composites

The mechanical properties and corrosion resistance of magnesium alloy composites were improved bythe addition of MgO surface modified tricalcium phosphate ceramic nanoparticles (m-β-TCP). Mg-3Zn-0.8Zr composites with unmodified (MZZT) and modified (MZZMT) nanoparticles were produced byhigh shear mixing technology. Effects of MgO m-β-TCP nanoparticles on the microstructure, mechanicalproperties, electrochemical corrosion properties and cytocompatibility of Mg-Zn-Zr/β-TCP compositeswere investigated. After hot extrusion deformation and dynamic recrystallization, the grain size ofMZZMT was the half size of MZZT and the distribution of m-β-TCP particles in the matrix was moreuniform than β-TCP particles. The yield tensile strength (YTS), ultimate tensile strength (UTS), andcorrosion potential (Ecorr) of MZZMT were higher than MZZT;the corrosion current density (Icorr) ofMZZMT was lower than MZZT. Cell proliferation of co-cultured MZZMT and MZZT composite sampleswere roughly the same and the cell number at each time point is higher for MZZMT than for MZZTsamples.