LHAASO Constraints on New-Source Cosmic Rays:Abundance Scenarios and Injection Spectral Indices

Abstract

Recent LHAASO observations indicate that microquasars and massive star clusters contribute to Galactic cosmic rays, and likely also to the pre-knee spectral hardening. Their markedly different astrophysical environments, compared with supernova remnants (SNRs), may imprint distinct cosmic-ray abundances. We investigate the spectra and composition of new-source cosmic rays under two abundance scenarios: Case-1, the SNR-like composition with enhanced heavy nuclei, and Case-2, the surface abundances of microquasar companion stars. We constrain injection spectral indices using LHAASO proton, helium, and all-particle spectra, and use their <lnA> data for model discrimination. Under the assumption of a unified heavy-element injection spectrum, Case-2 fits the <lnA> data significantly better than Case-1, but at the cost of large, physically unmotivated differences between heavy-element and He injection indices. This suggests the need to either relax the unified heavy-element injection-index assumption or explore alternative abundance patterns. We predict that if the new-source composition follows Case-1, the individual elemental spectral indices before the knee should remain nearly consistent; if it follows Case-2, the Mg, Si, and Fe spectra will be significantly softer than those of He, C, and O. Future element-resolved energy-spectrum measurements by HERD and LHAASO will provide a decisive test of this discriminating feature.