Energy-Dependent Shifts of Medium-Scale Anisotropies in Very-High-Energy Cosmic Rays Observed by LHAASO-KM2A

Abstract

Small deviations from isotropy in the arrival directions of Galactic cosmic rays serve as a unique probe of the local magnetic environment. In this Letter, we report observations of medium-scale anisotropies (MSA) at energies above 10 TeV using the LHAASO-KM2A array. Our analysis identifies four regions of excess and four regions of deficit, each spanning angular scales of approximately ten degrees. Crucially, we detect significant energy-dependent shifts in the centroids of two excess regions: Region B and the newly identified Region $\mathrm{\widetilde{D}}$. We also characterize the energy evolution of the fractional relative intensity across both excess and deficit regions. These findings imply that the observed anisotropies are shaped by the specific realization of the local turbulent magnetic field within the cosmic ray scattering length. Such energy-dependent behaviors impose strict constraints on local turbulence models and cosmic ray propagation theories.

Figures (4)

• Figure 1: Significance sky map of medium-scale anisotropy in equatorial coordinates with a median energy of $\sim35\,\mathrm{TeV}$. The map is smoothed with an $18^\circ$ radius top-hat function. The excess regions A, B, C, and $\mathrm{\widetilde{D}}$ exhibit significances of $14.2\,\sigma$, $30.6\,\sigma$, $14.4\,\sigma$, and $14.3\,\sigma$, respectively, while the deficit regions I, II, III, and IV show significances of $-16.7\,\sigma$, $-25.1\,\sigma$, $-29.1\,\sigma$, and $-15.1\,\sigma$.
• Figure 2: Significance sky maps of medium-scale anisotropy in equatorial coordinates. Each map is smoothed with an $18^\circ$ top-hat function. The maps in each row correspond to the bins in Table \ref{['tab:binning']} with median energies of 16.5 TeV, 23.8 TeV, 39.9 TeV, 69.9 TeV, and 159.2 TeV. The dashed line shows the evolution trend for Region $\mathrm{\widetilde{D}}$.
• Figure 3: Energy-dependent positions of Region B and Region $\mathrm{\widetilde{D}}$. The vertical axis represents declination for Region B and right ascension for Region $\mathrm{\widetilde{D}}$. The error bars indicate the total uncertainty, derived from the quadratic sum of the statistical and systematic components. The dashed-dotted lines is the best fit of 0th-order polynomial fitting. The dashed lines and light blue/red areas represent the best fit and the 68% confidence level for 1st-order polynomial fitting, respectively. For Region B, $\chi^{2}_{0} = 135.6$ and $\chi^{2}_{1} = 3.6$; for Region $\mathrm{\widetilde{D}}$, $\chi^{2}_{0} = 245.8$ and $\chi^{2}_{1} = 2.6$.
• Figure 4: Average fractional relative intensity of excess and deficit regions for each energy bin. The ROIs used to calculate the average fractional relative intensity are defined by a statistical significance of above $7\sigma$ for excess regions and below $-7\sigma$ for deficit regions. For Regions B and D, the definition of the ROIs is energy-dependent.