Deciphering the Dipole Anisotropy of Galactic Cosmic Rays

Recent measurements of the dipole anisotropy in the arrival directions of Galactic cosmic rays (CRs) indicate a strong energy dependence of the dipole amplitude and phase in the TeV–PeV range. We argue here that these observations can be well understood within standard diffusion theory as a combined effect of (i) one or more local sources at Galactic longitude $120°\lesssim l\lesssim 300°$ dominating the CR gradient below 0.1–0.3 PeV, (ii) the presence of a strong ordered magnetic field in our local environment, (iii) the relative motion of the solar system, and (iv) the limited reconstruction capabilities of ground-based observatories. We show that an excellent candidate of the local CR source responsible for the dipole anisotropy at 1–100 TeV is the Vela supernova remnant.

Figure 1

Summary plot of the reconstructed TeV–PeV dipole components ${\delta }_{0\mathrm{h}}^{\star }$ and ${\delta }_{6\mathrm{h}}^{\star }$ in the equatorial plane. The black arrow indicates the Compton-Getting effect from the solar motion with respect to the local standard of rest that we subtracted from the data following Eq. (3). We follow Eq. (8) to rescale the declination-averaged data of ARGO-YBJ [5], Tibet-AS$\gamma$ [9], and IceCube/IceTop [7, 8]. We also include results by EAS-TOP [6] derived by the East-West method [32]. The numbers attached to the data indicate the median energy of the bins as ${\log }_{10}(E_{\text{med}}/\mathrm{TeV})$. The colored disks show the $1\sigma$ error range estimated by Eq. (9). The dashed line and gray-shaded area indicate the magnetic field direction and its uncertainty (projected onto the equatorial plane) inferred from IBEX observations [21]. We also indicate the direction towards the Galactic center (GC).

Figure 2

An example showing the effect of the Vela SNR on the average anisotropy from all Galactic SNRs. The data points show the projected TeV–PeV DA in equal logarithmic energy steps. The red triangles and green stars shows the anisotropy evolution with and without magnetic field projections, respectively. The inset plot shows the amplitude of the two simulations. The blue dotted line shows the naive expectation of the full three-dimensional dipole amplitude without projection onto the magnetic field and into the equatorial plane.