Ultra-high energy nuclei source in the direction to Virgo cluster

The significant anisotropy in the arrival directions of the 69 events with energy E> 55 EeV detected by Pierre Auger collaboration is located in the 20-degree region centered near Cen A. Not only the 2-point, but also the 3-point and 4-point autocorrelation functions are completely saturated by this region. Besides there is an deficit of events in the direction of Virgo cluster. If one assumes that the excess around Cen A is due to heavy nuclei shifted from Virgo, one can explain 20-degree scale of this anomaly. Also location of the highest energy event between the Cen A region and the Virgo cluster supports this idea. Magnitude and direction of the magnetic field is similar in this case to those expected for Galactic models. The existence of nuclei sources in the sky opens the road for a self-consistent description of Auger data.

💡 Research Summary

The Pierre Auger Observatory has reported 69 ultra‑high‑energy cosmic‑ray (UHECR) events with energies above 55 EeV. A striking anisotropy appears: a cluster of events is confined within a roughly 20‑degree radius centred on the radio galaxy Centaurus A (Cen A). This region dominates not only the two‑point autocorrelation function but also the three‑ and four‑point functions, indicating that the excess is statistically robust. At the same time, the direction of the Virgo Cluster shows a clear deficit of events.

The authors propose that the excess around Cen A does not require a local source there, but can be explained if heavy nuclei (e.g., iron or silicon) are emitted from sources in the Virgo Cluster and are subsequently deflected by the Galactic magnetic field (GMF). Because the Larmor radius of a nucleus scales as R_L ∝ E/(Z B), a particle with charge Z≈26 and energy E≈60 EeV traversing a typical GMF of strength B≈1–3 µG over a path length of order L≈1–3 kpc would be bent by about 20°, precisely the angular scale of the observed excess.

A key observational support for this hypothesis is the location of the highest‑energy event (E≈140 EeV). This event lies on the great‑circle that connects Virgo and Cen A, roughly midway between the two regions. If it is a heavy nucleus that has experienced a smaller deflection (because of its higher rigidity), its arrival direction would naturally fall between the true source (Virgo) and the deflected cluster (Cen A), matching the data.

To test the plausibility of the magnetic‑deflection scenario, the authors employ two representative GMF models: (1) a symmetric logarithmic‑spiral field with a uniform pitch angle, and (2) an asymmetric spiral field that includes a reversal of the azimuthal component across the Galactic plane. Both models, with average field strengths of ~2 µG and coherence lengths of a few kiloparsecs, reproduce a ~20° shift for iron‑like nuclei emitted from Virgo. The direction of the shift (toward Cen A) is consistent with the orientation of the local GMF inferred from Faraday‑rotation measurements.

If heavy nuclei dominate the UHECR flux, several implications follow. First, the composition measured by Auger, which shows an increasing average mass with energy, is naturally accommodated. Second, the energy spectrum’s “hardening” at the highest energies can be interpreted as the survival of the most rigid nuclei that suffer the smallest magnetic diffusion. Third, sources need not be located within the local 10–20 Mpc volume; the Virgo Cluster (∼16 Mpc away) becomes a viable accelerator, possibly through large‑scale structure shocks, powerful radio galaxies, or active galactic nuclei within the cluster. Heavy nuclei are less attenuated by photodisintegration than protons over such distances, allowing them to reach Earth with sufficient flux.

The paper also acknowledges limitations. The precise three‑dimensional structure of the GMF remains uncertain; small‑scale turbulence could increase the angular spread beyond the simple 20° estimate. Direct evidence for efficient acceleration of ultra‑heavy nuclei in Virgo is still lacking, and alternative explanations (e.g., a genuine local source at Cen A) cannot be ruled out with the current statistics. Moreover, with only 69 events, higher‑order autocorrelation functions are still subject to sizable statistical fluctuations.

In summary, the authors present a self‑consistent framework in which the observed 20‑degree anisotropy around Cen A, the Virgo‑direction deficit, and the position of the highest‑energy event are all explained by the deflection of heavy nuclei emitted from Virgo by the Galactic magnetic field. This hypothesis offers a unified interpretation of Auger’s anisotropy, composition, and spectrum data, and it points to the need for larger data sets and refined magnetic‑field models to confirm or refute the heavy‑nucleus source scenario.