Rare Transients in Nearby Galaxies Explain Ultra-high-energy Cosmic Rays

Abstract

The origin of ultra-high-energy cosmic rays remains one of the central open questions in astroparticle physics. Recent measurements reveal anisotropies in arrival directions, a rigidity-dependent composition dominated by intermediate-mass nuclei, and striking hemispheric differences in the energy spectra. Here we show that \emph{rare transients in nearby galaxies} can naturally account for these features. In our fiducial neutron-star merger model, the cosmic ray flux above $25$~EeV is dominated by ten nearby galaxies within $8\,$Mpc. This accounts for the observed hotspots: seven of the ten brightest galaxies coincide with reported excess regions, a chance probability of $p\sim0.001$. Nearby transients can also explain the spectral excess of TA over Auger and modify the rigidity--aligned succession of isotopes.

Figures (4)

• Figure 1: Predicted UHECR skymap for $E>32$ EeV for our fiducial neutron star merger model. The locations of the top ten brightest galaxies are marked. Also shown are TA's Perseus--Pisces hotspot ($E>25$ EeV; near Andromeda; $20^\circ$ radius TAhotspot25), Auger's Cen A hotspot ($E>38$ EeV; near Cen A; $27^\circ$ radius; 2022ApJ...935..170A), TA's Ursa Major hotspot ($E>57$ EeV; near M81; $20^\circ$ radius; both from ta2014hotspot57 and 2016PhRvD..93d3011H), and Auger's excess around NGC253 (dashed circle; $E>40$ EeV; $25^\circ$ radius 2022ApJ...935..170A).
• Figure 2: Illustration of the transient transfer effect. The top panel illustrates the effect of a nearby transient source, with an intrinsic source spectrum following $E^{-2}$ and a uniform composition $Z=(1,7,26)$. The observable arrival time falls within a duration window $\Delta t \propto (Z/E)^2$, and the flux is suppressed by the inverse of this duration. Once the arrival time is outside this window, the flux drops to zero. The maximum observable energy is rigidity-dependent (see Eq. \ref{['eq:E_c']}). The bottom panel shows the case where multiple transients contribute to a hotspot. While the peaks are smeared out, a rigidity dependent features can persist for rates that are not too large.
• Figure 3: Spectrum from the combination of the two TA hotspots (inside the respective aperture of 25 and 20 deg), as shown in Kim:2025qmo. The southern sky average from Auger is also shown. Also shown is our flux model for Andromeda (M31) and M81, as well as the sum of the two with 0.75 of the Auger reference flux added in addition (see text for details).
• Figure A1: Cumulative distribution of the predicted neutron star merger rate with distance from Earth (solid line), compared to the expectation from a homogeneous source distribution (dashed line). The local overdensity produces a steep rise within the nearest tens of Mpc, implying that nearby galaxies dominate the expected UHECR flux.