Ultrahigh-energy cosmic rays from tidally-ignited white dwarfs

Ultrahigh-energy cosmic rays (UHECRs) can be accelerated by tidal disruption events of stars by black holes. We suggest a novel mechanism for UHECR acceleration wherein white dwarfs (WDs) are tidally compressed by intermediate-mass black holes (IMBHs), leading to their ignition and subsequent explosion as a supernova. Cosmic rays accelerated by the supernova may receive an energy boost when crossing the accretion-powered jet. The rate of encounters between WDs and IMBHs can be relatively high, as the number of IMBHs may be substantially augmented once account is taken of their likely presence in dwarf galaxies. Here we show that this kind of tidal disruption event naturally provides an intermediate composition for the observed UHECRs, and suggest that dwarf galaxies and globular clusters are suitable sites for particle acceleration to ultrahigh energies.

Figure 1

Spectrum (left), $⟨X_{\max }⟩$ (right), for models I and II. These plots are for the case of ${\mathcal{R}}_{\max }=8\mathrm{EV}$, assuming that sources evolve following the star formation rate. Spectral indices are $\alpha =1.2$ (sim II) and $\alpha =0.4$ (sim I). The spectrum is arbitrarily normalized with respect to Auger data at $E=10\mathrm{EeV}$. Hatched gray bands represent systematic uncertainties.