High Energy Astrophysical Phenomena (astro-ph.HE)

Abstract: The IceCube Observatory provides our highest-statistics picture of the cosmic-ray arrival directions in the Southern Hemisphere, with over 700 billion cosmic-ray-induced muon events collected between May 2011 and May 2022. Using the larger data volume, we find an improved significance of the PeV cosmic ray anisotropy down to scales of $6^\circ$. In addition, we observe a variation in the angular power spectrum as a function of energy, hinting at a relative decrease in large-scale features above 100 TeV. The data-taking period covers a complete solar cycle, providing new insight into the time variability of the signal. We present preliminary results using this up-to-date event sample.

Abstract: I consider the current sample of galaxy nuclei producing quasiperiodic eruptions (QPEs). If the quasiperiod results from the orbital motion of a star around the central black hole, the dearth of associated black hole masses $\gtrsim 10^6\msun$ places tight constraints on models. It disfavours those assuming wide orbits and small eccentricities, because there is ample volume within pericentre to allow significantly more massive holes in QPE systems than are currently observed. If instead the orbiting star is assumed to pass close to the black hole, the same lack of large black hole masses strongly suggests that the stellar orbits must be significantly eccentric, with $1 - e \lesssim {\rm few}\times 10^{-2}$. This favours a tidal disruption near-miss picture where QPEs result from repeated accretion from an orbiting star (in practice a white dwarf) losing orbital angular momentum to gravitational radiation, even though this is not assumed in deriving the eccentricity constraint. Given the tight constraints resulting from the current small observed sample, attempts to find QPE systems in more massive galaxies are clearly important.