# Primordial Black Holes and $r$-Process Nucleosynthesis

## TL;DR

This paper proposes that interactions between primordial black holes and neutron stars can produce r-process elements and observable electromagnetic transients, offering a novel explanation for certain astrophysical phenomena and the origin of heavy elements.

## Contribution

It introduces a new mechanism where primordial black holes induce neutron star destruction, leading to r-process nucleosynthesis and unique electromagnetic signals, distinct from traditional merger models.

## Key findings

- Primordial black hole interactions can produce r-process elements in neutron stars.
- Ejected neutron-rich matter can generate kilonova-like transients and radio bursts.
- The scenario aligns with pulsar statistics, dark matter distribution, and observed 511-keV emission.

## Abstract

We show that some or all of the inventory of $r$-process nucleosynthesis can be produced in interactions of primordial black holes (PBHs) with neutron stars (NSs) if PBHs with masses ${10}^{-14}\,{\rm M}_\odot < {\rm M}_{\rm PBH} < {10}^{-8}\,{\rm M}_\odot$ make up a few percent or more of the dark matter. A PBH captured by a neutron star (NS) sinks to the center of the NS and consumes it from the inside. When this occurs in a rotating millisecond-period NS, the resulting spin-up ejects $\sim 0.1-0.5\,{\rm M}_{\odot}$ of relatively cold neutron-rich material. This ejection process and the accompanying decompression and decay of nuclear matter can produce electromagnetic transients, such as a kilonova-type afterglow and fast radio bursts. These transients are not accompanied by significant gravitational radiation or neutrinos, allowing such events to be differentiated from compact object mergers occurring within the distance sensitivity limits of gravitational wave observatories. The PBH-NS destruction scenario is consistent with pulsar and NS statistics, the dark matter content and spatial distributions in the Galaxy and Ultra Faint Dwarfs (UFD), as well as with the $r$-process content and evolution histories in these sites. Ejected matter is heated by beta decay, which leads to emission of positrons in an amount consistent with the observed 511-keV line from the Galactic Center.