Isolated Black Holes as Potential PeVatrons and Ultrahigh-energy Gamma-ray Sources

TL;DR

This paper proposes that isolated black holes in molecular clouds could accelerate cosmic rays to PeV energies, explaining the dark gamma-ray sources detected by LHAASO without GeV-TeV counterparts.

Contribution

It introduces a novel scenario where isolated black holes with magnetically arrested disks produce PeV cosmic rays and dark gamma-ray sources, linking black hole physics with cosmic ray origins.

Findings

IBHs can accelerate protons to PeV energies.

Sub-PeV protons escape and produce gamma rays in molecular clouds.

The scenario explains LHAASO dark gamma-ray sources.

Abstract

The origin of PeV cosmic rays is a long-standing mystery, and ultrahigh-energy gamma-ray observations would play a crucial role in identifying it. Recently, LHAASO reported the discovery of ``dark'' gamma-ray sources that were detected above 100 TeV without any GeV--TeV gamma-ray counterparts. The origins of these dark gamma-ray sources are unknown. We propose isolated black holes (IBHs) wandering in molecular clouds as the origins of PeV cosmic rays and LHAASO dark sources. An IBH accretes surrounding dense gas, which forms a magnetically arrested disk (MAD) around the IBH. Magnetic reconnection in the MAD can accelerate cosmic-ray protons up to PeV energies. Cosmic-ray protons of GeV-TeV energies fall to the IBH, whereas cosmic-ray protons at sub-PeV energies can escape from the MAD, providing PeV CRs into the interstellar medium. The sub-PeV cosmic-ray protons interact with the surrounding molecular clouds, producing TeV-PeV gamma rays without emitting GeV-TeV gamma rays. This scenario can explain the dark sources detected by LHAASO. Taking into account the IBH and molecular cloud distributions in our Galaxy, we demonstrate that IBHs can provide a significant contribution to the PeV cosmic rays observed on Earth. Future gamma-ray detectors in the southern sky and neutrino detectors would provide a concrete test to our scenario.