Galactic Isolated Stellar-Mass Black Holes with the Magnetospheric Spark Gap as Possible GeV-TeV Gamma-ray Unidentified Sources

Abstract

Billions of isolated stellar-mass black holes (IBHs) are thought to wander through the interstellar medium (ISM) in the Galaxy, yet only one has been detected. IBHs embedded in ISM would accrete gas via Bondi-Hoyle-Littleton accretion, and with efficient magnetic flux accumulation, the magnetosphere would be formed in the vicinity of IBHs. We explore the detectability of such IBHs through high-energy gamma rays from spark gaps in their magnetospheres based on our recent numerical simulation. The gap gamma rays can be bright at the GeV-TeV energies when IBHs are in the dense ISM. About $10^3$ and $10$ IBHs might be contained in unidentified objects of the $\textit{Fermi}$ Large Area Telescope and the High Energy Stereoscopic System, respectively. A future Galactic plane survey by the Cherenkov Telescope Array Observatory would lead to $\sim10^2$ detections. We also evaluate the combined gamma-ray emission of IBHs in the Galaxy and find that the IBHs may contribute to the Galactic diffuse gamma rays. IBHs will emit optical and X-ray photons from their accretion disk as counterparts, potentially useful for identifying candidates.