X-ray Detectability of Accreting Isolated Black Holes in Our Galaxy

TL;DR

This study models the distribution and X-ray detectability of isolated black holes in our galaxy, suggesting future surveys could identify or constrain their population based on accretion signatures.

Contribution

It provides a realistic orbital and spatial distribution model of IBHs and estimates their X-ray detectability considering various galactic environments and parameters.

Findings

Detectable IBHs mainly near the Galactic Centre.

Current surveys may detect at most one IBH.

Future surveys could detect 30-100 IBHs under optimistic conditions.

Abstract

Detectability of isolated black holes (IBHs) without a companion star but emitting X-rays by accretion from dense interstellar medium (ISM) or molecular cloud gas is investigated. We calculate orbits of IBHs in the Galaxy to derive a realistic spatial distribution of IBHs, for various mean values of kick velocity at their birth $\upsilon_{\rm avg}$. X-ray luminosities of these IBHs are then calculated considering various phases of ISM and molecular clouds, for a wide range of the accretion efficiency $\lambda$ (a ratio of the actual accretion rate to the Bondi rate) that is rather uncertain. It is found that detectable IBHs mostly reside near the Galactic Centre (GC), and hence taking the Galactic structure into account is essential. In the hard X-ray band, where identification of IBHs from other contaminating X-ray sources may be easier, the expected number of IBHs detectable by the past survey by NuSTAR towards GC is at most order unity. However, 30--100 IBHs may be detected by the future survey by FORCE with an optimistic parameter set of $\upsilon_{\rm avg} = 50 \ \mathrm{km\ s^{-1}}$ and $\lambda = 0.1$, implying that it may be possible to detect IBHs or constrain the model parameters.