Observability of Isolated Stellar-mass Black Holes

TL;DR

This paper explores the potential for detecting electromagnetic signals from isolated stellar-mass black holes in the Milky Way, suggesting many may already be in catalogs but remain unidentified due to detection challenges.

Contribution

It models the electromagnetic emissions of isolated black holes in various environments and assesses their detectability with current telescopes, proposing strategies for future identification.

Findings

IsoBHs in dense environments are detectable with current telescopes.

Many existing astronomical sources may be undiscovered IsoBHs.

Detection of IsoBHs remains challenging despite their potential brightness.

Abstract

Stellar-mass black holes (BHs) represent the natural end states of massive stars. It is estimated that $10^8$ stellar-mass BHs are present in the Milky Way galaxy, a significant fraction of which are expected to be isolated. Despite their expected abundance, only about 20 have been detected so far - mostly in binary systems - with just one confirmed isolated black hole (IsoBH) identified via astrometric microlensing. In this study, we investigate the potential for detecting electromagnetic emissions from IsoBHs by generating synthetic model spectra of their emissions in different types of interstellar medium environments. These model spectra are then compared with current observational capabilities. We show that photons emitted by IsoBHs - especially those accreting material in dense environments or within the Solar neighborhood - should be readily detectable. However, confidently identifying these sources remains highly challenging. We conclude that a number of IsoBHs must already exist in current astronomical catalogs but have not been identified as such. We outline possible strategies for detection and identification of IsoBHs using the current and upcoming telescopes.