# Can Isolated Single Black Holes Produce X-ray Novae?

## TL;DR

This paper proposes that isolated single black holes, not just those in binary systems, can produce X-ray novae through accretion from interstellar matter, supported by distribution calculations and event rate estimates.

## Contribution

It introduces a novel mechanism for X-ray novae originating from isolated black holes, expanding the understanding beyond binary system models.

## Key findings

- Accretion rates from interstellar matter can trigger disk instability in isolated BHs.
- The estimated event rate matches observed X-ray nova occurrences.
- Potential candidate sources include specific X-ray sources like IGR J17454-2919.

## Abstract

Almost all black holes (BHs) and BH candidates in our Galaxy have been discovered as soft X-ray transients, so-called X-ray novae. X-ray novae are usually considered to arise from binary systems. Here we propose that X-ray novae are also caused by isolated single BHs. We calculate the distribution of the accretion rate from interstellar matter to isolated BHs, and find that BHs in molecular clouds satisfy the condition of the hydrogen-ionizaiton disk instability, which results in X-ray novae. The estimated event rate is consistent with the observed one. Possible candidates include IGR J17454-2919, XTE J1908-094, and SAX J1711.6-3808. Near infrared photometric and spectroscopic follow-ups can exclude companion stars for a BH census in our Galaxy.