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

Can Isolated Single Black Holes Produce X-ray Novae?