Can isolated single black holes produce X-ray novae?

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-ionization disk instability, which results in X-ray novae. The estimated event rate is consistent with the observed one. We also check an X-ray novae catalog and find that 16/59 ∼ 0.27 of the observed X-ray novae are potentially powered by isolated BHs. The 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.

Multi-wavelength astronomical searches for primordial black holes

“…As such we cannot be certain that such structures will not form around accreting isolated PBHs, and that this would not lead to the thermal instabilities expected in dense, thin accretion disks. The possibility of isolated BHs accreting from the ISM exhibiting such transient behaviour has been considered already [51,52]. If density perturbations within the CMZ exist on similar size scales to the Bondi sphere, the specific angular momentum of the gas may be sufficient for the gas to circularize prior to accreting [53].…”

“…If density perturbations within the CMZ exist on similar size scales to the Bondi sphere, the specific angular momentum of the gas may be sufficient for the gas to circularize prior to accreting [53]. Whilst [52] suggest that gas densities in the accretion flows of isolated BHs can become sufficient to trigger Hydrogen ionization instabilities (and thus transient outbursts), as shown by [51], the lack of detection of transient isolated systems (be they PBHs, astrophysical BHs, or Neutron stars) makes it likely that only a small fraction of isolated BHs actually exhibit outbursts of this nature. We have neglected this possibility, but note that folding in the predicted outburst rates of PBHs in the CMZ may have the effect of increasing the strength of the PBH DM bounds.…”

X-ray detectability of accreting isolated black holes in our Galaxy

“…Thus we cannot simply assume that IBHs would follow this constraint. Some studies (e.g., Fujita et al 1998;Agol & Kamionkowski 2002;Ioka et al 2017;Matsumoto et al 2018;Inoue & Kusenko 2017) used observations of the density (Armstrong et al 1995) or velocity fluctuations (Larson 1981) of the interstellar medium, deriving that R out is much smaller than the Bondi radius. They obtain R out ∼ 10 5 R s , which gives a range λ = 10 −4 -10 −2 for p = 0.5-1.…”

“…There have been many studies on the detectability of such accreting INSs (Ostriker et al 1970;Treves & Colpi 1991;Blaes & Madau 1993;Treves et al 2000;Perna et al 2003;Ikhsanov & Biermann 2007) and IBHs (Shvartsman 1971;Grindlay 1978;Carr 1979;McDowell 1985;Campana & Pardi 1993;Popov & Prokhorov 1998;Fujita et al 1998 Barkov et al 2012;Fender et al 2013;Ioka et al 2017;Matsumoto et al 2018). A number of observational searches have also been performed in the past for such accreting INSs and IBHs (Stocke et al 1995;Walter et al 1996;Wang 1997;Schwope et al 1999;Chisholm et al 2003;Muno et al 2006).…”

Radio emission from accreting isolated black holes in our galaxy

“…IBHs have thus mainly focused on accreting sources (Shvartsman 1971;McDowell 1985;Campana & Pardi 1993;Popov & Prokhorov 1998;Fujita et al 1998;Armitage & Natarajan 1999;Agol & Kamionkowski 2002;Maccarone 2005;Mii & Totani 2005;Barkov et al 2012;Fender et al 2013;Ioka et al 2017;Matsumoto et al 2018;Tsuna et al 2018). Although past searches using X-ray data have found candidates of accreting IBHs (Chisholm et al 2003;Muno et al 2006), no strong candidates are known at present.…”