Searching for Isolated Black Hole Candidates within 15 pc of the Solar System in Gaia DR3

TL;DR

This work assesses the feasibility of finding isolated stellar-mass black holes within 15 pc by examining ISM accretion physics and performing a Gaia DR3-based search. It combines Bondi-Hoyle-Lyttleton accretion with ADAF and jet models (including Park-Ricotti extensions) to predict electromagnetic signatures in hot and warm ISM phases, highlighting that detectable signals are most plausible if a BH resides in a Local Interstellar Cloud. The Gaia DR3 analysis yields five SIMBAD-absent candidates near the Galactic plane that are likely spurious, and non-accreting BH detection via stellar perturbations is deemed unlikely due to low encounter rates. The paper emphasizes the need for multi-wavelength follow-up and anticipates SKA-era data (2027) to enhance the discovery prospects of nearby isolated BHs.

Abstract

Theoretical models predict that the Galaxy hosts $10^8$-$10^9$ black holes formed from the complete gravitational collapse of heavy stars and that most of these black holes are isolated, without any companion. Within 15 pc of the Solar System ($\sim 50$ ly), there may be a few black holes. If located inside one of the Local Interstellar Clouds - which occupy 5-20% of this local volume - an isolated black hole could produce detectable electromagnetic emission via accretion from the interstellar medium, given the capabilities of current or near-future observatories. However, precise predictions remain challenging due to large uncertainties in the expected accretion spectra. Outside these clouds, the accretion rate would be too low in any standard model to yield a detectable electromagnetic signal. While astrometric detection via gravitational perturbation of nearby stars is conceivable, the local stellar density is too low for this method to be realistically successful. We have searched the Gaia DR3 catalog for candidate isolated black holes accreting from the interstellar medium and identified five sources. All candidates lie close to the Galactic plane, making them likely spurious astrometric solutions, for instance caused by unmodelled background sources (crowding) and/or unmodelled binarity; nevertheless, they cannot be definitively ruled out without follow-up observations.

Figures (4)

• Figure 1: Spectrum of an isolated black hole accreting from the interstellar medium as computed with the LLAGNSED model. We assume that the mass accretion rate is described by the BHL model with $\lambda = 1$ and we employ the following values for the parameters of the system: black hole mass $M_{\rm BH} = 10$$M_\odot$, black hole distance $D = 15$ pc, black hole speed $v_{\rm BH} = 50$ km s$^{-1}$, sound speed in the interstellar medium $c_{\rm s} = 10$ km s$^{-1}$, particle number density $n = 0.3$ cm$^{-3}$, and mean atomic mass $\mu = 0.75$. The resulting Eddington-scaled accretion luminosity is $\sim 10^{-9}$. The plot shows the three components of the spectrum: synchrotron radiation, inverse Compton scattering, and bremsstrahlung emission.
• Figure 2: Spectra of isolated black holes accreting from a hot and low-density interstellar medium at 3, 5, 10, and 15 pc from the Solar System as computed with LLAGNSED (ADAF model). We assume that the particle number density is $n = 0.05$ cm$^{-3}$, the mean atomic mass is $\mu = 0.5$, the sound speed is $c_{\rm s} = 200$ km s$^{-1}$, the black hole mass is $M_{\rm BH} = 10$$M_\odot$, and the black hole velocity is $v_{\rm BH} = 20$ km s$^{-1}$. In the left panel, we assume the BHL accretion model with $\lambda = 1$ (solid) and $\lambda = 0.1$ (dashed). In the right panel, we assume the PR accretion model with $c_{\rm ion} = 200$ km s$^{-1}$. The gray curve is the spectrum of a brown dwarf with surface temperature $T = 2,500$ K, radius $R = 60,000$ km, at a distance of 10 pc, approximated with a blackbody spectrum. We also report the detection thresholds of Gaia DR3 (black), eROSITA (green), 2MASS (red), WISE (blue), VLASS (dark blue), LoTSS (brown), and a 5 hour observation with SKA (cyan).
• Figure 3: Spectra of isolated black holes accreting from a warm and partially ionized interstellar medium at 3, 5, 10, and 15 pc from the Solar System as computed with LLAGNSED (ADAF model). We assume that the particle number density is $n = 0.3$ cm$^{-3}$, the mean atomic mass is $\mu = 0.75$, the sound speed is $c_{\rm s} = 10$ km s$^{-1}$, and the black hole mass is $M_{\rm BH} = 10$$M_\odot$. In the top panels, we assume the BHL accretion model with $\lambda = 1$ (solid) and $\lambda = 0.1$ (dashed). In the bottom panels, we assume the PR accretion model with $c_{\rm ion} = 25$ km s$^{-1}$. The black hole velocity is $v_{\rm BH} = 20$ km s$^{-1}$ in the left panels and $v_{\rm BH} = 60$ km s$^{-1}$ in the right panels. The gray curve is the spectrum of a brown dwarf with surface temperature $T = 2,500$ K, radius $R = 60,000$ km, at a distance of 10 pc, approximated with a blackbody spectrum. The detection thresholds are the same as in Fig. \ref{['f-hot']}.
• Figure 4: Spectra of isolated black holes accreting from a hot and low-density interstellar medium in the ADAF+Jet model. In the left panel, we vary the angle between the jet axis and our line of sight assuming that the mass-loss rate into jet is 10% of the black hole mass accretion rate. In the right panel, we vary the mass-loss rate into jet assuming that the angle between the jet axis and our line of sight is 35$^\circ$. In both panels, we assume the BHL accretion model with $\lambda = 0.3$ and we employ the following values for the parameters of the system: particle number density $n = 0.05$ cm$^{-3}$, mean atomic mass $\mu = 0.5$, sound speed $c_{\rm s} = 200$ km s$^{-1}$, black hole mass $M_{\rm BH} = 10$$M_\odot$, and black hole velocity $v_{\rm BH} = 20$ km s$^{-1}$. Solid curves are for a black hole distance $D = 3$ pc and dashed curves are for a black hole distance $D = 15$ pc. The detection thresholds are the same as in Fig. \ref{['f-hot']}.