Abstract
Abstract We discuss the prospects for identifying the nearest isolated black holes (IBHs) in our Galaxy. IBHs accreting gas from the interstellar medium likely form magnetically arrested disks (MADs). We show that thermal electrons in the MADs emit optical signals through the thermal synchrotron process while nonthermal electrons accelerated via magnetic reconnections emit a flat-spectrum synchrotron radiation in the X-ray to MeV gamma-ray ranges. The Gaia catalog will include at most a thousand IBHs within ≲1 kpc that are distributed on and around the cooling sequence of white dwarfs (WDs) in the Hertzsprung–Russell diagram. These IBH candidates should also be detected by eROSITA, with which they can be distinguished from isolated WDs and neutron stars. Follow-up observations with hard X-ray and MeV gamma-ray satellites will be useful to unambiguously identify IBHs.
Highlights
The existence of stellar-mass black holes (BHs) is confirmed by dynamical motion in X-ray binaries (Tetarenko et al 2016; Corral-Santana et al 2016) and gravitational-wave detection (Abbott et al 2021)
We have discussed strategy to identify isolated black holes (IBHs) based on multi-wavelength emission model of magnetically arrested disks (MADs)
Thermal and non-thermal electrons in MADs emit optical and X-ray signal, respectively, which are detectable by Gaia and eROSITA
Summary
The existence of stellar-mass black holes (BHs) is confirmed by dynamical motion in X-ray binaries (Tetarenko et al 2016; Corral-Santana et al 2016) and gravitational-wave detection (Abbott et al 2021). Vast majority of stellar-mass BHs in our Galaxy wandering in the interstellar medium (ISM) have yet to be identified. The other is to consider the prospects for detection by Gaia (Gaia Collaboration et al 2016) and eROSITA (Predehl et al 2021) These satellites will provide complete catalogs of Galactic objects more than ever before, and they likely contain accreting IBHs. In order to distinguish IBHs from other objects, we need to understand multi-wavelength spectra of accreting IBHs and develop a strategy for identifying them.
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