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Register a new userRadio Emission from Accreting Isolated Black Holes in Our Galaxy
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Apart from the few tens of stellar-mass black holes discovered in binary systems, an order of $10^8$ isolated black holes (IBHs) are believed to be lurking in our Galaxy. Although some IBHs are able to accrete matter from the interstellar medium, the accretion flow is usually weak and thus radiatively inefficient, which results in significant material outflow. We study electron acceleration generated by the shock formed between this outflow and the surrounding material, and the subsequent radio synchrotron emission from accelerated electrons. By numerically calculating orbits of IBHs to obtain their spatial and velocity distributions, we estimate the number of IBHs detectable by surveys using SKA1-mid (SKA2) as $sim 30$ ($sim 700$) for the most optimistic case. The SKAs parallax measurements may accurately give their distances, possibly shedding light on the properties of the black holes in our Galaxy.
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Detectability of isolated black holes (IBHs) without a companion star but emitting X-rays by accretion from dense interstellar medium (ISM) or molecular cloud gas is investigated. We calculate orbits of IBHs in the Galaxy to derive a realistic spatial distribution of IBHs, for various mean values of kick velocity at their birth $upsilon_{rm avg}$. X-ray luminosities of these IBHs are then calculated considering various phases of ISM and molecular clouds, for a wide range of the accretion efficiency $lambda$ (a ratio of the actual accretion rate to the Bondi rate) that is rather uncertain. It is found that detectable IBHs mostly reside near the Galactic Centre (GC), and hence taking the Galactic structure into account is essential. In the hard X-ray band, where identification of IBHs from other contaminating X-ray sources may be easier, the expected number of IBHs detectable by the past survey by NuSTAR towards GC is at most order unity. However, 30--100 IBHs may be detected by the future survey by FORCE with an optimistic parameter set of $upsilon_{rm avg} = 50 mathrm{km s^{-1}}$ and $lambda = 0.1$, implying that it may be possible to detect IBHs or constrain the model parameters.
I outline the theory of accretion onto black holes, and its application to observed phenomena such as X-ray binaries, active galactic nuclei, tidal disruption events, and gamma-ray bursts. The dynamics as well as radiative signatures of black hole accretion depend on interactions between the relatively simple black-hole spacetime and complex radiation, plasma and magnetohydrodynamical processes in the surrounding gas. I will show how transient accretion processes could provide clues to these interactions. Larger global magnetohydrodynamic simulations as well as simulations incorporating plasma microphysics and full radiation hydrodynamics will be needed to unravel some of the current mysteries of black hole accretion.
We consider radio emission from a newborn black hole (BH), which is accompanied by a mini-disk with a mass of $lesssim M_odot$. Such a disk can be formed from an outer edge of the progenitors envelope, especially for metal-poor massive stars and/or massive stars in close binaries. The disk accretion rate is typically super-Eddington and an ultrafast outflow with a velocity of $sim 0.1mbox{-}0.3,c$ will be launched into the circumstellar medium. The outflow forms a collisionless shock, and electrons are accelerated and emit synchrotron emission in radio bands with a flux of $sim 10^{26-30} rm erg s^{-1} Hz^{-1}$ days to decades after the BH formation. The model predicts not only a fast UV/optical transient but also quasi-simultaneous inverse-Compton X-ray emission $sim$ a few days after the BH formation, and the discovery of the radio counterpart with coordinated searches will enable us to identify this type of transients. The occurrence rate can be $0.1-10 %$ of the core-collapse supernova rate, which makes them a promising target of dedicated radio observations such as the Jansky VLA Sky Survey.
The role of bremsstrahlung in the emission from hot accretion flows around slowly accreting supermassive black holes is not thoroughly understood. In order to appraise the importance of bremsstrahlung relative to other radiative processes, we compute spectral energy distributions (SEDs) of accretion disks around slowly accreting supermassive black holes including synchrotron radiation, inverse Compton scattering, and bremsstrahlung. We compute SEDs for (i) four axisymmetric radiative general relativistic magnetohydrodynamics (RadGRMHD) simulations of $10^{8}M_{odot}$ black holes with accretion rates between $10^{-8}dot{M}_{text{Edd}}$ and $10^{-5}dot{M}_{text{Edd}}$, (ii) four axisymmetric RadGRMHD simulations of M87$^ast$ with varying dimensionless spin $a_ast$ and black hole mass, and (iii) a 3D GRMHD simulation scaled for Sgr A$^ast$. At $10^{-8}dot{M}_{text{Edd}}$, most of the luminosity is synchrotron radiation, while at $10^{-5}dot{M}_{text{Edd}}$ the three radiative processes have similar luminosities. In most models, bremsstrahlung dominates the SED near $512text{ keV}$. In the M87$^ast$ models, bremsstrahlung dominates this part of the SED if $a_{ast} = 0.5$, but inverse Compton scattering dominates if $a_{ast}= 0.9375$. Since scattering is more variable than bremsstrahlung, this result suggests that $512text{ keV}$ variability could be a diagnostic of black hole spin. In the appendix, we compare some bremsstrahlung formulae found in the literature.
A common consequence of accretion onto black holes is the formation of powerful, relativistic jets that escape the system. In the case of supermassive black holes at the centres of galaxies this has been known for decades, but for stellar-mass black holes residing within galaxies like our own, it has taken recent advances to arrive at this conclusion. Here, a review is given of the evidence that supports the existence of jets from accreting stellar-mass black holes, from observations made at optical and infrared wavelengths. In particular it is found that on occasion, jets can dominate the emission of these systems at these wavelengths. In addition, the interactions between the jets and the surrounding matter produce optical and infrared emission on large scales via thermal and non-thermal processes. The evidence, implications and applications in the context of jet physics are discussed. It is shown that many properties of the jets can be constrained from these studies, including the total kinetic power they contain. The main conclusion is that like the supermassive black holes, the jet kinetic power of accreting stellar-mass black holes is sometimes comparable to their bolometric radiative luminosity. Future studies can test ubiquities in jet properties between objects, and attempt to unify the properties of jets from all observable accreting black holes, i.e. of all masses.
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