Abstract. Magnetic field behaviour in a spherically-symmetric accretion flow for parameters typical of single black holes in the Galaxy is discussed. It is shown that in the majority of Galaxy volume, accretion onto single stellar-mass black holes will be spherical and have a low accretion rate (10 −6 −10 −9 of the Eddington rate). An analysis of plasma internal energy growth during the infall is performed. Adiabatic heating of collisionless accretion flow due to magnetic adiabatic invariant conservation is 25% more efficient than in the standard non-magnetized gas case. It is shown that magnetic field line reconnections in discrete current sheets lead to significant nonthermal electron component formation. In a framework of quasi-diffusion acceleration, the "energy-radius" electron distribution is computed and the function describing the shape of synchrotron radiation spectrum is constructed. It is shown that nonthermal electron emission leads to formation of a hard (UV, X-ray, up to gamma), highly variable spectral component in addition to the standard synchrotron optical component first derived by Shvartsman generated by thermal electrons in the magnetic field of accretion flow. For typical interstellar medium parameters, a black hole at 100 pc distance will be a 16-25 m optical source coinciding with the highly variable bright X-ray counterpart, while the variable component of optical emission will be about 18-27 m . The typical time scale of the variability is 10 −4 s, with relative flare amplitudes of 0.2-6% in various spectral bands. Possible applications of these results to the problem of search for single black holes are discussed.

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The ejecta composition is an open question in gamma-ray bursts (GRB) physics 1 . Some GRBs possess a quasi-thermal spectral component in the time-resolved spectral analysis 2 , suggesting a hot fireball origin. Others show a featureless non-thermal spectrum known as the "Band" function 3-5 , consistent with a synchrotron radiation origin 5,6 and suggesting that the jet is Poynting-flux-dominated at the central engine and likely in the emission region as well 7,8 . There are also bursts showing a subdominant thermal component and a dominant synchrotron component 9 , suggesting a likely hybrid jet composition 10 . Here we report an extraordinarily bright GRB 160625B, simultaneously observed in gamma-rays and optical wavelengths, whose prompt emission consists of three isolated episodes separated by long quiescent intervals, with the durations of each "sub-burst" being ∼ 0.8 s, 35 s, and 212 s, respectively. Its high brightness (with isotropic peak luminosity L p,iso ∼ 4 × 10 53 erg/s) allows us to conduct detailed time-resolved spectral analysis in each episode, from precursor to main burst and to extended emission. The spectral properties of the first two sub-bursts are distinctly different, allowing us to observe the transition from thermal to non-thermal radiation between well-separated emission episodes within a single GRB. Such a transition is a clear indication of the change of jet composition from a fireball to a Poynting-flux-dominated jet.

General properties of accretion onto isolated stellar-mass black holes in the Galaxy are discussed. An analysis of plasma internal energy growth during the infall is performed. Adiabatic heating of collisionless accretion flow due to magnetic adiabatic invariant conservation is 25% more efficient than in the standard non-magnetized gas case. It is shown that magnetic field line reconnections in discrete current sheets lead to significant nonthermal electron component formation, which leads to a formation of a hard (UV, X-ray, up to gamma), highly variable spectral component in addition to the standard synchrotron optical component first derived by Shvartsman generated by thermal electrons in the magnetic field of the accretion flow. Properties of accretion flow emission variability are discussed. Observation results of two single black hole candidates -gravitational lens MACHO-1999-BLG-22 and radio-loud X-ray source with featureless optical spectrum J1942+10 -in optical band with high temporal resolution are presented and interpreted in the framework of the proposed model.

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