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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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We imaged the position of the naked-eye burst, GRB080319B, before, during, and after its gamma-ray activity with sub-second temporal resolution using the TORTORA wide-field camera. The burst optical prompt emission, which reached 5.3 mag, has been detected, and its periodic optical variability has been discovered in the form of four equidistant flashes with a duration of several seconds. We also detected a strong correlation (r ≈ 0.82) between optical and gamma-ray light curves with a 2 s delay of the optical emission with respect to the gamma-ray emission. The revealed temporal structure of the optical light curve in comparison with the gamma-ray light curve can be interpreted in the framework of the model of shell collisions in the ejecta containing a significant neutron component. All observed emission features reflect the non-stationary behavior of the burst internal engine-supposedly, a hyperaccreting solar-mass black hole formed in the collapse of a massive stellar core.

GRANDMA is a network of 25 telescopes of different sizes, including both photometric and spectroscopic facilities. The network aims to coordinate follow-up observations of gravitational-wave candidate alerts, especially those with large localisation uncertainties, to reduce the delay between the initial detection and the optical confirmation. In this paper, we detail GRANDMA’s observational performance during Advanced LIGO/Advanced Virgo Observing Run 3 (O3), focusing on the second part of O3; this includes summary statistics pertaining to coverage and possible astrophysical origin of the candidates. To do so, we quantify our observation efficiency in terms of delay between gravitational-wave candidate trigger time, observations, and the total coverage. Using an optimised and robust coordination system, GRANDMA followed-up about 90% of the gravitational-wave candidate alerts, i.e. 49 out of 56 candidates. This led to coverage of over 9000 deg2 during O3. The delay between the gravitational-wave candidate trigger and the first observation was below 1.5 hour for 50% of the alerts. We did not detect any electromagnetic counterparts to the gravitational-wave candidates during O3, likely due to the very large localisation areas (on average thousands of degrees squares) and relatively large distance of the candidates (above 200 Mpc for 60% of BNS candidates). We derive constraints on potential kilonova properties for two potential binary neutron star coalescences (GW190425 and S200213t), assuming that the events’ locations were imaged.