Issue |
A&A
Volume 440, Number 1, September II 2005
|
|
---|---|---|
Page(s) | 223 - 238 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361:20040572 | |
Published online | 19 August 2005 |
Low-rate accretion onto isolated stellar-mass black holes
1
Special Astrophysical Observatory, Nizhnij Arkhyz, Karachaevo-Cherkesia 369167, Russia e-mail: beskin@sao.ru
2
Isaac Newton Institute of Chile, SAO Branch, Russia
Received:
1
April
2004
Accepted:
28
April
2005
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
( 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
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 16–25m optical source coinciding with the highly variable
bright X-ray counterpart, while the variable component of optical emission will
be about 18–27m. 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.
Key words: accretion, accretion disks / magnetic fields
© ESO, 2005
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