"Magnetic
field generation during accretion into black holes
Prof. G.S. Bisnovaty-Kogan (IKI)
Abstract:
The
problem of magnetic field generation in accretion flows onto black holes
is discussed, including the field amplification in the falling gas, and
due to the action of the excess radiation force on electrons. This
excess force may arise from the Poynting-Robertson effect.
Instead of a recent claim of the generation of
dynamically important magnetic fields, we establish the validity of
earlier results from 1977 which show only small magnetic fields are
generated.
The radiative force causes the magnetic field to
initially grow linearly with time. However, this linear growth holds for
only a restricted time interval which is of the order of the accretion
time of the matter.
Very large magnetic fields may appear in the accretion disk when large
scale magnetic field is present in the falling gas. Jet formation from
accretion disks in AGN and microquasars is probably connected with this
fields.
A model of the Poynting-Robertson magnetic field
generation close to the horizon of a Schwarzschild black hole is solved
exactly using General Relativity, and the field is also found to be
dynamically insignificant. These weak magnetic fields may however be
important as seed fields for dynamos.