Electric charge of black holes: Is it really always negligible?
  We discuss the problem of the third black hole parameter, an electric charge.
While the mass and the spin of black holes are frequently considered in the
majority of publications, the charge is often neglected and implicitly set
identically to zero. However, both classical and relativistic processes can
lead to a small non-zero charge of black holes. When dealing with neutral
particles and photons, zero charge is a good approximation. On the other hand,
even a small charge can significantly influence the motion of charged
particles, in particular cosmic rays, in the vicinity of black holes.
Therefore, we stress that more attention should be paid to the problem of a
black-hole charge and hence, it should not be neglected a priori, as it is done
in most astrophysical studies nowadays. The paper looks at the problem of the
black-hole charge mainly from the astrophysical point of view, which is
complemented by a few historical as well as philosophical notes when relevant.
In particular, we show that a cosmic ray or in general elementary charged
particles passing a non-neutral black hole can experience an electromagnetic
force as much as sixteen times the gravitational force for the mass of the
Galactic centre black hole and its charge being seventeen orders of magnitude
less than the extremal value (calculated for a proton). Furthermore, a
Kerr-Newman rotating black hole with the maximum likely charge of 1 Coulomb per
solar mass can have the position of its innermost stable circular orbit (ISCO)
moved by both rotation and charge in ways that can enhance or partly cancel
each other, putting the ISCO not far from the gravitational radius or out at
more than 6 gravitational radii. An interpretation of X-ray radiation from near
the ISCO of a black hole in X-ray binaries is then no longer unique.
