  Recent work has linked the quiescent luminosities and hard/soft spectral
state switches of neutron stars (NSs) to their spinning magnetic fields. It is
shown here that the quiescent luminosities and spectral switches of galactic
black hole candidates (BHCs) could be produced in the same way for spin rates
below 100 Hz and magnetic fields above 10^10 G. It is also shown that the
ultrasoft peaks and large flickering amplitudes of the BHCs would be expected
from the surfaces of massive NSs. None of the few spectral characteristics that
distinguish BHCs from low mass NSs have been explained in terms of event
horizons. Serious consideration of the possibility that they might simply be
massive NSs opens an avenue for proof of event horizons by negation, but
requires the use of a space-time metric that has no event horizon. The Yilmaz
exponential metric used here is shown to have an innermost marginally stable
orbit with radius, binding energy and Keplerian frequency that are within a few
percent of the same quantities for the Schwarzschild metric. A maximum NS mass
of ~10 solar mass is found for the Yilmaz metric. The two metrics essentially
differ only by the presence/absence of a surface for the BHCs, thus enabling
proof or disproof of the existence of an event horizon.
