  We investigate several aspects of black hole formation in failing
core-collapse supernovae using 1D general-relativistic hydrodynamic
simulations. We use the open-source code GR1D and incorporate into it
nucleon-nucleon Bremsstrahlung, a crucial neutrino pair-production channel. We
focus on how various thermal effects can influence the postbounce supernova
evolution towards black hole formation. By performing simulations with and
without nucleon-nucleon Bremsstrahlung, we investigate the sensitivity of black
hole formation to thermal support in the protoneutron star. We also investigate
delayed black hole formation by artificially driving explosions in an extreme
model where the protoneutron star is initially thermally supported above the
maximum baryonic cold neutron star mass but then collapses to a black hole
after neutrino cooling removes sufficient thermal support.
