Populations of Supersoft X-ray Sources: Novae, tidal disruption, Type Ia
  supernovae, accretion-induced collapse, ionization, and intermediate-mass
  black holes?
  Observations of hundreds of supersoft x-ray sources (SSSs) in external
galaxies have shed light on the diversity of the class and on the natures of
the sources. SSSs are linked to the physics of Type Ia supernovae and
accretion-induced collapse, ultraluminous x-ray sources and black holes, the
ionization of the interstellar medium, and tidal disruption by supermassive
black holes. The class of SSSs has an extension to higher luminosities:
ultraluminous SSSs have luminosities above 10^39 erg/s. There is also an
extension to higher energies: quasisoft x-ray sources (QSSs) emit photons with
energies above 1 eV, but few or none with energies above 2 keV. Finally, a
significant fraction of the SSSs found in external galaxies switch states
between observations, becoming either quasisoft or hard. For many systems
``supersoft'' refers to a temporary state; SSSs are sources, possibly including
a variety of fundamentally different system types, that pass through such a
state. We review those results derived from extragalactic data and related
theoretical work that are most surprising and that suggest directions for
future research.
