  Supercritical accretion onto compact objects may drive massive winds that are
nearly spherical, optically thick, and Eddington limited. Blackbody emission
from the photosphere is the direct observational signature of the wind. Here we
investigate whether or not it can explain the soft emission component seen in
the energy spectra of ultraluminous X-ray sources (ULXs). Based on high-quality
XMM-Newton spectra of 15 ULXs, we find that the soft component can be modeled
as blackbody emission with a nearly constant luminosity, and the 5 known
pulsating ULXs (PULXs) in the sample display a blackbody luminosity among the
lowest. These are consistent with the scenario that the soft emission
originates from the photosphere of the optically thick wind. However, the
derived blackbody luminosity for PULXs is significantly above the Eddington
limit for neutron stars. A possible explanation is that a considerable fraction
of the optically thick wind roots in the inner accretion flow, where the
radiative flux could exceed the Eddington limit due to a reduced scattering
cross-section or enhanced radiation transfer with magnetic buoyancy. Based on a
wind model, the inferred mass accretion rate in these standard ULXs overlaps
but is on average lower than that in luminous and very soft X-ray sources,
which are also candidates with supercritical accretion. Alternatively, it
cannot be ruled out that the soft emission component is a result of the hard
component, e.g., via down-scattering in a cool medium, as a weak correlation
may exist between them.
