  The atomic features in the X-ray spectra of the ultraluminous X-ray source
(ULX) may be associated with the outflow (Middleton et al. 2015), which may
provide a way to explore the physics of the ULXs. We construct a conical
outflow model, and calculate the thermal X-ray Fe emission lines from the
outflows. Our results show that thermal line luminosity decreases with
increasing outflow velocity or/and opening angle of the outflow for a fixed
kinetic power of the outflows. Assuming the kinetic power of the outflows to be
comparable with the accretion power in the ULXs, we find that the equivalent
width can be several eV for the thermal X-ray Fe emission line from the
outflows in the ULXs with stellar mass black holes. The thermal line luminosity
is proportional to 1/M (M is the black hole mass of the ULX). The equivalent
width decreases with the black hole mass, which implies that the Fe line
emission from the outflows can hardly be detected if the ULXs contain
intermediate mass black holes. Our results suggest that the thermal X-ray Fe
line emission should be preferentially be detected in the ULXs with high
kinetic power slowly moving outflows from the accretion disks surrounding
stellar mass black holes/neutron stars. The recent observed X-ray atomic
features of the outflows in the ULX may imply that it contains a stellar mass
black hole/neutron star (Pinto et al. 2016).
