  The lack of unambiguous detections of atomic features in the X-ray spectra of
ultraluminous X-ray sources (ULXs) has proven a hindrance in diagnosing the
nature of the accretion flow. The possible association of spectral residuals at
soft energies with atomic features seen in absorption and/or emission and
potentially broadened by velocity dispersion could therefore hold the key to
understanding much about these enigmatic sources. Here we show for the first
time that such residuals are seen in several sources and appear extremely
similar in shape, implying a common origin. Via simple arguments we assert that
emission from extreme colliding winds, absorption in a shell of material
associated with the ULX nebula and thermal plasma emission associated with star
formation are all highly unlikely to provide an origin. Whilst CCD spectra lack
the energy resolution necessary to directly determine the nature of the
features (i.e. formed of a complex of narrow lines or intrinsically broad),
studying the evolution of the residuals with underlying spectral shape allows
for an important, indirect test for their origin. The ULX NGC 1313 X-1 provides
the best opportunity to perform such a test due to the dynamic range in
spectral hardness provided by archival observations. We show through highly
simplified spectral modelling that the strength of the features (in either
absorption or emission) appears to anti-correlate with spectral hardness, which
would rule out an origin via reflection of a primary continuum and instead
supports a picture of atomic transitions in a wind or nearby material
associated with such an outflow.

High Energy Astrophysical Phenomena