Searching for Massive Outflows in Holmberg IX X-1 and NGC 1313 X-1: The
  Iron K Band
  We have analysed all the good quality XMM-Newton data publicly available for
the bright ULXs Holmberg IX X-1 and NGC 1313 X-1, with the aim of searching for
discrete emission or absorption features in the Fe K band that could provide
observational evidence for the massive outflows predicted if these sources are
accreting at substantially super-Eddington rates. We do not find statistically
compelling evidence for any atomic lines, and the limits that are obtained have
interesting consequences. Any features in the immediate Fe K energy band (6-7
keV) must have equivalent widths weaker than ~30 eV for Holmberg IX X-1, and
weaker than ~50 eV for NGC 1313 X-1 (at 99 per cent confidence). In comparison
to the sub-Eddington outflows observed in GRS 1915+105, which imprint iron
absorption features with equivalent widths of ~30 eV, the limits obtained here
appear quite stringent, particularly when Holmberg IX X-1 and NGC 1313 X-1 must
be expelling at least 5-10 times as much material if they host black holes of
similar masses. The difficulty in reconciling these observational limits with
the presence of strong line-of-sight outflows suggests that either these
sources are not launching such outflows, or that they must be directed away
from our viewing angle.
