  The origin of the variable X-ray emission in the $(0.3-30)\,$keV energy range
of ultraluminous X-ray sources (ULXs) remains unclear, making it difficult to
constrain the mass of the central compact object. X-ray luminosities of bright
ULXs can be explained with sub-critical accretion ($L<L_{\rm Edd}$) on to an
intermediate-mass BH, with the alternative being super-critical accretion on to
a stellar-mass BH. Broadband X-ray emission in the former scenario can be
explained using the canonical disk plus Comptonizing corona model, whereas in
the latter scenario radiation pressure driven massive winds lead to complex
spectra that are inclination angle dependent. Here we fit the broadband
(optical/UV to X-ray) spectrum of the persistently bright ULX Holmberg IX X-1
with the disk-corona plus irradiated outer disk model in an effort to constrain
the BH mass. We use a one-zone time-dependent numerical code to exactly solve
for the steady-state properties of the optically thick coronal
photon-electron-positron plasma. Our modelling suggests that Holmberg IX X-1
hosts a stellar mass BH, with mass $4\lesssim(\hat M_{\rm BH}\equiv\alpha
M_{\rm BH}/M_\odot)\lesssim10$ where $1/6\leq\alpha<1$ for a spinning (Kerr)
BH, undergoing super-critical accretion ($L_{\rm Bol}/L_{\rm
Edd}\sim20\alpha$). In our model, the X-ray spectrum below $10\,$keV is
explained with an absorbed multi-colour disk spectrum having inner disk
temperature $k_BT_{\rm in}\sim(2.2-2.9)\,$keV. An additional cooler thermal
spectral component, as found in many works and not included in our modeling, is
required. The hard excess above $10\,$keV, as seen by NuSTAR, arises in a
photon-rich optically-thick Comptonizing spherical corona with optical depth
$\tau_T\sim3.5$ and particle temperature $k_BT_e\sim14\,$keV.
