  We performed timing and spectral analysis of multi-epoch Suzaku, XMM-Newton
and NuSTAR observations of the ultraluminous X-ray source (ULX) Circinus ULX5,
to put constraints on the mass of the central object and the accretion mode
operating in this source. We aim to answer whether the source contains a
stellar mass black hole with a super-Eddington accretion flow or an
intermediate mass black hole accreting matter in a sub-Eddington mode.
Moreover, we search for major observed changes in spectra and timing occur, and
determine if they are associated with major structural changes in the disk,
similar to those in black hole X-ray binaries. We performed timing and spectral
analysis to study the relation between luminosity and inner disk temperature.
We constructed the hardness ratio versus intensity diagram to reveal spectral
state transitions in Circinus ULX5. Our spectral analysis revealed at least
three distinctive spectral states of Circinus ULX5, in analogy to state
transitions in Galactic black hole X-ray binaries. Disk-dominated spectra are
found in high flux states and the power-law dominated spectra are found in
lower flux states. The source was also observed in an intermediate state, where
the flux was low, but spectrum is dominated by a disk component. Over eighteen
years of collected data, ULX5 appeared two times in the high, three times in
the low, and two times in the intermediate state. The fastest observed
transition was $\sim$7 months. Our analysis suggests that the central object in
Circinus ULX5 is a stellar mass BH ($<10\ \rm M_{\odot}$), or possibly a NS
even though we do not detect pulsations in the lightcurves. Fractional
variability amplitudes are consistent with state transitions in Circinus ULX5
wherein higher variability from the power law-like Comptonized emission gets
suppressed in the thermal disk-dominated state.
