2021A&A...651A..54M

Context. We performed timing and spectral analyses of multi-epoch Suzaku, XMM-Newton, and NuSTAR observations of the ultraluminous X-ray source (ULX) Circinus ULX5 with the aim of putting constraints on the mass of the central object and the accretion mode operating in this source.
Aims. We investigate whether the source contains a stellar mass black hole (BH) 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 and determine whether they are associated with major structural changes in the disk, similarly to those in black hole X-ray binaries.
Methods. We collected all available broadband data from 2001 to 2018 including Suzaku, XMM-Newton, and NuSTAR. We a performed timing and spectral analyses to study the relation between luminosity and inner disk temperature. We proceeded with time-averaged spectral analysis using phenomenological models of different accretion modes. Finally, we constructed the hardness ratio versus intensity diagram to reveal spectral state transitions in Circinus ULX5.
Results. Our spectral analysis revealed at least three distinctive spectral states of Circinus ULX5 that are analagous 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 the 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 ∼seven months.
Conclusions. Our analysis suggests that the central object in Circinus ULX5 is a stellar mass BH (<10M_☉) or, possibly, a neutron star (NS) despite there being no detection of pulsations in the light curves. The fractional variability amplitudes are consistent with state transitions in Circinus ULX5, wherein higher variability from the power law-like Comptonized emission becomes suppressed in the thermal disk-dominated state.