Spectral state transitions in Circinus ULX5

Abstract

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.