Unveiling the accretion scenario of BH-ULXs using XMM-Newton observations

We present a comprehensive spectrotemporal analysis of five ultraluminous X-ray sources (ULXs) with central object likely being a black hole, using archival XMM-Newton observations. These sources, namely NGC1313 X-1, NGC5408 X-1, NGC6946 X-1, M82 X-1, and IC342 X-1, reveal short-term variability with fractional variance of 1.42$-27.28~{{\ \rm per\ cent}}\, $ and exhibit Quasi-periodic Oscillations (QPOs) with frequency ν_QPO ~ 8-667 mHz. Long-term evolution of ULXs energy spectra (0.3-10 keV; excluding M82 X-1) are described satisfactorily with a model combination that comprises a thermal Comptonization component (nthComp, yielding Γ_nth ~ 1.48-2.65, kT_e ~ 1.62-3.76 keV, τ ~ 8-20, y-par~1.16-6.24) along with a standard disc component (diskbb, kT_in ~ 0.16-0.54 keV). We find that these ULXs generally demonstrate anticorrelation between disc luminosity and inner disc temperature as $L_{\rm disc} \propto T_{\rm in}^\alpha$, where α = -3.58 ± 0.04 for NGC1313 X-1 and IC342 X-1, α = -8.93 ± 0.11 for NGC6946 X-1, and α = -10.31 ± 0.10 for NGC5408 X-1. We also obtain a linear correlation between bolometric luminosity L_bol and Γ_nth that indicates spectral softening of the sources when L_bol increases. We observe that in presence of QPO, Comptonized seed photon fraction varies in between $\sim 5-20~{{\ \rm per\ cent}}\, $, while the Comptonized flux contribution (50$-90~{{\ \rm per\ cent}}\, $) dominates over disc flux. Utilizing ν_QPO and L_bol, we constrain ULXs mass by varying their spin (a_k) and accretion rate ($\dot{m}$). We find that NGC6946 X-1 and NGC5408 X-1 seem to accrete at sub-Eddington accretion rate provided their central sources are rapidly rotating, whereas IC342 X-1 and NGC1313 X-1 can accrete in sub/super-Eddington limit irrespective to their spin values.