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

Abstract

We present a comprehensive spectro-temporal analysis of five ultraluminous X-ray sources (ULXs) with central object likely being a black hole, using archival {\it XMM-Newton} observations. These sources, namely NGC 1313 X-1, NGC 5408 X-1, NGC 6946 X-1, M82 X-1 and IC 342 X-1, reveal short-term variability with fractional variance of $1.42-27.28\%$ and exhibit QPOs with frequency $\nu_{\rm QPO} \sim 8-667$ 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 (\texttt{nthComp}, yielding $\Gamma_{\rm nth} \sim 1.48 - 2.65$, $kT_{\rm e} \sim 1.62 - 3.76$ keV, $\tau \sim 8 - 20$, y-par $\sim 1.16 - 6.24$) along with a standard disc component (\texttt{diskbb}, $kT_{\rm in} \sim 0.16 - 0.54$ keV). We find that these ULXs generally demonstrate anti-correlation between disc luminosity and inner disc temperature as $L_{\rm disc} \propto T_{\rm in}^\alpha$, where $\alpha = - 3.58 \pm 0.04$ for NGC 1313 X-1 and IC 342 X-1, $\alpha = - 8.93 \pm 0.11$ for NGC 6946 X-1, and $\alpha = - 10.31 \pm 0.10$ for NGC 5408 X-1. We also obtain a linear correlation between bolometric luminosity $L_{\rm bol}$ and $\Gamma_{\rm nth}$ that indicates spectral softening of the sources when $L_{\rm bol}$ increases. We observe that in presence of QPO, Comptonized seed photon fraction varies in between $\sim 5 - 20 \%$, while the Comptonized flux contribution ($50 - 90\%$) dominates over disc flux. Utilizing $\nu_{\rm QPO}$ and $L_{\rm bol}$, we constrain ULXs mass by varying their spin ($a_{\rm k}$) and accretion rate ($\dot m$). We find that NGC 6946 X-1 and NGC 5408 X-1 seem to accrete at sub-Eddington accretion rate provided their central sources are rapidly rotating, whereas IC 342 X-1 and NGC 1313 X-1 can accrete in sub/super-Eddington limit irrespective to their spin values.