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 ${\nu }_{\mathrm{QPO}}\sim 8-667$ 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 ${\Gamma }_{\mathrm{nth}}\sim 1.48-2.65$, $k{T}_{\mathrm{e}}\sim 1.62-3.76$ keV, $\tau \sim 8-20$, y-par $\sim 1.16-6.24$) along with a standard disc component (\texttt{diskbb}, $k{T}_{\mathrm{in}}\sim 0.16-0.54$ keV). We find that these ULXs generally demonstrate anti-correlation between disc luminosity and inner disc temperature as $L_{\mathrm{disc}}\propto T_{\mathrm{in}}^{\alpha }$, 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 $L_{\mathrm{bol}}$ and ${\Gamma }_{\mathrm{nth}}$ that indicates spectral softening of the sources when $L_{\mathrm{bol}}$ 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 ${\nu }_{\mathrm{QPO}}$ and $L_{\mathrm{bol}}$, we constrain ULXs mass by varying their spin ($a_{\mathrm{k}}$) 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.

21 pages, 11 figures, 6 tables, accepted for publication in MNRAS