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

Abstract

ABSTRACT 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 νQPO ∼ 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 Γnth ∼ 1.48–2.65, kTe ∼ 1.62–3.76 keV, τ ∼ 8–20, y-par∼1.16–6.24) along with a standard disc component (diskbb, kTin ∼ 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 Lbol and Γnth that indicates spectral softening of the sources when Lbol 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}}\, $) dominates over disc flux. Utilizing νQPO and Lbol, we constrain ULXs mass by varying their spin (ak) and accretion rate ($\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.