Long-term evolution of BH-ULX candidates: An ‘unusual’ L_disc − T_col correlation associated with spectral states

Abstract

We present the long-term spectral evolution of eight black hole ultra-luminous X-ray sources (BH-ULXs), namely NGC1313 X−1, NGC5408 X−1, NGC6946 X−1, IC342 X−1, NGC55 ULX1, NGC4395 ULX1, NGC5204 X−1 and NGC4190 ULX1 using XMM-Newton monitoring data spanning over a decade or more. An in-depth spectral modeling with thermal Comptonization (nthComp) and standard disc (diskbb) components reveals NGC5204 X−1, IC342 X−1, NGC4190 ULX1 and NGC1313 X−1 exhibiting harder spectral characteristics with dominant effect of Comptonization (F_nth > F_disc, Γ_nth ≲ 2). However, NGC6946 X−1 and NGC55 ULX1 remain in a disc-dominated state (F_disc ∼ 2F_nth, Γ_nth ≳ 2), while NGC5408 X−1 shows intermediate spectral characteristics. The spectral analyses indicate an anti-correlation between disc luminosity (L_disc) and temperature (T_col) for all sources except NGC5204 X−1. These anti-correlations follow a relation |$L_{\rm disc} \propto T_{\rm col}^{\alpha }$| with steeper exponents of α = −6.01 ± 0.25 (NGC55 ULX1), −8.93 ± 0.11 (NGC6946 X−1), and −10.31 ± 0.10 (NGC5408 X−1) for sources with softer or intermediate spectral characteristics. For harder sources, NGC1313 X−1 and IC342 X−1, the combined results provide α = −3.58 ± 0.04. However, for NGC5204 X−1, a positive correlation is found, yielding α = 1.4 ± 0.1, suggesting that the emission mechanism is associated with the transition from the ‘standard disc’ to the ‘slim disc’ scenario. These findings suggest that the observed L_disc − T_col correlations, along with the overall spectro-temporal properties of BH-ULXs, seems to be governed by disc-corona-wind driven accretion processes at various inclinations. Finally, we report a QPO-like feature (∼20 mHz) with rms% ∼ 6.6, Q-factor ∼6.7 and significant 2.8σ in NGC55 ULX1.

accretion, accretion disc, black hole physics, X-rays: galaxies, radiation mechanisms: general, stars: individual

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