A spectral-timing model for ULXs in the supercritical regime

Abstract

Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. ABSTRACTUltraluminous X-ray sources (ULXs) with luminosities lying between ∼3 × 10 39 and 2 × 10 40 erg s −1 represent a contentious sample of objects as their brightness, together with a lack of unambiguous mass estimates for the vast majority of the central objects, leads to a degenerate scenario where the accretor could be a stellar remnant (black hole or neutron star) or intermediate-mass black hole (IMBH). Recent, high-quality observations imply that the presence of IMBHs in the majority of these objects is unlikely unless the accretion flow somehow deviates strongly from expectation based on objects with known masses. On the other hand, physically motivated models for supercritical inflows can re-create the observed X-ray spectra and their evolution, although have been lacking a robust explanation for their variability properties. In this paper, we include the effect of a partially inhomogeneous wind that imprints variability on to the X-ray emission via two distinct methods. The model is heavily dependent on both inclination to the line of sight and mass accretion rate, resulting in a series of qualitative and semiquantitative predictions. We study the time-averaged spectra and variability of a sample of well-observed ULXs, finding that the source behaviours can be explained by our model in both individual cases as well as across the entire sample, specifically in the trend of hardness-variability power. We present the covariance spectra for these sources for the first time, which shed light on the correlated variability and issues associated with modelling broad ULX spectra.Key words: accretion, accretion discs -X-rays: binaries. I N T RO D U C T I O NUltraluminous X-ray sources (ULXs) have been widely observed in the local Universe, with inferred isotropic luminosities above 10 39 erg s −1 (Roberts 2007;Feng & Soria 2011). Those below ∼3 × 10 39 erg s −1 can be readily associated with accretion on to stellar mass black holes (BHs) (∼10 M ) accreting close to or at their Eddington limit (see Sutton, Roberts & Middleton 2013, and references therein). There is now strong evidence to support this assertion, with the discovery of extremely bright ballistic jets from a ULX in M31 Middleton, Miller-Jones & Fender 2014b), which unambiguously links the flow with Eddington rate accretion (Fender, Belloni & Gallo 2004), and the first dynamical mass measurement of the compact object in a ULX, from M101 E-mail: mjm@ast.cam.ac.uk ULX-1 (Liu et al. 2013). Ob...

The ultraluminous X-ray source NGC 5643 ULX1: a large stellar mass black hole accreting at super-Eddington rates?

“…Indeed, a number of other ULXs showed similar degeneracies between spectral shape, luminosity and short-term variability when comparing low and highquality data, and it has been proposed that these ULXs may be sMBHs or massive stellar BHs accreting at super-Eddington rates (e.g. Stobbart et al 2006;Roberts 2007;Gladstone et al 2009;Heil et al 2009;Middleton et al 2011;Sutton et al 2013b;Pintore et al 2014;Middleton et al 2015). If the power-law-dominated spectra of NGC 5643 ULX1 are indeed simply due to the poor data quality and the intrinsic spectra are actually similar to the highest quality observation, then the marginally different levels of short-term variability could be explained by the effects of an outflow ejected by the accretion disc at radii where it is locally super-Eddington (Poutanen et al 2007;Ohsuga & Mineshige 2011;Takeuchi et al 2014).…”

“…However, if both components are present in NGC 5643 ULX1 and the inclination angle is suitable (see e.g. Middleton et al 2015), then we would expect to observe both of these features in its spectrum. Hence, although it is not required by the data, we tentatively added a multicolour disc component to the Comptonization model.…”

“…The Comptonization component originates from the hot, inner accretion disc (e.g. Middleton et al 2011;Kajava et al 2012;Middleton et al 2015). However, if both components are present in NGC 5643 ULX1 and the inclination angle is suitable (see e.g.…”

Two Eclipsing Ultraluminous X-Ray Sources in M51

“…Luminosity (or more precisely, mass accretion rate) and viewing angle are thought to be the main parameters that determine the observational appearance of ULXs in the X-ray band (e.g., Sutton et al 2013;Middleton et al 2015a;Urquhart & Soria 2016). Disentangling and quantifying their roles is still an unsolved problem.…”

“…Despite being often modeled with a diskbb component for practical purposes, it is by no means clear whether or not it originates from the disk. It could come instead from the more optically thick parts of the outflow (Middleton et al 2015a;Urquhart & Soria 2016). On the other hand, the "disk" in ULX-2 is the dominant continuum emission component.…”