Long-term X-ray spectral evolution of ultraluminous X-ray sources: implications on the accretion flow geometry and the nature of the accretor

Abstract

Context. The discovery of pulsations in several ultraluminous X-ray sources (ULXs) has demonstrated that a fraction of them are powered by super-Eddington accretion onto neutron stars (NSs). This has raised questions regarding the NS to black hole (BH) ratio within the ULX population and the physical mechanism that allows ULXs to reach luminosities well in excess of their Eddington luminosity. Is this latter the presence of strong magnetic fields or rather the presence of strong outflows that collimate the emission towards the observer? Aims. In order to distinguish between these scenarios, namely, supercritically accreting BHs, weakly magnetised NSs, or strongly magnetised NSs, we study the long-term X-ray spectral evolution of a sample of 17 ULXs with good long-term coverage, 6 of which are known to host NSs. At the same time, this study serves as a baseline to identify potential new NS-ULX candidates. Methods. We combine archival data from Chandra, XMM-Newton, and NuSTAR observatories in order to sample a wide range of spectral states for each source. We track the evolution of each source in a hardness–luminosity diagram in order to identify spectral changes, and show that these can be used to constrain the accretion flow geometry, and in some cases the nature of the accretor. Results. We find NS-ULXs to be among the hardest sources in our sample with highly variable high-energy emission. On this basis, we identify M 81 X-6 as a strong NS-ULX candidate, whose variability is shown to be akin to that of NGC 1313 X-2. For most softer sources with an unknown accretor, we identify the presence of three markedly different spectral states, which we interpret by invoking changes in the mass-accretion rate and obscuration by the supercritical wind/funnel structure. Finally, we report on a lack of variability at high energies (≳10 keV) in NGC 1313 X-1 and Holmberg IX X-1, which we argue may offer a means to differentiate BH-ULXs from NS-ULXs. Conclusions. We support a scenario in which the hardest sources in our sample might be powered by strongly magnetised NSs, meaning that the high-energy emission is dominated by the hard direct emission from the accretion column. Instead, softer sources may be explained by weakly magnetised NSs or BHs, in which the presence of outflows naturally explains their softer spectra through Compton down-scattering, their spectral transitions, and the dilution of the pulsed-emission should some of these sources contain NSs.

XMM-Newton campaign on the ultraluminous X-ray source NGC 247 ULX-1: outflows

“…From Eq. (38) in Poutanen et al (2007), assuming MBH = 10M andṁ = 10, we estimated a temperature for the spherisation radius T sph ∼ 0.3 keV, which is comparable to the warm blackbody component in our fits (see Table 2), with the cooler (∼ 0.1 keV) blackbody associated with the outer disc and, likely, the wind photosphere as suggested by recent work (see, e.g., Qiu & Feng 2021, Gúrpide et al 2021. We notice, however, that the source is being seen at high inclination with a substantial fraction of the hard X-ray photons obscured by the funnel.…”

“…For the modelling of EPIC spectra of individual observations we adopted a simple continuum model consisting of two blackbody (bb) components, which is often used as a proxy for more complex models (see, e.g., Walton et al 2014, Pinto et al 2017, Koliopanos et al 2017, and Gúrpide et al 2021. We did not model the hard tail in the individual spectra because it is so weak that any model would be highly unconstrained, but for the time-averaged SED modelling we took it into account (see Sect.…”

Broadband X-ray spectral variability of the pulsing ULX NGC 1313 X-2

“…The behaviour of the soft component has also been recently examined by Gúrpide et al (2021). As they report, the significantly softer spectra of NGC 1313 X-2 can be explained by the scenario consistent with the wind structure responsible for highly anisotropic emission, given the wide HR variability the source spans.…”

Discovery of a recurrent spectral evolutionary cycle in the ultra-luminous X-ray sources Holmberg II X–1 and NGC 5204 X–1

“…The atypical ULX spectral states were thus empirically classified as hard or soft ultra-luminous (HUL and SUL regimes), which were argued to depend on whether the observer had a clean view of the inner regions of the accretion flow or instead the source was viewed through the wind (Sutton et al 2013). The wind cone is expected to narrow with the accretion rate (King 2009;Kawashima et al 2012) and may explain the spectral transitions frequently observed in ULXs (Sutton et al 2013;Middleton et al 2015a;Gúrpide et al 2021). It has been argued that at higher inclinations or mass-accretion rates a ULX may appear as an ultra-luminous super-soft source (ULSs; e.g.…”

“…In Gúrpide et al (2021), we studied the long-term variability of a sample of 17 ULXs in order to understand the accretion flow geometry that could be responsible for the spectral variability observed in each source. In particular, we showed that certain ULXs followed an interesting evolutionary track in the hardness-luminosity diagram (HLD).…”