The unusual broad-band X-ray spectral variability of NGC 1313 X-1 seen with XMM–Newton, Chandra, and NuSTAR

Abstract

We present results from the major coordinated X-ray observing programme on the ULX NGC 1313 X-1 performed in 2017, combining XMM–Newton, Chandra, and NuSTAR, focusing on the evolution of the broad-band (∼0.3–30.0 keV) continuum emission. Clear and unusual spectral variability is observed, but this is markedly suppressed above ∼10–15 keV, qualitatively similar to the ULX Holmberg IX X-1. We model the multi-epoch data with two-component accretion disc models designed to approximate super-Eddington accretion, allowing for both a black hole and a neutron star accretor. With regards to the hotter disc component, the data trace out two distinct tracks in the luminosity–temperature plane, with larger emitting radii and lower temperatures seen at higher observed fluxes. Despite this apparent anticorrelation, each of these tracks individually shows a positive luminosity–temperature relation. Both are broadly consistent with L ∝ T4, as expected for blackbody emission with a constant area, and also with L ∝ T2, as may be expected for an advection-dominated disc around a black hole. We consider a variety of possibilities for this unusual behaviour. Scenarios in which the innermost flow is suddenly blocked from view by outer regions of the super-Eddington disc/wind can explain the luminosity–temperature behaviour, but are difficult to reconcile with the lack of strong variability at higher energies, assuming this emission arises from the most compact regions. Instead, we may be seeing evidence for further radial stratification of the accretion flow than is included in the simple models considered, with a combination of winds and advection resulting in the suppressed high-energy variability.

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

“…8) between the temperature of the soft component and its normalisation and therefore these relationships cannot be used reliably to support this scenario. Indeed, it has been shown that depending on the assumption of the underlying model, the correlation can disappear entirely (see e.g., Luangtip et al 2016; Walton et al 2020; see also Gonçalves & Soria 2006 for a discussion on the weaknesses of associating the soft component with an accretion disk). Nevertheless, these correlations might be useful to identify sources evolving in a similar fashion.…”

“…luminosity, radius, temperature, etc.). As the latest studies revealed that the 0.3−10 keV band can be modelled by two thermal components (e.g., Mukherjee et al 2015;Koliopanos et al 2017Koliopanos et al , 2019Walton et al 2020), which can reproduce the curvature seen at high-energies, we first considered a phenomenological model based on two multi-colour blackbody disks (diskbb in xspec; Mitsuda et al 1984) to fit the data in this band, taking into account interstellar absorption by neutral hydrogen with two absorbing components tbabs in xspec. One was frozen at the Galactic value along the source line of sight (see Table 1), and the other one was left free to vary to take into account possible absorption from the host galaxy and the system itself.…”

“…By doing so we were able to use the broadband information provided by NuSTAR to have some constraints on those epochs where this information is not available. For NGC 1313 X-1, after checking the consistency of Γ across epochs and bearing in mind that its variability above 10 keV has been shown to be small (Walton et al 2020), we tied Γ for all observations where the simpl model is used. For NGC 5408 X-1 and NGC 6946 X-1, we also found very little variability at high-energies and therefore we again tied Γ across all observations where we added the simpl model.…”

“…The physical processes at play to produce these high-energy photons are still poorly understood (e.g., Walton et al 2020) but it is generally accepted that this emission is produced in the vicinity of the accretor (e.g., Kawashima et al 2012;Takahashi et al 2016;Walton et al 2020). For the remainder of this part, we assume that this high-energy component is indeed produced in the inner regions of the accretion flow and focus on the influence of the wind and funnel structure on the spectra, rather than on the origin of this emission, which is discussed in Sect.…”

“…This in turn provides clues about the nature of the accretor, and by comparing its evolution with the known PULXs, we can identify potential strong NS-ULX candidates. To do so, we build a phenomenological model taking into account the new information obtained on the ULX broadband emission with NuSTAR (Bachetti et al 2014;Walton et al 2015Walton et al , 2020Mukherjee et al 2015) and track their spectral changes in a hardness-luminosity diagram (HLD).…”