The ultraluminous state

Abstract

We revisit the question of the nature of ultraluminous X‐ray sources (ULXs) through a detailed investigation of their spectral shape, using the highest quality X‐ray data available in the XMM–Newton public archives (≳10 000 counts in their EPIC spectrum). We confirm that simple spectral models commonly used for the analysis and interpretation of ULXs (power‐law continuum and multicolour disc blackbody models) are inadequate in the face of such high‐quality data. Instead we find two near ubiquitous features in the spectrum: a soft excess and a rollover in the spectrum at energies above 3 keV. We investigate a range of more physical models to describe these data. Slim discs which include radiation trapping (approximated by a p‐free disc model) do not adequately fit the data, and several objects give unphysically high disc temperatures (kTin > 3 keV). Instead, disc plus Comptonized corona models fit the data well, but the derived corona is cool and optically thick (τ∼ 5–30). This is unlike the τ∼ 1 coronae seen in Galactic binaries, ruling out models where ULXs are powered by sub‐Eddington accretion on to an intermediate‐mass black hole despite many objects having apparently cool disc temperatures. We argue that these observed disc temperatures are not a good indicator of the black hole mass as the powerful, optically thick corona drains energy from the inner disc and obscures it. We estimate the intrinsic (corona‐less) disc temperature, and demonstrate that in most cases it lies in the regime of stellar mass black holes. These objects have spectra which range from those similar to the highest mass accretion rate states in Galactic binaries (a single peak at 2–3 keV) to those which clearly have two peaks, one at energies below 1 keV (from the outer, un‐Comptonized disc) and one above 3 keV (from the Comptonized, inner disc). However, a few ULXs have a significantly cooler corrected disc temperature; we suggest that these are the most extreme stellar mass black hole accretors, in which a massive wind completely envelopes the inner‐disc regions, creating a cool photosphere. We conclude that ULXs provide us with an observational template for the transition between Eddington and super‐Eddington accretion flows, with the latter occupying a new ultraluminous accretion state.

Spectral state transitions of the Ultraluminous X-ray Source IC 342 X-1

“…Based on high-energy spectral curvature, X-1 appears to be in a super-Eddington state in its highest luminosity observations, as has been suggested by previous authors (Watarai et al 2001;Ebisawa et al 2003;Feng & Kaaret 2009;Yoshida et al 2012). This spectral curvature has been theorized to be the result of heavy Comptonization by a cool, optically thick corona when the source is in a high accretion state (Gladstone et al 2009). However, at low luminosity X-1 appears to transition to a sub or near-Eddington accretion state where high energy curvature is less significant.…”

“…Spectral curvature above ∼6 keV in the hard component of a large fraction of ULX spectra (Stobbart et al 2006;Gladstone et al 2009;Miyawaki et al 2009) distinguishes them from BHBs, whose hard component is generally modelled by a powerlaw which extends unbroken to much higher energies, often above ∼100 keV. With sensitivity above 10 keV, recent observations with the Nuclear Spectroscopic Telescope Array (NuStar ) have confirmed a spectral cutoff in ULXs such as NGC 1313 X-1 (Bachetti et al 2013).…”

Irradiated, colour-temperature-corrected accretion discs in ultraluminous X-ray sources

“…sources in the original ultraluminous state description of ULX accretion, as suggested by Gladstone et al (2009). In this scenario, the two components in the X-ray spectrum correspond to emission from a soft truncated disc and a hard corona.…”

“…They use this to calculate a mass function, and find that it likely contains an ∼20-30 M black hole. Gladstone, Roberts & Done (2009) identified three types of ULX X-ray spectra, which they suggested may represent a spectral sequence with increasing accretion rate. They suggested that around the Eddington limit ULXs appear with broad disc-like X-ray spectra, whilst at higher accretion rates a two-component X-ray spectrum emerges.…”

An Optically Thick Disk Wind in Gro J1655–40?

“…Recent deep optical spectroscopy revealed that the broad emission lines of H and He observed in ULXs (and also SS 433, which is the only known Galactic X-ray binary thought to power persistent supercritical accretion; see Fabrika 2004 and references therein) likely originate in dense, strong winds launched on supercritical accretion disks (Fabrika et al 2015). 7 Moreover, Gladstone et al (2009) suggest that massive optically thick winds could be the origin of the soft Comptonization component usually detected in ULXs. This component has a low temperature (≈3 keV) and a large optical depth (t » 5-30), similar to those estimated for the thermal Comptonization component in GRO J1655−40.…”