Optically thick outflows in ultraluminous supersoft sources

Abstract

Ultraluminous supersoft sources (ULSs) are defined by a thermal spectrum with colour temperatures ∼0.1 keV, bolometric luminosities ∼ a few 10 39 erg s −1 , and almost no emission above 1 keV. It has never been clear how they fit into the general scheme of accreting compact objects. To address this problem, we studied a sample of seven ULSs with extensive Chandra and XMM-Newton coverage. We find an anticorrelation between fitted temperatures and radii of the thermal emitter, and no correlation between bolometric luminosity and radius or temperature. We compare the physical parameters of ULSs with those of classical supersoft sources, thought to be surfacenuclear-burning white dwarfs, and of ultraluminous X-ray sources (ULXs), thought to be super-Eddington stellar-mass black holes. We argue that ULSs are the sub-class of ULXs seen through the densest wind, perhaps an extension of the soft-ultraluminous regime. We suggest that in ULSs, the massive disk outflow becomes effectively optically thick and forms a large photosphere, shrouding the inner regions from our view. Our model predicts that when the photosphere expands to 10 5 km and the temperature decreases below ≈50 eV, ULSs become brighter in the far-UV but undetectable in X-rays. Conversely, we find that harder emission components begin to appear in ULSs when the fitted size of the thermal emitter is smallest (interpreted as a shrinking of the photosphere). The observed short-term variability and absorption edges are also consistent with clumpy outflows. We suggest that the transition between ULXs (with a harder tail) and ULSs (with only a soft thermal component) occurs at blackbody temperatures of ≈150 eV.

Two Eclipsing Ultraluminous X-Ray Sources in M51

“…However, neither of them is an ultraluminous supersoft source (ULS). This supports our earlier suggestion (Urquhart & Soria 2016) that ULSs require not only a high viewing angle, but an accretion rate high enough to produce effectively optically thick outflows.…”

“…Both positions are subject to the standard uncertainty in the absolute astrometry of Chandra pointings, ≈0 6 at the 90% confidence level. 3 A more precise determination of their positions is left to a follow-up study (R. Soria et al 2016, in preparation) of their optical and radio counterparts.…”

“…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.…”

“…We will also briefly discuss more general implications and opportunities provided by the detection of eclipses for our modeling of these systems. In a companion paper (R. Soria et al 2016, in preparation) we will present a study of the optical counterparts and other interesting, newly discovered properties of those same two ULXs, which show optical and radio evidence of jets and outflows.…”