Soft extragalactic X-ray binaries at the Eddington Threshold

Abstract

The 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. ABSTRACTThe luminosity range at and just below the 10 39 erg s −1 cut-off for defining ultraluminous X-ray sources (ULXs) is a little-explored regime. It none-the-less hosts a large number of X-ray sources, and has great potential for improving our understanding of sources with ∼Eddington accretion rates. We select a sample of four sources in this Eddington Threshold regime with good data for further study; these objects possess a variety of soft spectral shapes. We perform X-ray spectral and timing analysis on the XMM-Newton and Chandra data for these objects to gain insight into their accretion mechanisms, and also examine their optical counterparts using HST images. NGC 300 X-1 is a highly luminous and well-known example of the canonical steep power-law accretion state. M51 ULS exhibits a cool blackbody-like spectrum and is consistent with being an ultraluminous supersoft source (ULS), possibly a super-Eddington accreting object viewed at a high inclination through an optically thick outflowing wind. NGC 4395 ULX-1 and NGC 6946 ULX-1 have unusually steep power-law tails, for which we discuss a variety of possible physical mechanisms and links to similar features in Galactic microquasars, and we conclude that these sources are likely intermediate objects between the soft ultraluminous regime of ULXs and classic ULSs.

A Broadband Look at the Old and New ULXs of NGC 6946

“…Both sources are found to be very soft and at a low enough flux to be a little below the ULX threshold luminosity (both have L X ∼ 6 − 7 × 10 38 erg s −1 ), consistent with these sources being persistently bright Eddington threshold objects that only occasionally reach the ULX luminosity regime. ULX-1 is consistent with a steep power-law spectral shape with soft residuals, as previously found in Earnshaw & Roberts (2017), where it was suggested that it is a super-Eddington accreting source viewed at high accretion rate and/or at a high inclination, with most of the high-energy central emission obscured and down-scattered by the surrounding wind and outer disc. While the most typical examples of ultraluminous supersoft sources have spectra that are almost entirely dominated by thermal emission (e.g.…”

“…It is often referred to in literature as the only ULX of NGC 6946, as the others are less frequently observed at ULX luminosities. However, NGC 6946 also contains the soft sources NGC 6946 ULX-1 (Devi et al 2008;Earnshaw & Roberts 2017) and ULX-2 (Liu & Bregman 2005) that have previously been observed as ULXs.…”

“…Note-1 Fabbiano & Trinchieri (1987), most commonly used name, 2 Associated nebula (Matonick & Fesen 1997) Which component is dominant in the spectrum depends on the inclination and/or mass accretion rate in this model, and at very high inclinations/accretion rates the high-energy emission may be completely obscured and reprocessed, leaving an ultraluminous supersoft source with a spectrum entirely dominated by a soft thermal component (e.g. Di Stefano et al 2004;Urquhart & Soria 2016;Earnshaw & Roberts 2017). Conversely, ULXs with spectra dominated by hard emission are more likely to be sources observed at low inclinations in this picture, and make particularly good targets for NuSTAR given its sensitivity to energies > 10 keV.…”

The hypersoft state of Cygnus X–3

“…The observation coincided with a prolonged state of a quenched radio emission (<21 µJy) and zero Swift/BAT flux (Rushton et al 2016). The hypersoft state also bears similarities with ultraluminous supersoft sources that have very soft, thermal, or steep power law (Γ ∼ 3−4) spectra, which produce disk winds due to super-Eddington accretion (Earnshaw & Roberts 2017).…”

Searching for propeller-phase ULXs in the XMM–Newton Serendipitous Source Catalogue

“…The spectra for NGC 7479 ULX-1 have previously been found to be consistent with it being an extreme super-Eddington accreting stellar-mass object like the majority of the ULX population as a whole (Wang, Zhang & Fan 2010;Sutton et al 2012). Similarly, while NGC 6946 ULX-1 is fainter and much softer, it can also be interpreted as a super-Eddington source viewed at very high inclinations (Earnshaw & Roberts 2017). This shows that super-Eddington sources can be capable of high amounts of inter-observation variability, but does not in itself provide any evidence for a NS nature of the central compact object.…”