Nature of the Soft Ulx in NGC 247: Super-Eddington Outflow and Transition Between the Supersoft and Soft Ultraluminous Regimes

Abstract

We report on XMM-Newton/Chandra/Swift/Hubble Space Telescope observations of the ultraluminous X-ray source (ULX) in NGC 247, which is found to make transitions between the supersoft ultraluminous (SSUL) regime with a spectrum dominated by a cool (∼0.1 keV) blackbody component and the soft ultraluminous (SUL) regime with comparable luminosities shared by the blackbody and power-law components. Multi-epoch observations revealed an anti-correlation between the blackbody radius and temperature, µ - R T bb bb 2.8 0.3 , ruling out a standard accretion disk as the origin of the soft X-ray emission. The soft X-ray emission is much more variable on both short and long timescales in the SSUL regime than in the SUL regime. We suggest that the SSUL regime may be an extension of the ultraluminous state toward the high accretion end, being an extreme case of the SUL regime, with the blackbody emission arising from the photosphere of thick outflows and the hard X-rays being emission leaked from the embedded accretion disk via the central low-density funnel or advected through the wind. However, the scenario that the supersoft ULXs are standard ULXs viewed nearly edge-on cannot be ruled out. Flux dips on a timescale of 200 s were observed. The dips cannot be explained by an increase of absorption, but could be due to the change of accretion rate or related to thermal fluctuations in the wind or disk. The optical emission of NGC 247 ULX exhibits a blackbody spectrum at a temperature of 19,000K with a radius of 20  R , likely arising from an OB supergiant companion star.

From ultraluminous X-ray sources to ultraluminous supersoft sources: NGC 55 ULX, the missing link

“…In the last decade it has been proposed that a substantial fraction of the population of ultraluminous X-ray sources (ULXs) and ultraluminous supersoft sources (ULSs) are powered by super-Eddington accretion onto compact objects such as neutron stars and black holes (see, e.g., King et al 2001, Roberts 2007, Urquhart and Soria 2016, Feng et al 2016, and references therein). In particular, ULSs could be a category of ULXs observed at high inclination angles, possibly edge on, where a thick layer of material is obscuring the innermost hard X-ray emitting regions (e.g., Kylafis &Xilouris 1993 andPoutanen et al 2007).…”

“…NGC 247 ULX is a very interesting source which oscillate around the boundary between the ULS and the soft ULX regime (see also, e.g., Feng et al 2016). Unfortunately, no deep grating observations are available for this object and for ULSs in general (see Sect.…”

“…Classical X-ray binaries and ULXs, either have broad-band emission over the 1-10 keV range or a peak disk-blackbody temperature of a few keV. Alternative models have tried to describe the X-ray spectra of ULSs with accreting intermediate-mass and stellar-mass black holes like for ULXs or with extreme supersoft sources powered by surface-nuclear-burning on white dwarf accretors (see, e.g., Di Stefano & Kong 2004, Soria & Kong 2016, Feng et al 2016, and references therein).…”

“…It is therefore speculated that ULXs and ULSs are simply two types of super-Eddington accretors, with respectively geometrically thinner and thicker outflows along the line of sight due to different viewing angles or mass accretion rates (e.g. Poutanen et al 2007;Urquhart and Soria 2016;Feng et al 2016 and reference therein). To some extent, this scenario is similar to the unification scenario proposed for active galactic nuclei (e.g., Elvis 2000, and references therein).…”

A Wind-disk Self-irradiation Model for Supercritical Accretion

“…As mentioned above, UV spectrum is the key to testing the model. By searching the literature, there is only one source, NGC 247 X-1 (aka NGC 247 ULX), with such data available (Tao et al 2012;Feng et al 2016). The SED consists of a slitless UV spectrum and broadband photometry covering a wavelength range from 1350Å to 15000Å, measured with the Hubble Space Telescope (HST).…”