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

Abstract

In recent work with high-resolution grating spectrometers (RGS) aboard XMM-Newton Pinto et al. (2016) have discovered that two bright and archetypal ultraluminous X-ray sources (ULXs) have strong relativistic winds in agreement with theoretical predictions of high accretion rates. It has been proposed that such winds can become optically thick enough to block and reprocess the disk X-ray photons almost entirely, making the source appear as a soft thermal emitter or ultraluminous supersoft Xray source (ULS). To test this hypothesis we have studied a ULX where the wind is strong enough to cause significant absorption of the hard X-ray continuum: NGC 55 ULX. The RGS spectrum of NGC 55 ULX shows a wealth of emission and absorption lines blueshifted by significant fractions of the light speed (0.01 − 0.20)c indicating the presence of a powerful wind. The wind has a complex dynamical structure with the ionization state increasing with the outflow velocity, which may indicate launching from different regions of the accretion disk. The comparison with other ULXs such as NGC 1313 X-1 and NGC 5408 X-1 suggests that NGC 55 ULX is being observed at higher inclination. The wind partly absorbs the source flux above 1 keV, generating a spectral drop similar to that observed in ULSs. The softening of the spectrum at lower (∼ Eddington) luminosities and the detection of a soft lag agree with the scenario of wind clumps crossing the line of sight, partly absorbing and reprocessing the hard X-rays from the innermost region.

Lense-Thirring precession in ULXs as a possible means to constrain the neutron star equation of state

“…We now know from high resolution imaging of an archetypal ULX that it cannot be associated with gas beyond ∼25pc of the source and in Middleton et al (2014) the residuals were re-interpreted as low resolution, blue-shifted atomic features from an outflowing wind for the first time. This was subsequently confirmed in a high energy-resolution spectral analysis using XMM-Newton's RGS at soft energies (Pinto et al 2016(Pinto et al , 2017) and a CCD-energy-resolution spectral analysis at higher energies in a combined XMM-Newton/NuSTAR study (Walton et al 2016a). The features at low (CCD-quality) energyresolution appear ubiquitous in the population of bright ULXs and in one well studied case decrease in strength with spectral hardness (Middleton et al 2015b), thought to indicate an equatorial (i.e.…”

Super-Eddington accretion on to the neutron star NGC 7793 P13: Broad-band X-ray spectroscopy and ultraluminous X-ray sources

“…Poutanen et al 2007). Indeed, unambiguous evidence for powerful outflows has now been seen through resolved, blueshifted atomic features in a handful of other ULX systems with deep XMMNewton coverage, primarily in the soft X-ray band (<2 keV) but in the case of NGC 1313 X-1 also in the iron Kα band (Pinto et al 2016(Pinto et al , 2017Walton et al 2016b). If our proposed scenario for P13 is correct, and thick inner regions of the flow do begin to form, then we might expect to see evidence for a similar wind in P13, either through ionised absorption features if the outflow crosses our line of sight, or ionised emission otherwise.…”

The Variable Fast Soft X-Ray Wind in PG 1211+143

“…A wealth of absorption lines were also observed in the RGS spectrum of NGC 4051 with velocity components of up to ∼−10,000 km s −1 (Pounds & Vaughan 2011), interpreted as a cooling shocked flow (Pounds & King 2013). Finally, in addition to such detections in AGNs, evidence for highvelocity outflows in the soft X-ray band has also been seen in ultraluminous X-ray sources (ULXs)-e.g., in NGC 1313 X-1 and NGC 5408 X-1 (Pinto et al 2016(Pinto et al , 2017a(Pinto et al , 2017c, with measured outflow velocities of ∼−0.2c. This paper focuses on PG 1211+143, which is a luminous narrow-line Seyfert galaxy/quasi-stellar object (QSO) at a distance of 331 Mpc (z = 0.0809; Marziani et al 1996).…”