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

TL;DR

This study investigates NGC 55 ULX, a bright X-ray source with a powerful wind, supporting the idea that winds can obscure hard X-ray emission and transform ULXs into ultraluminous supersoft sources.

Contribution

It provides observational evidence of a strong, complex wind in NGC 55 ULX that causes spectral softening, supporting the wind obscuration model for ULSs.

Findings

Detection of blueshifted emission and absorption lines indicating a powerful wind

Wind causes spectral drop similar to ultraluminous supersoft sources

Spectral softening and soft lag support wind clump absorption scenario

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 X-ray 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.