Resolved atomic lines reveal outflows in two ultraluminous X-ray sources

TL;DR

High-resolution X-ray spectroscopy of ultraluminous X-ray sources reveals fast outflows and emission lines, providing insights into accretion processes and wind properties around compact objects in these extreme systems.

Contribution

First detection of blueshifted absorption lines indicating high-velocity outflows in ultraluminous X-ray sources using high-resolution spectra.

Findings

Detection of highly ionized emission lines with >5 sigma significance.

Identification of blueshifted absorption lines indicating outflows at ~0.2c.

Evidence for powerful winds around the compact objects.

Abstract

Ultraluminous X-ray sources are extragalactic, off-nucleus, point sources in galaxies with an X-ray luminosity above 3x10^39 erg/s, thought to be powered by accretion onto a compact object. Possible explanations include accretion onto neutron stars with strong magnetic fields, stellar-mass black holes (< 20 Msolar) at or in excess of the classical Eddington limit or intermediate-mass black holes (10^3-5 Msolar). The lack of sufficient energy resolution in previous analyses has hitherto prevented an unambiguous identification of any emission or absorption lines in the X-ray band, thereby precluding a detailed analysis of the accretion flow. Here we report the presence of X-ray emission lines arising from highly ionized iron, oxygen and neon with a cumulative significance > 5 sigma, and blueshifted (~0.2c) absorption lines (5 sigma) in the high-resolution X-ray spectrum of the ultraluminous X-ray source NGC 1313 X-1. In a similar source, NGC 5408 X-1, we also detect emission lines at rest and blueshifted absorption. The blueshifted absorption lines must occur in a fast outflowing gas, whereas the emission lines originate in slow-moving gas around the source. We conclude that the compact object is surrounded by powerful winds with an outflow velocity of about 0.2c as predicted by models of accreting supermassive black holes and hyper-accreting stellar mass black holes.