In both cases, the gas forms a swirling disk around the compact and dense central object; friction in the disk causes the gas to heat up and emit light at many wavelengths, with a peak in X-rays.
Not all of the gas is swallowed by the central object though, and some of it might even be pushed away by powerful winds and jets.
But an intermediate class of objects was discovered in the 1980s and is still not well understood. Ten to a hundred times brighter than ordinary X-ray binaries, these sources are nevertheless too faint to be linked to accreting supermassive black holes and, in any case, are usually found far from the center of their host galaxy.
“We think these ‘ultra-luminous X-ray sources’ are somewhat special binary systems, sucking up gas at a much higher rate than an ordinary X-ray binary,” said Ciro Pinto from the Institute of Astronomy in Cambridge, United Kingdom.
“Some host highly magnetized neutron stars, while others might conceal the long-sought-after intermediate-mass black holes, which have masses around 1,000 times the mass of the Sun. But in the majority of cases, the reason for their extreme behavior is still unclear.”
Ciro and his colleagues delved into the XMM-Newton archives and collected several days’ worth of observations of three ultra-luminous X-ray sources, all hosted in nearby galaxies located less than 22 million light-years from our Milky Way.
The data were obtained over several years with the Reflection Grating Spectrometer, a highly sensitive instrument that allowed them to spot subtle features in the spectrum of the X-rays from the sources.
In all three sources, the scientists were able to identify X-ray emission from gas in the outer portions of the disk surrounding the central compact object, slowly flowing towards it.
But two of the three sources — known as NGC 1313 X-1 and NGC 5408 X-1 — also show clear signs of X-rays being absorbed by gas that is streaming away from the central source at an extremely rapid 43,500 miles per second (70,000 kilometers), almost a quarter of the speed of light.
“This is the first time we’ve seen winds streaming away from ultra-luminous X-ray sources,” said Ciro.
While the hot gas is pulled inwards by the central object’s gravity, it also shines brightly, and the pressure exerted by the radiation pushes it outwards. This is a balancing act: the greater the mass, the faster it draws the surrounding gas. But this also causes the gas to heat up faster, emitting more light and increasing the pressure that blows the gas away.
There is a theoretical limit to how much matter can be accreted by an object of a given mass, called the “Eddington luminosity”. It was first calculated for stars by astronomer Arthur Eddington, but it can also be applied to compact objects like black holes and neutron stars.
Eddington’s calculation refers to an ideal case in which both the matter being accreted onto the central object and the radiation being emitted by it do so equally in all directions.
But the sources studied by Ciro and his collaborators are being fed through an accretion disk that is likely being puffed up by internal pressure of the gas flowing at a fast pace towards the central object.
In such a configuration, the material in the disk can shine 10 times or more above the Eddington limit and, as part of the gas eludes the gravitational grasp from the central object, high-speed winds can arise like the ones observed by XMM-Newton.
“By observing X-ray sources that are radiating beyond the Eddington limit, it is possible to study their accretion process in great detail, investigating by how much the limit can be exceeded and what exactly triggers the outflow of such powerful winds,” said Norbert Schartel from ESA.
The nature of the compact objects hosted at the core of the sources observed in this study is, however, still uncertain, although the scientists suspect it might be stellar-mass black holes, with masses of several to a few dozen times that of the Sun.
To investigate further, the team is still scrutinizing the data archive of XMM-Newton, searching for more sources of this type and are also planning future observations in X-rays as well as at optical and radio wavelengths.
“With a broader sample of sources and multi-wavelength observations, we hope to finally uncover the physical nature of these powerful peculiar objects,” said Ciro.