Researchers are gaining ground in the struggle to understand the mysterious objects known as UltraLuminous X-ray sources (ULXs). These objects, named for their extreme brightness at X-ray wavelengths, are thought to be dense, compact objects like black holes or neutron stars. Their luminosity (which extends to other wavelengths) arises as they actively draw matter from an orbiting companion.

“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, a research associate from the Institute of Astronomy in Cambridge, UK, and author of a recent study. “Some host highly magnetised neutron stars, while others might conceal the long-sought-after intermediate-mass black holes, which have masses around 1000 times the mass of the Sun. But in the majority of cases, the reason for their extreme behaviour is still unclear.”

It’s been difficult to study them in detail because we've lacked the sensitivity needed to identify the emission lines and/or absorption lines created by specific elements. When light passes through material such as gas, certain wavelengths are absorbed by elements in the gas, leaving a blank line in the light source’s spectrum. Emission lines are light emitted by the element itself.

Imaging these lines would allow researchers to learn a lot about the environment around the source of ULXs, which might help clear up one of the weirdest mysteries about them: they’re brighter than they should be. Depending on their masses, their intense light output could indicate that they’re drawing in matter faster than something called the Eddington limit. Beyond the Eddington limit, the light generated by the infalling matter pushes new material away.

There are scenarios in which an object can get around this limit. But it would be nice to know which, if any, are acting here.

ULX marks the spot

A new study tackled this problem with better-resolution observations, courtesy of the European Space Agency’s XMM-Newton Observatory. XMM-Newton provided high-resolution X-ray imaging of ULXs in binary systems in three different galaxies. These sources were chosen because they were close enough to Earth (all within 22 million light-years of the Milky Way) and bright enough to produce high-quality data. One of them turned out to have a poorly resolved spectrum, so the researchers focused on the other two.

For both these sources, they detected both emission and absorption lines. The emission lines were coming from ionized iron, oxygen, and neon. Their spectra was characteristic of gas moving relatively slowly around the ULX. That wasn't unusual. Black holes and neutron stars typically are surrounded by a disk of infalling material known as an accretion disk.

The absorption lines, meanwhile, were extremely blue-shifted. This happens when the source is moving toward us (a source moving away would be red-shifted). It means that an outflow of material is being shot away from the ULX (more or less) toward us. And based on how far the lines are shifted toward the blue end of the spectrum, the researchers are able to tell how fast this material is moving: about twenty percent the speed of light.

Such outflows are predicted for supermassive black holes and also for ordinary (stellar-mass) black holes that have high accretion rates.

Progress!

This doesn't tell us what sort of objects the ULXs are. Stellar-mass black holes, intermediate-mass black holes, and neutron stars are still all on the table. However, this study was the first one that allowed researchers to piece together a picture of the immediate environment around a ULX. Since we now know that some ULXs have powerful outflows, we can watch for this behavior in other objects like these.

Additionally, the team is still combing the XMM-Newton archives for more data to build on their work, as well as planning new observations in X-ray, optical, and radio wavelengths.

“This is the first time we’ve seen winds streaming away from ultra-luminous X-ray sources,” said Pinto. “And there's more, since the very high speed of these outflows is telling us something about the nature of the compact objects in these sources, which are frantically devouring matter. With a broader sample of sources and multi-wavelength observations, we hope to finally uncover the physical nature of these powerful, peculiar objects.”

Nature, 2015. DOI: doi:10.1038/nature17417 (About DOIs)