Astronomy is the scientific study of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earth's atmosphere (such as the cosmic background radiation). It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the formation and development of the universe.

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Team leader Dr Klaas Wiersema of the University of Leicester will present the discovery on Wednesday 22nd April at the European Week of Astronomy and Space Science conference at the University of Hertfordshire.

The RXTE satellite has been scanning the centre of our galaxy every few days for the last years, searching for variable X-ray sources. Through these scans it has found a multitude of varying X-ray sources, most of which are thought to be X-ray binaries. These systems consist of a compact star (a neutron star or black hole) that pulls material away from a "normal" companion star. This material forms hot disks, which emit X-rays. X-ray binaries are also known to spout jets of gas at velocities very close to the speed of light.

While most of them are highly variable in intensity, there is also a subclass found by RXTE which is nearly constant in brightness and rather faint. It is this class of sources that Dr Wiersema and his team set out to study. They obtained accurate positions of the X-ray sources using the NASA Chandra X-ray space telescope and used the European Southern Observatory’s 3.6-m telescope at La Silla in Chile to search for the corresponding optical signals. The sources were then confirmed as X-ray binaries.

But one of these sources surprised the team. In addition to a faint optical source a bright large nebula (cloud of gas and dust) was visible on the optical images. This nebula consists of two stripes, and is like no other nebula seen before – it is a completely new class of object.

Careful measurements of the shape of the nebula helped the team to understand the origin of the nebula: it appears to be made by the powerful jets of the X-ray binary. The jets of the binary slam into the interstellar medium (ISM - the tenuous gas between the stars), where they make the gas radiate. As the binary moves rapidly through the galaxy, the jet-ISM interaction points move with it, creating the so-called "jet trails" we see in the image.

These trails had been predicted by theorists in the past, but despite searches were not seen before in other sources, as they require a rare set of circumstances to form: the X-ray binary has to move very rapidly (in this case about 100 km per second across the line of sight), and the interstellar medium has to be denser than normal.

Dr Wiersema compares the nebula pattern to garden hoses on soil. “Imagine holding two powerful hoses, pointing to the ground. Where the water hits the ground, mud splashes up. If you stand still, a large circular patch of mud would form and slowly spread out. But if you walk quickly across the garden, you make two parallel stripes of mud. The jets from the X-ray binary make the nebula in the same way.”

The accidental discovery of this nebula gives astronomers a powerful new tool to help them understand how X-ray binaries live their life. The power of the jet now and in the past can be derived from the shape and brightness of the nebula and shapes a new view of the way X-ray binaries produce these jets.

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