"Garden hose" jet trail nebula pictured

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 22 April, 2009

A jet trail nebula, a once theoretical object never before seen has been discovered by NASA's Rossi X-ray Timing Explorer (RXTE).

The fan-like nebulosity is clearly visible in this image from the ESO 3.6-m telescope. As the binary powers through the ISM, it leaves two long trails behind it. Image: K. Wiersema / ESO / University of Leicester.

RXTE is a dedicated X-ray satellite, scanning the centre of our Galaxy for a multitude of variable X-ray sources, such as X-ray binary systems. These comprise a compact neutron star or black hole that grabs material from a normal companion star, swirling it into hot, X-ray emitting discs. X-ray binaries are also known to spew out jets of gas at velocities very close to the speed of light.

The majority of X-ray binaries are highly variable in intensity but RXTE has uncovered a subclass that is nearly constant in brightness and surprisingly faint.

Dr Wiersema and colleagues obtained positions of the X-ray sources using the NASA Chandra X-ray space telescope, and optical signals from the European Southern Observatory's 3.6-m telescope at La Silla in Chile to confirm the sources as X-ray binaries.

One of these sources threw up a surprise: in addition to a faint optical source, a bright large nebula unlike any other was present. After meticulous study, the team concluded that it appears to be made by the powerful jets of the X-ray binary as they slam into the interstellar medium (ISM), lighting up the tenuous gas there. As the binary system moves rapidly through the galaxy, the jet-ISM interaction points move with it, creating the so-called "jet trails" visible in the image.

These jet trails were once just a prediction from theory, and are extremely rare. They require that the X-ray binary has to move very rapidly - in this case about 100 kilometres 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 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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