Using a Danish telescope, scientists from DTU Space have caught rare, longer lasting X-ray flashes and have helped to identify a white dwarf star approx. 25,000 light years away from Earth. The discovery can bring astrophysicists a step closer to providing an answer to how the heaviest elements in the Universe were formed.
It was three powerful X-ray flashes 25,000 light years from Earth towards the centre of the Milky Way that revealed the white dwarf. Each flash lasted up to half an hour, which is surprisingly long, and it became clear for the astrophysicist Jérôme Chenevez from DTU Space and his international partners that the source must be a white dwarf. The surface of white dwarves consists of helium, which in large volumes can develop these rare, long X-ray flashes, which have only been observed on very few occasions.
A neutron star is 'eating' the dwarf
In fact, the flashes do not come from the white dwarf itself, but from its neighbour, which is a neutron star. Together, the dwarf and the neutron star form a double star system, and the neutron star’s extremely powerful gravitational field ‘sucks’ the helium to it from the dwarf. When the neutron star has gathered sufficient helium to cover its entire surface with a layer in excess of 100 m, it explodes due to the pressure from the star’s enormous gravitational field. During the explosion, the neutron star’s radius expands from 10 km to as much as 50 km.
Danish X-ray camera revealed the dwarf
The X-ray flashes were observed with JEM-X, an X-ray camera developed by DTU Space. JEM-X is located on board the INTEGRAL satellite, which has been in orbit around the Earth since 2002. JEM-X is behind the sighting of half of the twelve long-lasting X-ray flashes which have ever been observed. The camera has a field of view of 5 degrees, which makes it possible to monitor large areas of the sky at any one time. JEM-X therefore has much more of a chance of catching the rare flashes than other X-ray instruments, which usually only have a fraction of a degree’s field of view.
X-ray flashes reveal how heavy elements are formed
The discovery of the white dwarf and its neutron star neighbour can bring astrophysicists a step closer to providing an answer to how the heaviest elements in the Universe were formed. The lighter elements were formed in stars such as our own sun, but ordinary stars do not possess sufficient energy to form elements heavier than iron. The heavy elements were probably formed in supernovas and in violent explosions on neutron stars, like that which emitted the three long X-ray flashes.
Scientists still don’t know exactly which elements are formed in the various cosmic explosions, but this is one of the mysteries these X-ray flashes can perhaps help to solve. The article about the white dwarf and its neighbouring neutron star has just been highlighted as the best of the week in the renowned periodical Astronomy & Astrophysics.
For further information, please contact Jérôme Chenevez.