Image credit: NASA/JPL-Caltech/SAO/NOAO

Voracious neutron star discovered

Friday 10 Oct 14


Finn Erland Christensen
Senior Scientist Emeritus
DTU Space
Astronomers have discovered a neutron star—a pulsating dead star which emits X-rays with an energy equivalent to around 10 million suns. The discovery was published in the latest issue of Nature and was made using the telescope on NASA's NuSTAR satellite, with DTU playing a key role.

The article in Nature describes an X-ray source observed in the relatively close M82 galaxy, only 12 million light-years away. X-ray sources such as these are extremely clear. When stellar mass from a companion star is consumed by the X-ray source, it shines brightly. Until now, it was believed that these X-ray sources were black holes.

But the X-ray source in the M82 galaxy turned out to be a neutron star, also called a pulsar. Neutron stars emit X-rays from two hot spots. As the star rotates this radiation hits the Earth like a light from a lighthouse, such that the signal detected by the satellite pulsates.

Finn Christensen, Senior Researcher at DTU Space and co-author of the article, explains DTU's contribution to the observations:

"We developed an X-ray reflecting surface for the mirrors in the NuSTAR telescopes, which has made it possible to see these energy releases at higher energy levels than previously possible. NuSTAR has therefore been able to make these observations in a new way, essentially enabling the discovery."

This neutron star (M82 X-2) has an extremely powerful magnetic field which rotates about its axis every 1.37 seconds, while a companion star orbits it every 2.5 days. The system emits huge energy amounts—equivalent to around 10 million suns.

Like black holes, neutron stars are the burned-out cores of exploded stars, but with much less mass. M82 X-2 has a mass roughly equal to the sun. If it were a black hole, its mass would be a few hundred times greater. This leaves researchers with more questions than answers, as there are indications that it devours as much stellar matter as a black hole—growing at ten times the rate measured for a neutron star to date.

The discovery therefore challenges our accepted knowledge, explains Finn Christensen:

"It was believed that medium-sized black holes were the source of the huge X-ray radiation from these systems. So we expected to have to find their 'trails'. But that is not what is happening at all. Since we have been able to make these observations in a new way, we have found something which is basically simple, yet something we have been unable to see before. We now have a better idea what to look for and that will give us much more knowledge," says Finn Christensen.

"I am confident that in the years ahead we will find many more neutron stars in the these systems, which we had thought were purely the domain of black holes."

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