Sometimes observations confirm a scientific theory perfectly, yet at other times telescopes bring completely new phenomena to light. That is what happened in the case of SRON astronomer Peter den Hartog. ‘I was looking for new sources of high energy X-rays on a celestial chart, made using the space telescope INTEGRAL. To our surprise, at the edge of this chart a star was visible that we knew was a magnetar. However, we never expected that it would emit this type of radiation,' says the researcher, who upon making this discovery immediately requested additional observation time with INTEGRAL for follow-up research.

Magnetars are small compact neutron stars with a magnetic field that is one billion times stronger than what can be artificially made on Earth. They are the strongest magnets in the universe. They have a mass one-and-a-half times that of the Sun but this is squeezed into a sphere with a radius of 10 kilometres. How they form exactly is a mystery. As they emit enormous quantities of energy in the form of X-rays, they have a lifespan of only 10,000 years. The magnetars rotate like mad around their axes, as a result of which they regularly sling a bundle of radiation into space like a lighthouse emitting a beacon of light. Although these X-rays to not reach the Earth's surface, they are nevertheless visible in space with the aid of an X-ray telescope.

For a long time astronomers thought that they had understood the nature of magnetars. The internal energy of a magnetar, stored in the extreme internal magnetic field that spirals through the star, was emitted as relatively low energy X-rays. However, that image was overturned several years ago by SRON astronomer Lucien Kuiper, when he used observations from INTEGRAL to demonstrate that the magnetars emit far more radiation of a far higher energy level. The phenomenon of the magnetars was once again shrouded in mystery. And Peter den Hartog's research has only added to this by revealing even more striking properties.

‘By converting the observations from INTEGRAL, XMM-Newton and RXTE into a type of short film, we could see how the characteristics of the X-rays changed over the course of time,’ explains Den Hartog. The characteristics of the radiation were found to drastically change during the rotation of the magnetar. Den Hartog: ‘Three different processes were found to be taking place in the magnetar that gave rise to three different pulses’. For the time being, the meaning of this Morse code remains a mystery. This is why astronomers look with high expectations forward to the first data of space observatory GLAST due for launch by NASA on the 2nd of June. GLAST will study the high energy radiation from the universe in detail.

SRON is strongly involved in both INTEGRAL and XMM-Newton. SRON astronomer Wim Hermsen is a mission scientist in the INTEGRAL team and as such is closely involved in the satellite's scientific programme. SRON has also built an instrument for XMM-Newton that unravels the X-rays picked up by the telescope and then analyses these in detail.

Peter den Hartog defended his PhD thesis entitled ‘Non-thermal X-ray emission from Anomalous X-ray Pulsars’ on Wednesday 21 May 2008 at the Universiteit van Amsterdam.

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