A scientific researcher from the University of Southampton is leading an international team that has discovered the fastest-yet-seen accreting X-ray pulsar.
Dr Simon Shaw of the University’s School of Physics and Astronomy is UK representative to the INTEGRAL Science Data Centre near Geneva, Switzerland (ISDC is part of the Geneva University Observatory). There he co-ordinates a team that receives and monitors data from INTEGRAL, a European Space Agency (ESA) satellite designed to detect X and gamma-ray radiation from space.
A previously unknown, bright source of X-rays was first spotted in INTEGRAL data at the ISDC in December 2004. It was named ‘IGR J00291+5934’ and its discovery was announced to astronomers around the world shortly after. Follow-up observations made in the next few weeks, during which time the source slowly faded, showed that IGR J00291+5934 was the fastest known accreting binary X-ray pulsar.
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A binary system is formed of two stars orbiting each other. If one of these stars undergoes a super-nova explosion it may collapse to form a ‘neutron star’ – an object composed entirely of neutrons. Neutron stars are incredibly dense, weighing slightly more than our Sun but compacted into a sphere with a size similar to Southampton; a spoonful of neutron star material would weigh about the same as the total weight of every person on Earth.
The strong gravitational field around the neutron star causes material to be pulled off the orbiting star, which spirals onto the neutron star, in a process known as ‘accretion’. The magnetic field of the neutron star causes the accreted matter to be channelled onto small ‘hot-spots’ on the neutron star surface where they radiate X and gamma-rays. A ‘pulsar’ is observed when regular flashes, or pulsations, are seen from the hot-spots as the neutron star spins; this can be thought of in exactly the same way as the periodic flashes seen from the rotating beam of light in a lighthouse.
However, this particular lighthouse is rotating approximately 600 times a second, equivalent to the surface of the pulsar moving at 30,000 km/second (10 per cent of the speed of light) – the fastest of its kind yet observed. The orbital period of the system is also impressive; the two stars orbit each other every 2.5 hours, but are separated by roughly the same distance as the Moon and the Earth. On the pulsar in IGR J00291+5934 a day lasts 0.0016 seconds and a year is 147 minutes!
‘The rate at which this object is spinning is truly amazing,’ commented Dr Shaw. ‘It gives us an opportunity to study the effects of such extreme forces of this rotation on the exotic material found in neutron stars, which does not exist on Earth. It is possible that there are more of these objects waiting to be discovered, possibly even faster ones; if they are there, INTEGRAL will find them.’
Dr Shaw is the lead author of a paper on the object accepted for publication by the journal Astronomy and Astrophysics. Pre-print available from http://arxiv.org/abs/astro-ph/0501507
University of Southampton News Release