A neutron star tumbling through space

Not all pulsars rotate with the stability that astrophysicts had previously assumed

April 20, 2006

It's not easy to spin a top. You need the right kind of snap of the finger to get it moving around an axis. Usually it ends up tumbling over the table, because the push you give it comes too much from one side. An international team including Frank Haberl from the Max Planck Institute for Extraterrestrial Physics has now found a neutron star which ended up also being propelled in the twisted motion of a tumbling top, after being created in a supernova. The researchers observed changes in RX J0720.4-3125's x-ray spectrum over the course of a few years. These kinds of changes happen when the temperature varies on the surface of the neutron star. This leads the scientists to believe that the pulsar has been tumbling slightly through space; its axis is precessing. It thus turns towards us hotter and cooler spots on its surface. Until now, scientists had always thought neutron stars and pulsars rotated very stably. (Astronomy and Astrophysics).

Auf der Röntgen-Aufnahme, die XMM-Newton gemacht hat, erscheint der Neutronenstern RX J0720.4-3125 als helles rötlich leuchtendes Objekt in der Mitte.

Frank Haberl and an international team of scientists evaluated measurements from an ESA satellite, XMM-Newton. Among these measurements were the signals sent out by neutron star RX J0720.4-3125. This pulsar is located about 1000 light years from us in the constellation Canis Major. The pulsar's light is so weak that only the strongest telescopes can detect it. However, as a source of x-rays, RX J0720.4-3125 is quite powerful. Haberl and colleagues from the University of Padua, the Institute for Space Research in Holland, and the Mullar Space Science Laboratory at University College in London have closely inspected the pulsar's x-ray spectra from various years. The researchers determined that the spectra changed in the course of a few years. Between May 2000 and May 2004, the percentage of higher-energy x-rays increased, then later decreased.

This could indicate that the surface temperature of RX J0720.4-3125 also fluctuates as much as 100,000 degrees celsius, about 10%. The surface temperature determines how strongly a heavenly body emits rays from a particular part of the electromagnetic spectrum. Haberl says, "we think it is highly improbable that the surface temperature of the neutron star changes so much in so few years." He and his colleagues guess, rather, that the axis of RX J0720.4-3125 is moving in a circle and that the star is not remaining stable in space. Physicists call this kind of tumbling gyroscopic "precessing". Each of the two poles are moving back and forth in XMM-Newton's visual field. Because the poles are of varying temperatures, they emit various percentages of hard x-rays. It is still not clear to the scientists why one pole is hotter than the other. "Possibly," says Haberl, "the magnetic field is not symmetrical." The magnetic field's strength changes over the surface of the neutron star and influences how much heat is transported out of the interior of the pulsar.

Temperaturschwankungen, die sich aus den Messungen von XMM-Newton für die Oberfläche von RX J0720.4-3125 ergeben.

For years, physicists have observed similar fluxuations - but these only lasted a few seconds. RX J0720.4-3125's x-ray energy profile also changes over seconds. The pulsar rotates at this tempo, and we can observe sides with varying temperatures.

Haberl and his colleagues can only speculate about how RX J0720.4-3125 ended up tumbling in this way. It could be that the neutron star flattened out, as it rotated about its axis 100 times a second shortly after its was created. Haberl suggests, "the effect would have been much smaller than on Earth, because it is so dense." That effect could have been enough, however, to cause the star to tumble, if the supernova that created it also gave a little push.

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