Representation of a pulsar with intense, strongly focused radio radiation. (Image: ESA / ATG medialab)
Reading aloud Pulsars are the beacons of the cosmos: the extremely compact neutron stars surrounded by a strong magnetic field send out rotating bundles of X-rays and radio waves far into space like a kind of lighthouse. Some of them turn out to be real chameleons: they seem to suddenly change the intensity and composition of their radio wave pulses. Why, so far remained unclear. An international team of astronomers has now observed something surprising in one of these chameleon pulsars - and may have come a step closer to explaining the moods of neutron stars. A pulsar is essentially nothing more than the remnant of a massive star. When exploded in a supernova, it hurls most of its outer matter into space, but its nucleus collapses to form an extremely compressed, fast-rotating neutron star. To achieve its density, one would have to compress the sun on a sphere of only 20 kilometers in size. Its magnetic field is extremely compressed and amplified in a pulsar. It ensures that a large part of the radiation emitted by it, often in the area of ​​radio waves or X-rays, is bundled into two intense cones at the poles. "However, what gives rise to these radiation beams and makes them so extremely energetic is still largely unclear, " explains first author Wim Hermsen from the University of Amsterdam.

And something else is a mystery. Some pulsars are extremely moody: their radio radiation suddenly drops extremely within seconds and changes their frequency. The timing of the jump is not predictable. After a few hours or days, the pulsars then return to their original state, as if nothing had happened. "What causes these stars to change their condition, we do not know - we are completely in the dark, " says astronomer Joanna Rankin of the University of Vermont.

Global toggle switch

Hermsen, Rankin and her team have now examined one of these chameleon pulsars in more detail - not only its radio-wave emissions, but in parallel also the X-rays it emits with the help of the X-ray satellite XMM-Newton of the European Space Agency ESA. They discovered something surprising: Whenever the radio radiation of the star suddenly decreased, its emissions in the X-ray range increased drastically - similar to when a toggle between the different wavelength ranges of radiation is switched. "To our great surprise, we found that as the signal brightness decreased by half, the X-ray brightness was doubled, " says Hermsen. And only in its bright phase does the X-ray pulsar pulsate measurably. display

This observation, however, torpedoes previous assumptions about the origin of the fluctuations, which were thought to be due to a local influence of the radio radiation by the magnetic field at the poles of the star. However, the now discovered change between the short-wave X-ray and the long-wave radio radiation can not be explained by such local processes. "Something has to happen globally, " says Rankin. The entire global environment of the pulsar must undergo a transformation in order to be able to switch so strongly in just over one second. According to current theory, the radio waves arise when high-energy particles are thrown outwards into the magnetic field, but the X-rays are formed when these particles are accelerated inwards and strike the surface of the pulsar.

What exactly animates the chameleon pulsars to their changeable behavior, is therefore not clear even after the new observations. The astronomers now know, after all, that the process probably covers the entire magnetic field of the neutron star. How exactly, should now further investigations on other, also between radio and X-radiation changing pulsars clarify.

Wim Hermsen (University of Amsterdam) et al .: Science, doi: 10.1126 / science.1230960

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