Astronomers find new evidence for magnetars
BY JEFF FOUST
Posted: December 9, 2000
In a paper published in the December 7 issue of the journal Nature, astronomers from Utrecht University in the Netherlands and the California Institute of Technology reported the discovery of an optical component to a class of stars known as anomalous x-ray pulsars (AXPs) that had previously been linked to the hypothetical magnetars.
The optical counterpart to the AXP, designated 4U 0142+61, was discovered in images taken by the two 10-meter Keck telescopes in Hawaii in 1994 and 1999. The visible-light star seen in the Keck images matches the position of 4U 0142+61, the brightest of the handful of known AXPs.
AXPs are objects that emit regular bursts of x-rays. Earlier this year other astronomers definitively linked AXPs to pulsars, rapidly spinning neutron stars that also emit regular bursts of energy, but at radio wavelengths rather than the more energetic x-rays of the AXPs. The AXP's source of energy, though, has remained a mystery.
The discovery of the visible counterpart to 4U 0142+61 allowed astronomers to consider two possible sources of energy for the object: an accretion disk of matter surrounding the pulsar left over from its creation by a supernova explosion, and the decay of an intense magnetic field generated by the pulsar. The former explanation was ruled out when models of accretion disks computed a brightness for the optical counterpart much higher than observed.
The alternative would require that the x-ray and visible light come from a powerful magnetosphere generated by the pulsar. Such an object would have to have a magnetic field measuring 1015 -- one million billion -- gauss. By comparison, the Sun's magnetic field, the most powerful in the solar system, is only 1000 gauss, while the Earth's is less than one gauss.
Scientists had hypothesized the existence of such intensely magnetized pulsars for nearly a decade, dubbing them magnetars. They were first proposed to explain a class of gamma-ray bursts known as soft gamma-ray repeaters (SGRs) that sporadically generate bursts of gamma rays over the course of several thousand years. The first strong evidence for magnetars came in 1998, when astronomers detected a slowdown in rotation rate of one SGR that could not be explained by accretion disks or other models, but could be explained if the star had a magnetic field powerful enough to distort the star's surface, triggering "starquakes" that released gamma rays. AXPs are thought to be related to SGRs, as both have similar pulse periods as well as x-ray luminosities and spectra.
Astronomers cautioned, though, that their discovery is not outright evidence that magnetars exist. "Basically our work shows that competing models for anomalous x-ray pulsars have difficulties in explaining the optical results," said Ferdi Hulleman of Utrecht University, lead author of the Nature paper. "However, since there is presently no prediction about optical emission from magnetars, we cannot prove or disprove the magnetar model."
Further observations at both optical and x-ray wavelengths are required to determine if 4U 0142+61 truly is a magnetar. "If one would find strong pulsations in the optical emission of the AXP counterpart, this would be a clue that it is indeed a magnetar," said Hulleman. "A strong clue could also be provided by detailed X-ray spectra, which might show features that can be ascribed to the presence of these high magnetic fields."