Even ‘Weakling’ Magnetars are Strong and Powerful

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The name alone, “magnetar” elicits a magnificent, powerful and strong astronomical object, and most of these “magnetic stars” are whirling, X-ray blasting dynamos, shooting out strong bursts of energy. But there are some magnetars which seem to have a softer, quieter side, and are called soft gamma repeaters and anomalous X-ray pulsars. However, they might not be as soft as they appear. A team of astronomers using the several different space- and Earth-based observatories have found a supposed ‘weakling’ was only masking its superpowers. The new findings indicate the presence of a huge internal magnetic field in these seemingly less powerful pulsars, which is not matched by their surface magnetic field.

Magnetars are a type of neutron stars, which are the collapsed remains of massive, rapidly rotating stars. They collapses down to tiny cores, with the hot neutron liquid rising and falling from the center to the crust setting up a dynamo effect, creating that incredible magnetic field. Although they are on average only about 30km in diameter, a magnetar can have a magnetic field billions of times that of our Sun.

It was thought that dramatic flares and bursts of energy came from only the strong class of magnetars, but these same features have been observed emanating from a weakly magnetized, slowly rotating pulsar.

“We have now discovered bursts and flares, i.e. magnetar-like activity, from a new pulsar whose magnetic field is very low,” said Dr Silvia Zane, from UCL’s (University College London) Mullard Space Science Laboratory, and an author of the research.

The neutron star, SGR 0418+5729, was discovered on June 5, 2009 when the Fermi Gamma-ray Space Telescope detected bursts of gamma-rays from this object. Follow-up observations four days later with the Rossi X-Ray Timing Explorer (RXTE) showed that, in addition to sporadic X-ray bursts, the neutron star exhibits persistent X-ray emission with regular pulsations that indicate that the star has a rotational period of 9.1 seconds.

What makes SGR 0418 different from similar neutron stars is that, unlike those stars that are observed to be gradually rotating more slowly, continued monitoring of SGR 0418 over a span of 490 days has revealed no evidence that its rotation is decreasing.

“It is the very first time this has been observed and the discovery poses the question of where the powering mechanism is in this case. At this point, we are also interested in how many of the other normal, low field neutron stars that populate the galaxy can at some point wake up and manifest themselves as a flaring source,” said Zane.

The team of astronomers, led by Dr. Nanda Rea of Institut de Ciencies de l’Espai (ICE-CSIC, IEEC) in Barcelona, wonder how large an imbalance can be maintained between the surface and interior magnetic fields. SGR 0418 represents an important test case.

“If further observations by Chandra and other satellites push the surface magnetic field limit lower, then theorists may have to dig deeper for an explanation of this enigmatic object,” said Rea.

Sources: Chandra Blog, University College, London (via Eurekalert)