The Magnetar in Pulsar's Clothing

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Rarely do you read an article where pulsars are considered “run of the mill.” Let’s face it, they are pretty extreme. However, new observations of an even more extreme astrophysical object suggest that it may have been masquerading as a “normal” pulsar all along.

Pulsars are a subset of neutron stars, the dense remnants of stellar cores left behind after supernovae. A pulsar doesn’t actually pulse, but it does spin rapidly. Hotspots on the surface, thought to be at the magnetic poles, are often detected with radio telescopes as they rotate into our line of sight. So really, pulsars are a bit like lighthouses.

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Magnetars are pulsars with much stronger magnetic fields. As a result, they exhibit all kinds of weird characteristics, such as pulsing in the x-ray and giving off sporadic bursts of high energy emission. Sixteen such magnetars are known.

It is interesting to note how you even measure the magnetic field of such a distant and weird object. There is a well-known relation in pulsar physics that allows you to determine the dipole magnetic field strength (illustrated in the artist’s conception above, outside the star) just from the period of the pulsar spin and the spin-down rate. The spin-down rate is the measure of how the pulsar rotation slows down as it ages and loses energy.

Enter SGR 0418+5729, detected in June 2009 when it went through several bursts indicative of magnetar behavior. This new x-ray pulsar and “soft-gamma repeater” had a period of 9.1 seconds, also typical for a magnetar. Follow-up observations continued for five months as astronomers waited for signs of pulsar spin-down. However, it never seemed to slow, to the detection limit of their instruments, before it got too close to the sun to be observed.

Starting in July of this year, astronomers marshalled the forces of several x-ray telescopes to monitor this strange pulsar again. In an article on Science Express, they report a new upper limit on the spin-down rate, which means they still have not reliably detected a slowing of the rotation. When you take this through the magnetic field calculations, you find that the dipole magnetic field has to be much lower than that of a magnetar, and that it instead lies in the range of normal pulsars.

A plot for the real pulsar nerds. The x-axis is the spin period of the pulsar, and the y-axis is the spin-down rate. Normal pulsars are black dots and magnetars are red stars. SGR 0418+5729 is plotted with the upper limit for its spin-down and magentic field strength.

Astronomers are currently unable to determine what is powering the x-ray outbursts of SGR 0418+5729, thought it may be a much stronger interior magnetic field that has not been detected (see top illustration again). This find may also mean that other “run of the mill” pulsars are tantrum-throwing magnetars just waiting to happen! However, as the paper’s first author, Nanda Rea, notes, more study will be needed to see if this not-so-magnetized magnetar is the exception or the rule.

Images: Top: Artist’s conception of the new magnetar, showing the outer dipole magentic field and a possible inner magnetic field. Credit: NASA/CXC/M.Weiss. Bottom: Credit: N. Rea.

A preprint of the paper can also be found on arxiv.org.

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