A Pulsar Eases Off the Brakes

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In 2006, pulsar PSR 1846–0258 unexpectedly launched into a series of energetic X-ray outbursts. Now a study has determined that this event may have permanently changed the behavior of this pulsar, raising questions about our understanding of how pulsars evolve.

Between Categories

A pulsar — a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation — can be powered by one of three mechanisms:

  1. Rotation-powered pulsars transform rotational energy into radiation, gradually slowing down in a predictable way.
  2. Accretion-powered pulsars convert the gravitational energy of accreting matter into radiation.
  3. Magnetars are powered by the decay of their extremely strong magnetic fields.

Astronomical classification often results in one pesky object that doesn’t follow the rules. In this case, that object is PSR 1846–0258, a young pulsar categorized as rotation-powered. But in 2006, PSR 1846–0258 suddenly emitted a series of short, hard X-ray bursts and underwent a flux increase — behavior that is usually only exhibited by magnetars. After this outburst, it returned to normal, rotation-powered-pulsar behavior.

Since the discovery of this event, scientists have been attempting to learn more about this strange pulsar that seems to straddle the line between rotation-powered pulsars and magnetars.

Unprecedented Drop

One way to examine what’s going on with PSR 1846–0258 is to evaluate what’s known as its “braking index,” a measure of how quickly the pulsar’s rotation slows down. For a rotation-powered pulsar, the braking index should be roughly constant. The pulsar then slows down according to a fixed power law, where the slower it rotates, the slower it slows down.

In a recent study, Robert Archibald (McGill University) and collaborators report on 7 years’ worth of timing observations of PSR 1846–0258 after its odd magnetar-like outburst. They then compare these observations to 6.5 years of data from before the outburst. The team finds that the braking index for this bizarre pulsar dropped suddenly by 14.5σ after the outburst — a change that’s unprecedented both in how large and how long-lived it’s been.

Why is this a problem? Many of the quoted properties of pulsars (like ages, magnetic fields, and luminosities) are determined based on models that envision pulsars as magnetic dipoles in a vacuum. But if this is the case, a pulsar’s braking index should be constant — or, in more realistic scenarios, we might expect it to change slightly over the span of thousands of years. The fact that PSR 1846–0258 underwent such a drastic change during its outburst poses a significant challenge to these models of pulsar behavior and evolution.

 

Citation

R. F. Archibald et al 2015 ApJ 810 67. doi:10.1088/0004-637X/810/1/67

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