From Stellar Corpses to Zombie Pulsars

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Astronomy lets us explore some of the most bizarre places and things that we can imagine, as well as many things that we can’t quite wrap our heads around. Pulsars definitely qualify as bizarre and unintuitive, and only seem more so the every time we look.

Astronomers used data from the Fermi Space Telescope to find nine new pulsars that shine with gamma-ray light. Gamma-rays are the highest energy form of light that we know and thus are usually created by very powerful processes.

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A pulsar is a neutron star, or a dense core left after a large star explodes as a supernova, that rotates very rapidly with a “hotspot” that blinks in and out of view.

The new pulsars were discovered in pre-existing data using a new, more powerful algorithm to find fainter gamma-ray emitting pulsars than were found in the initial search with Fermi. Since pulsars appear to blink on and off, a blind search for them requires a large computer to test multiple pulse periods for every point on the sky. Add to that the fact that gamma-ray photons are few and far between, and this makes for a painstaking process.

Many pulsars are also studied with radio telescopes, though only one of these new pulsars was shown to emit radio waves.

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Even when a given pulsar does show up in both radio and gamma-rays, the signals don’t exactly match up. Thus, it is hypothesized that the different types of radiation can come from different zones around the magnetic pole hotspots on the neutron star.

Another group of astronomers discovered what they are calling a “super-energetic millisecond pulsar” with Fermi, as well.

When a pulsar is born, it usually rotates a few times a second, and slows its spin over hundreds of thousands of years as the magnetic field weakens. When the field is too weak, the pulsar stops emitting, or “dies.”

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Occasionally, an old — or dead — pulsar in a binary system with another star can have gas dumped onto it by its companion, which causes to pulsar to “spin up” to a few hundred times a second, thus becoming a millisecond pulsar. In a way, the dead pulsar is “reanimated,” hence the shameless zombie reference.

Millisecond pulsars are often found in globular clusters which have some of the oldest star populations in the galaxy. This pulsar, affectionately called J1823-3021A, is the first millisecond pulsar to be seen giving off gamma-rays as well as radio light.

This indicates that the magnetic field for this pulsar is much higher than expected by our hypothesis for how millisecond pulsars are born. Once again, these enigmatic objects are even more mysterious every time we look.

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Image: Still from a simulation of a pulsar showing he magentic fields and light from the hotspots. Credit: NASA

These results will be published in the Astronomical Journal and Science Express. If you don’t mind a little math, a great introduction to pulsars can be found here as part of a radio astronomy course designed by Condon and Ransom.