The Peculiar Pulsar in the Crab Nebula

by Nicholos Wethington on October 23, 2007

Crab Nebula. Image credit: Hubble
In 1054, Chinese and Arab astronomers noticed a new star shining brilliantly in the sky. This was one of the first recorded observations of a supernova, and the remnants of that explosion are what we now call the Crab Nebula. The nebula still interests astronomers today, as in addition to its spectacular beauty, it’s host to a pulsar with curious properties that may change drastically our understanding of the physics of pulsars.

A team led by Dr. Jean Eilek and Tim Hankins at New Mexico Tech looked at the Crab pulsar in the radio spectrum using the Very Large Array and Aricebo telescopes, and discovered to their surprise that the pulsar’s radio emissions are much more complex than previously thought.

Their article, What makes the Crab pulsar shine?, appeared in the proceedings of the meeting Forty Years of Pulsars: Millisecond Pulsars, Magnetars and More.

The Crab pulsar – a rapidly spinning neutron star that emits radiation from its magnetic poles like a lighthouse – actually emits two different pulses; one is called the main pulse, and the other, which is about 160 degrees away in rotation from the main one, is called the interpulse.

They looked at individual pulses over very short periods of time to tease apart their properties, and found that the interpulse differs in three significant ways from the main pulse on certain radio frequencies: it produces light that is more polarized – the wavelengths of light are more aligned together – the interpulse lasts longer, and the radiation emitted is more dispersed.

“Because similar physics should lead to similar emission processes, these models suggest that the main pulse and interpulse should be the same in their observable quantities. We were – and remain – quite surprised that this turns out not to be the case in the Crab pulsar,” Eilek wrote.

In other words, it is curious that the interpulse should have different properties, given that it is thought to come from the same mechanism as the main pulse. This raises a lot of questions as to our knowledge of pulsar physics: Are other pulsars like this? How do pulsars create such different radio emissions?

“The Crab is clearly special as far as the number of components in its mean profile, and also the fact that it has a larger number of very-bright pulses than most pulsars (those “giant” pulses are the ones bright enough for us to study individually). But no one knows if that means its emission physics is special, or ordinary… there’s still argument about that,” Eilek wrote in an email response.

Eilek’s team plans to look at other pulsars with the aim of answering this very question, among the host of others raised by their research.

Source: Arxiv paper

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