The Case of the Cosmic Crab

A NEW MOVIE FROM NASA’S Chandra X-ray Observatory shows a sequence of images of the Crab Nebula, taken over an interval of seven months and showing dramatic variations.

The Crab Nebula is one of the most famous objects in the sky. It is the remnant cloud from a supernova (exploding star) that was seen by astronomers in China and other countries in the year 1054.

At the centre of the nebula is a pulsar, a rapidly spinning neutron star. It has a mass greater than our Sun but is only tens of kilometres wide, and is spinning at the rate of 30 times per second.

The pulsar’s spin is gradually slowing down, and as it does so large amounts of energy are injected into its surroundings. In particular, a high-speed wind of matter and anti-matter particles ploughs into the surrounding nebula, creating a shock wave that forms the expanding ‘ring’ seen in the movie.

In addition, ‘jets’ shooting out from the poles of the pulsar spew X-ray emitting matter and antimatter particles in a direction at right angles to the ring.

The goal of the latest Chandra observations was to pinpoint the location of remarkable flares spotted by NASA’s Fermi Gamma Ray Observatory satellite and Italy’s AGILE satellite.

A strong gamma-ray flare was detected from the Crab in September 2010, followed by an even stronger series of “superflares” in April 2011. The gamma-ray satellites were not able to locate the source of the flares within the nebula, but it was hoped that Chandra, with its high-resolution images, would.

Scientists have put together a short sequence of the images taken by Chandra, showing the remarkable changes in the nebula:

Chandra began observing the Crab on monthly intervals beginning six days after the discovery of the gamma-ray flare in September 2010. This established a baseline of seven images before the superflare was seen last month.

What was unexpected, though, was that nothing significant showed up in the Chandra observations as compared with the Fermi observations. Astronomers are now trying to figure out why that is so.

One possible explanation is that the gamma-ray flares picked up by Fermi happened very close to the pulsar, in which case they would have been missed by Chandra, because the Crab pulsar is so bright that the detectors are in essence “overexposed” so variations from that region cannot be observed. (Note that in the movie an artificial source of constant brightness is included to show the position of the pulsar.)

Adapted from information issued by CXC. Crab Nebula image courtesy (X-ray) NASA / CXC / SAO / F. Seward; (optical) NASA / ESA / ASU / J. Hester & A. Loll; (infrared) NASA / JPL-Caltech / Univ. Minn. / R. Gehrz.

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