The Crab, the Pulsar and the Gamma-Ray 'Burp'

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For most people, the night sky seems to be placid and changes only in a predictable way. However, we are excited to see things things that flare, blow up, or race through the sky. So, imagine the excitement of astronomers when an old standby leftover from a massive explosion starts to flare unpredictably.

I’m talking about the Crab Nebula, the rather gorgeous leftovers of a star that detonated in a supernova long ago. In fact, it was observed by Chinese astronomers and labelled a “guest star” in 1054.

BIG PIC: Dissecting the Crab Nebula.

Once the “star” faded, it wasn’t until 1731 that amateur astronomer John Bevis cataloged the remnant of the explosion from his telescopic observations of the sky. It is better known now as Messier 1 due to its rediscovery nearly thirty years later by Charles Messier for his catalog of “fuzzy things that look like comets but aren’t.” (At least, that’s what I would name it!)

As astronomy opened up new parts of the electromagnetic spectrum, M1, or the Crab Nebula, showed a host of new properties. Energetic x-rays were discovered emanating from it in 1963 by an instrument on sounding rocket, and the radio pulsar was discovered in 1968 from my favorite place, the Green Bank Observatory in West Virginia.

I even remember a particularly gorgeous image of the Crab Nebula and central pulsar being released when I was in high school, and cutting it out of the science section of my local newspaper to post on my wall.

Today, the Crab Nebula is often used as a calibrator for x-ray and gamma-ray observation because it appears to be quite stable at those wavelengths.

However, in 2007, an Italian gamma-ray telescope called AGILE detected a flare, but astronomers attributed it to bugs being worked out in the new satellite. Determined not to be missed, however, the Crab sent out another gamma-ray “burp” that was detected in September of 2010. Several astronomers raced to follow up these observations with x-ray and optical telescopes to find that the gamma-ray flare might be associated with a new, brighter spot in the nebula.

What caused the flare? Astronomers aren’t sure yet, though hypotheses abound. One has the flare being produced by particles circling around with 100 times the energy of the Large Hadron Collider! Whatever it is, this observation is a nice reminder to never expect the universe to be placid and boring.

Images: Top: The many faces of the Crab Nebula. Credit – Chandra, Palomar, Keck, and VLA. Bottom: Though no pretty images exist in gamma-ray, the flare is easily seen in September 2010. Credit – Tavani et al. (2011)

Many, many, many lovely thanks to Craig Heinke, co-author of this work and one of my favorite astronomers, for posting lots of background and images about this study. For more information, please visit his website about this flare.

This work has been published in Science and a preprint is available at arxiv.org.

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