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Posts Tagged ‘1054

Neutron star at heart of the Crab Nebula pumping out gamma rays

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In 1054, a supernova went off in our galactic neighborhood and was recorded in a number of historical accounts. Today, the remnants of that blast form the spectacular Crab Nebula shown above. Buried within it is a rapidly rotating neutron star, which we can detect by its pulsed emissions. Now, researchers have used a rather unusual telescope — one that incorporates our own planet into the optics—to catch a glimpse of the pulsar using very high energy gamma rays…

The Crab Nebula, being only about 1,000 years old, contains a pulsar that’s both spinning rapidly and is relatively close to us, which makes for an excellent test of various theories about neutron star behavior. Based on measurements from other pulsars, it has looked like pulsar emissions tended to remain fairly even until they hit energies of 100MeV to a few GeV, at which point they underwent an exponential decay (above those energies, emissions rapidly tail off and there are few photons of higher energies). Observations of the Crab Nebula had picked up a few high energy photons, but these were erratic and not clearly packaged into pulses, so they simply set an upper limit on possible emissions at high energies.

To get a better view of this pulsar, a research team turned to telescope called VERITAS, located in the Arizona desert. VERITAS consists of four 12m telescopes, but these are never focused on the stars. Instead, they are used to detect light in the Earth’s atmosphere that is caused by incoming gamma rays.

When a highly energetic gamma ray strikes the atmosphere, it sets off a shower of energetic particles, some of which end up moving faster than the speed of light in that medium. The particles (at least, the ones that aren’t neutrinos) slow down rapidly by emitting light called Cherenkov radiation. Telescopes like VERITAS use this light to reconstruct the path of the particles back to the source, providing information about its direction and energy. In short, the entire atmosphere gets used as something roughly analogous to a CCD…

…When the data was analyzed, a clear pattern of pulses became apparent at energies above 120GeV, and the timing of the pulses lined up nicely with observations at lower energies made using the Fermi space telescope. The object there is pulsing at much higher energies than any previously detected…

In any case, the authors make a compelling case that we should be looking at the Crab Nebula at higher energies than we generally do, since a careful study of gamma rays may help us constrain or eliminate various models of neutron stars. And, if possible, looking at other pulsars might give us a greater sense as to whether the one in the Crab Nebula is an anomaly.

And it rocks!

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Written by Ed Campbell

October 8, 2011 at 2:00 pm

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