Tikalon Blog by Dev Gualtieri

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Gamma Rays from the Crab Nebula

October 19, 2011

You can assume that a scientific paper is important if it's published in a notable journal ; and it has ninety-five authors! [1] That's the number of authors on the paper, "Detection of Pulsed Gamma Rays Above 100 GeV from the Crab Pulsar," published in the October 7, 2011, issue of Science . So, what's the big deal? These are the highest gamma ray energies ever detected from a pulsar ; so high that that they can't be explained by current pulsar models.[1-6]

Gamma ray emitting pulsars are common. Like all other pulsars, they are rotating neutron stars , the remnants of supernovae . Composed entirely of neutrons , they are extremely dense, having a density slightly greater than that of an atomic nucleus , which is about 3 x 10¹⁷ kg/m³. Most pulsars emit radiation at radio wavelengths , also.

The Crab Pulsar , located 6,500 light-years from Earth , is the result of a supernova explosion that was observed on Earth with the unaided eye in 1054. This supernova explosion, visible for about a week, was recorded by Chinese and Native American observers.[3]

The Crab Nebula , the expanding region of gas ejected by the supernova, is an easy target for astronomers , so it's been observed at all available wavelengths , often generating striking images, as in the photograph below.

Hubble Space Telescope image of the Crab Nebula

Hubble Space Telescope image of the Crab Nebula.

This is a mosaic of 24 images of the six-light-year-wide remnant of the supernova explosion of 1054.

(Via Wikimedia Commons).

The Crab Pulsar emits its radiation in twin beams. One beam directs a radiation pulse towards Earth 30.2 times a second because of the star's rotation . One interesting observation is that the pulse rate is slowing, which also means that the star is spinning down. It's losing rotational energy, since it's emitting energy in the radiation beams.

The Crab Pulsar was found to emit gamma rays when the first spaceborne gamma-ray detectors were launched in 1971. The mechanism is the same as that for particle accelerators built here, on Earth. Particles moving in curved paths in the huge magnetic field of the neutron star emit the gamma rays.[2,6] Detailed observations by the Energetic Gamma Ray Experiment Telescope (EGRET), active from 1991 to 2000, have shown that most of the Crab Pulsar's radiation is emitted by gamma rays with energies greater than 10 MeV .

The successor gamma ray telescope to EGRET is the Fermi space telescope , launched on 11 June 11, 2008. I wrote about one of Fermi's discoveries in a previous article (Cosmic Ice and Galactic Flatulence, November 16, 2010). Although Fermi's detectors will only respond to gamma radiation with energy below 20 GeV , the energy spectrum suggested that things might be happening at higher energies.[2]

The gamma ray telescope, called VERITAS (Very Energetic Radiation Imaging Telescope Array System), used in the Science study was not a spaceborne instrument. It's a ground-based gamma ray observatory composed of four, twelve meter telescopes at the Southern Arizona Whipple Observatory .[6] The telescopes detect the Cherenkov radiation emitted by high velocity electron-positron pairs created by gamma rays.[4,6] Cherenkov radiation is the same bluish light seen in pool-type nuclear reactors .

Artist's conception of the Crab Pulsar with VERITAS

An artist's conception of the Crab Pulsar with VERITAS.

Gamma rays from the Crab Pulsar create electron-positron pairs with high velocity that produce Cherenkov radiation as they pass through Earth's atmosphere. This creates a cone of visible light detected by VERITAS.

(Image: Jose Francisco Salgado, Adler Planetarium).

VERITAS began full-scale observations in September, 2007, and 107 hours of data were obtained over the course of three years.[4,6] Gamma ray pulses were detected with energy up to 400 GeV,[1] and the pulse width is almost three times shorter than observed for pulses at other gamma ray energies.[3,6] The high energy gamma rays could be described by a power law , and they appear to be generated more than ten stellar radii from the neutron star.[1]

Says VERITAS Collaboration spokesman, Rene Ong , a UCLA professor of physics and astronomy , "This was very surprising and means this new radiation is probably coming from a different physical region of the star's outer magnetosphere."[6]

This discovery may add impetus for funding of a larger gamma ray observatory, the Cherenkov Telescope Array (CTA). The CTA will be ten times more sensitive than VERITAS, since it will be composed of dozens of telescopes covering more than a half square mile .[6]

The 95 authors of the Crab Pulsar paper in Science are from twenty-six institutions in five countries.[6] VERITAS receives funding from the National Science Foundation , the U.S. Department of Energy , the Smithsonian Institution , the Natural Sciences and Engineering Research Council of Canada , the Science Foundation Ireland , and the Science and Technology Facilities Council of the United Kingdom .[5-6]

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