Where Cosmic Rays Come From

HESS gamma ray image of supernova remnant
HESS gamma-ray image of supernova remnant
Credit: HESS

European astronomers have produced the first image of an object using high energy gamma rays – the most penetrating form of radiation known. The image is of a supernova remnant called RX J1713.7-3946, which exploded 1,000 years ago.

Over time, a ring of material has expanded to twice the diameter of the Moon in the sky. If you had gamma ray eyes, you would be able to see a large ring in the sky every night. This also helps solve a 100 year mystery about the origin of cosmic rays; the remnant seems to be acting as a particle accelerator.

During Earth’s earliest history, its surface also was bombarded by high-energy particles associated with solar activity (from a solar wind that was enhanced during early history and from solar flares) and galactic cosmic rays, and possibly from nearby supernovae and events associated with gamma-ray bursts. This bombardment must have had deleterious effects on life at the Earth’s surface, and may have severely affected the formation and earliest evolution of life.

The latest result, published in the Journal Nature on November 4th, was carried out using the High Energy Stereoscopic System (H.E.S.S.), an array of four telescopes, in Namibia, South-West Africa. Dr Paula Chadwick of the University of Durham said "This picture really is a big step forward for gamma-ray astronomy and the supernova remnant is a fascinating object. If you had gamma-ray eyes and were in the Southern Hemisphere, you could see a large, brightly glowing ring in the sky every night."

Dr. John Horack, who led the assembly, testing and calibration program for the gamma ray burst experiment on NASA’s Compton Gamma-Ray Observatory
Credit: D. Rezabek

Professor Ian Halliday, CEO of the UK-based Particle Physics and Astronomy Research Council said "These results provide the first unequivocal proof that supernovae are capable of producing large quantities of galactic cosmic rays – something we have long suspected, but never been able to confirm."

To put the finding in perspective, Astrobiology Magazine had the opportunity to talk with John Horack, who led the assembly, testing and calibration program for the gamma ray burst experiment on NASA’s Compton Gamma-Ray Observatory.

Astrobiology Magazine (AM): Have people been proposing a gamma-ray telescope for a long time, or is this possible now with newer technologies?

John Horack (JH): There have been gamma-ray telescopes in the past, for example the COMPTEL experiment aboard NASA’s Compton Gamma-Ray Observatory imaged the cosmos in gamma rays for nearly 10 years from low earth orbit. The breakthrough here, as I understand it, is that we are now making gamma ray images from the ground.

AM: Presumably, seeing in a small wavelength window highlights the higher energy processes in the sky. Does it make sense to look at gamma ray bursts with a gamma ray telescope?

JH: Gamma ray bursts are very different from supernova remnants, of course, but yes, it makes sense to try — and scientists have been trying since their discovery in 1967. The problem with gamma ray bursts is that you do not know where the next one is coming from, and therefore trying to guess by pointing your telescope is kind of a shot in the dark. Early gamma-ray burst experiements used very wide fields fo view, and found the direction to bursts by working out the direction by studying the time difference in detection by widely-separated experiments in space.

Later, experiements like BATSE on the Compton GRO used detectors that could see the entire sky at once, thereby capturing any gamma-ray burst of sufficient strength and determining the direction by the relative brightness in each of the eight detectors pointed in different directions. The Dutch-Italian BeppoSAX actually was able to catch gamma-ray bursts in real time by quickly slewing the telescope on the spacecraft after a detection.

Today, experiments like the High Energy Transient Experiment (HETE) and SWIFT offer the best chances we’ve ever had to catch a gamma ray burst "in the act" either by detecting the gamma-ray or x-ray emission as the burst is occurring, or shortly after it has begun.

HESS Gamma Ray Telescope. Cherenkov light develops within an air shower. Because the particle moves faster than the speed of light in air, there is a sonic boom or shock wave, which sends out a flash of blue light in the direction of the primary gamma quantum and lasts a few billionths of a second. This happens about ten kilometers (6.3 miles) above the earth’s surface.
Credit: Hess Collection

AM: Because atmospheres shield gamma rays, could such gamma telescopes be turned to closer objects like Venus or Titan to pick off novel features of their atmospheres as opaque in this wavelength?

JH: The Earth’s atmosphere does prevent gamma-ray radiation from reaching the ground, and gamma-rays are usually associated with far more energetic objects such as black-holes, neutron stars, supernovae, and quasars. Turning a space-based gamma-ray telescope at a planet is not terribly useful, since these planets are "dark" in gamma-rays, and their angular sizes as seen from the Earth are incredibly small compared to the resolution of a gamma-ray telescope.

In gamma-rays, the Sun isn’t even the brightest object in the sky. The Crab Nebula, Cygnus X-1, and a host of other sources typically are brighter in gamma-rays.

The universe is just far too plentiful in exotic, energetic, and explosive phenomena to spend time trying to detect gamma-rays from Venus. One thing that was learned from BATSE, however, is that thunderstorms on Earth do create large but short-lived flashes of intense, upward-moving gamma radiation observable from low-earth orbit. This was a total surprise, and one of those kinds of discoveries that happen in science that one would never have predicted prior to the launch of BATSE in 1991.

AM: One question that is curious about HESS and its relation to gamma ray detectors of the past, is the basis for having a space telescope. How does HESS see such objects when the atmosphere is opaque–is it looking for byproducts in the upper atmosphere, like Cerenkov radiation, thus making it more of a Cerenkov eye, not a true gamma eye?

JH: HESS does not detect the gamma-rays directly, since they do not reach the ground. So yes, it is detecting the evidence of a gamma-ray, not the gamma-ray itself.

Crab Nebula in X-rays showing its main central jet.
Credit: NASA

This evidence is called Cerenkov radiation, produced when an object such as a high-energy particle moves faster than the speed of light in the medium it is travelling through. Please note, this speed is NEVER faster than the speed of light in a vacuum. As an example, the speed of light in water is slower than the speed of light in a vacuum. Nothing can travel faster than the speed of light in a vacuum. But if you move an electron through water at a speed less than the vacuum speed, but more than water velocity, it will emit radiation that can be detected, called Cerenkov radiation. This is what HESS is detecting, from particles created through interactions between the incoming gamma-rays and the upper atmosphere.

AM: The origin of cosmic rays are being attributed to supernova, because of a glowing ring. Aren’t there theories that cosmic rays are just accelerated in the plasma of a supernova, not originating there?

JH: The origin of cosmic rays is a long-standing question, and the prediction of an association between cosmic rays and supernovae dates back to the late 1930′s. So this result helps put that question to bed, and may allow scientists to get on with understanding the detailed physics of how the production takes place in this environment.

AM: Any ideas for where to point this next, like at our Sun or even burst afterglows?

JH: I’d love to see some Northern hemisphere objects, such as the Crab Nebula, Cygnus X-1, or the possibility of detection of things like X-ray transients from the Galactic Center. In fact, this is one of the most interesting things about the high-energy sky: it changes very frequently, and the brightest object in the sky today, might not be the brighest object in the sky tomorrow.

Things can change on timescales of minutes, days, or weeks.

The HESS telescopes are ten times more sensitive than earlier Cherenkov telescopes. Each HESS collector has a diameter of twelve meters (~40 feet) and 380 individual round mirrors that make up a light-collecting surface area of 108 square meters (~1000 sq. ft.). The camera enables exposure times of a mere one hundred millionth of a second. The HESS acronym alludes to the Austrian physicist Viktor Franz Hess (1883-1964) who discovered cosmic rays during ten balloon flights between 1911 and 1913. In 1936, he was awarded the Nobel Prize for Physics.

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