Optical image of the Milky Way and a multi-wavelength (optical, Hα) zoom into the Large Magellanic Cloud with superimposed H.E.S.S. sky maps.
Optical image of the Milky Way and a multi-wavelength (optical, Hα) zoom into the Large Magellanic Cloud with superimposed H.E.S.S. sky maps. (Milky Way image: © H.E.S.S. Collaboration, optical: SkyView, A. Mellinger; LMC image: © H.E.S.S. Collaboration, Hα: R. Kennicutt, J.E. Gaustad et al. (2001), optical (B-band): G. Bothun

Astronomers using the HESS telescopes have discovered a new source of high-energy gamma rays. Dubbed a superbubble, it appears to be a massive shell of gas and dust 270 light-years in diameter being blown outward by the radiation from multiple stars and supernovas. HESS also discovered two other gamma-ray sources, each a giant of its kind. One is a powerful supernova remnant and the other a pulsar wind nebula. All three objects are located in the Large Magellanic Cloud, a small satellite galaxy orbiting the Milky Way at a distance of 170,000 ly. As a result, these objects are not only the most luminous gamma-ray sources discovered to date but also the first sources discovered outside the Milky Way.

Gamma-rays are emitted when very energetic charged particles collide with other particles, such as in a cloud of gas. Therefore, gamma radiation in the sky is often used as a proxy for high-energy phenomena. And astronomers have for long known that the Large Magellanic Cloud houses many such clusters of frenzied activity: weight for weight of their stars, the Cloud’s supernova rate is five times that of the Milky Way. It also hosts the Tarantula Nebula, which is the most active star-forming region in the Local Group of galaxies (which includes the Milky Way, Andromeda, the Cloud and more than 50 others).

Super-luminous sources

It is in this environment that the superbubble – designated 30 Dor C – thrives. According to the HESS team’s notice, it “appears to have been created by several supernovae and strong stellar winds”. In the data, it is visible as a strong source of gamma-rays because it is filled by highly energetic particles. The notice adds that this freak of nature

“represents a new class of sources in the very high-energy regime.”

The other two super-luminous sources are familiar to astronomers. Pulsars, especially, are the extremely dense remnants of stars that have run out of hydrogen to fuse and imploded, resulting in a rapidly spinning core composed of neutrons and wound by fierce magnetic fields. They emit a jet of energetic particles from polar points on their surface that form nebulaic clouds. One such cloud is N 157B, emitted by PSR J0537 – 6910. According to the HESS team, N 157B outshines the Crab Nebula in gamma-rays. The Crab Nebula is Milky Way’s most famous and most powerful source of gamma-rays.

The third is a supernova remnant: the rapidly expanding shell of gas that a once-heavy dying star blows away as its core collapses. The shell can be expelled at more than thousand times the speed of sound, resulting in a shockwave that can accelerate nearby particles and heat up upstream gas clouds to millions of kelvin. The resulting glow can last for thousands of years – but the one HESS has seen in the Cloud seems to going strong for 2,500-6,000 years, much longer than astronomers thought possible. It’s called N132D.

“Obviously, the high star formation rate of the LMC causes it to breed very extreme objects,” said Chia Chun Lu, a student at the Max Planck Institute for Astronomy in Heidelberg who analyzed the data for her thesis.

Imaging Cherenkov radiation

Detecting gamma-rays is no easy task because it requires the imaging of Cherenkov radiation. Just as when a jet flies through air at faster than the speed of sound and results in a sonic boom, a charged particle traveling at faster than the speed of light in that medium results in a shockwave of energy called Cherenkov radiation. This typically lasts a few billionths of a second and requires extremely sensitive cameras to capture.

When high-energy particles collide with the upper strata of Earth’s atmosphere, they percolate through while triggering the release of Cherenkov radiation. The five ground-based HESS telescopes – whose name stands for High Energy Stereoscopic System – quickly capture their bluish flashes before they disappear, and reconstruct their sources’ energy based on theirs. So, while gamma-rays can be a proxy for high-energy phenomena in the distant reaches of the cosmos, Cherenkov radiation in the upper atmosphere is a proxy for the gamma radiation itself.

Very-high-energy gamma-rays, of the order emitted by the Crab pulsar at the center of its nebula, are often the result of events that have made astronomers redefine what they consider anomalous. A good example is of GRB 080916C, a gamma-ray burst spotted in 2009 at about 12 billion ly from Earth. It was the result of a star collapsing into a black hole, with consequent ‘burp’ of energy lasting for a whopping 23 minutes. Valerie Connaughton, of the University of Alabama, Huntsville, and one of the members of the team studying the burst, said of its energy: “… it would be equivalent to 4.9 times the mass of the sun being converted to gamma rays in a matter of minutes”.

Natural particle accelerators

Such profuse emissions can behave like natural particle accelerators, often reaching energies the Large Hadron Collider can only dream of. They give scientists the opportunity to study particles as well as the vacuum of space in conditions closer to that prevalent at the time of the Big Bang, in effect rendering the telescopes that study them as probes of fundamental physics. In the case of GRB 080916C, for example, low-energy gamma-rays dominated the first five seconds of emissions, following by the high-energy gamma-rays for the next twenty minutes. As astronomy-blogger Paul Gilster interpreted this,

They might also give us a read on theories of quantum gravity that suggest empty space is actually a froth of quantum foam, one that would allow lighter, lower-energy gamma rays to move more quickly than their higher-energy cousins. Future observations to study unusual time lags like these should help us pin down a plausible explanation.

The Fermi orbiting telescope that spotted the burst is also used to look for dark matter. When certain hypothetical particles of dark matter annihilate or decay, they yield high-energy antielectrons that could then annihilate upon colliding with electrons and yield gamma-rays. These are measured by Fermi. Then, astronomers use preexisting data as a filter to extrude anomalous observations and use it inform their theories of dark matter.

In this sense, the HESS telescopes are important observers of the universe. They comprise five telescopes, of which four, each 12 meters in diameter, are situated on the corners of a square of side 120 m. At the center is the fifth telescope of diameter 28 m. The array, fixed up with computers to work as one big telescope, is located in Namibia, and is capable of observing gamma-ray fluxes in the range 30 GeV to 100 TeV. In 2015, in fact, construction for the more-impressive $268-million Cherenkov Telescope Array will start. Upon completion, it will be able to study gamma-ray fluxes of 100 TeV but with a wider angle of observation and much larger collecting area.

Whether or not the CTA can pinpoint the existence of dark matter, it will likely allow astronomers to discover more superbubbles, pulsar wind nebulae, supernova remnants and gamma-ray bursts, each more revealing than the last about the universe’s deepest secrets.

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