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Gamma Ray Astronomy: The Good, The Bad And The Ugly

The current golden age for gamma ray astronomy is creating more questions than answers.

  • November 4, 2009

Gamma ray bursts have provided a constant source of excitement since they were discovered in the 1960s by US military satellites hunting for evidence of secret nuclear weapons tests.

When they light up the sky, gamma ray bursts are the brightest objects in the Universe. They emit so much light that astronomers believe it must be collimated in some way, otherwise the total emission could not arise from currently understood astrophysical phenomenon. As it is, they release in a few seconds, the energy equivalent to the rest mass of the Sun.

That makes them of more than a passing interest to humanity. Gamma ray bursts in the Milky Way may have triggered mass extinctions on Earth in the past and so could threaten us in future.

However a gamma ray burst has never been seen in the Milky Way. In fact, they are generally the most distant, and therefore the oldest, astronomical objects we can see. Astronomers last week said they’d spotted a gamma ray burst that occurred just 630 million years after the Big Bang.

All this information and much more is the result of two revolutions that have occurred in gamma ray astronomy. First is the launch of the Swift and Fermi gamma ray telescopes in 2004 and 2008 respectively. Second is a global co-ordination project that alerts the community to gamma ray bursts so that their afterglows can be observed at other frequencies.

As a result astronomers have gone from being starved of data about gamma ray bursts to suddenly drowning in it. And as the amount of poorly understood data grows day by day, it’s slowly becoming clear that gamma ray bursts are much more complex and mysterious than anybody imagined.

Today, Maxim Lyutikov at Purdue University in Indiana, outlines the mysteries that astronomers are puzzling over and it makes for fascinating reading. There seem to be two types of gamma ray burst: long ones lasting thousands of seconds and short ones that flash on and off in less than a second. How these different types occur is not yet known. And don’t bet against other types of burst being discovered soon.

These bursts have x-ray afterglows which sometimes decay quickly and in other cases plateau for tens of thousands of seconds. Some bursts later flare up again and others cut out momentarily, like a backfiring Ford Model T.

Each of these observations requires a separate explanation and the theorists are struggling. The consensus is that gamma ray bursts are created in some kind of gravitational collapse in which gravitational energy is converted into kinetic energy and then into light. So supernovas are generally agreed to be one type of source. Where the others come from, nobody knows.

Then there is the question of how such a collapse occurs. A gravitational collapse implies the existence of a shock wave but the structure of this wave and how it interacts with anything in its path is poorly understood.

Even the physical mechanism by which gamma rays are formed is disputed. One possibility is by synchrotron emission, charged particles accelerated in a magnetic field. Where this magnetic field comes from and how it interacts with a shock wave is not known. Another option is inverse Compton emission in which high energy electrons boost the energy of photons to higher frequencies. Take your pick.

The hope is that these mechanisms can be put together in some way that they will explain the structure of the data that astronomers see: the flares, the afterglows and the varying timescales over which these happen.

But the fear that Lyutikov raises is that these processes are so complex that they will forever be beyond mortal understanding.

That’s unduly pessimistic. Advances in many areas of astrophysics are rate-limited by lack of data. Gamma ray astronomy is an exception, at least for the moment. There’s no denying the complexity that this data represents. But what this state of affairs represents is a golden opportunity for a new generation astrophysicists: an exciting problem just asking to be solved.

Ref: arxiv.org/abs/0911.0349: Gamma Ray Bursts: Back to the Blackboard

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