Scientific Method / Science & Exploration

Intense gamma-ray burst spells doom—for our models of gamma-ray bursts

Closest, most intense one we've observed yet.

A massive star goes boom, leaving behind a black hole and sending off a burst of gamma rays.

Back in April, orbiting observatories started picking up the first indications of a gamma-ray burst. By the time observations wrapped up, the event (GRB 130427A) produced the largest outpouring of photons of any yet detected, and it set a record for the highest energy photon we've seen from these events. And because it was unusually close to Earth, GRB 130427A provided a wealth of information about these extreme events—and told us that we don't really understand how they produce the gamma-rays that are their signature.

Yesterday's issue of Science contains four papers that describe the event, partly because it was unusually well-documented. The enormous stars that produce gamma-ray bursts were much more common in the early Universe and, as a result, most of them occur out at the edge of the observable Universe. But GRB 130427A is an exception; the Universe was already about 10 billion years old when it happened, meaning the supernova that produced the gamma rays occurred less than four billion light years from Earth. As a result, ground-based instruments that were directed to the right area of the sky by the orbiting instruments were quickly able to identify the supernova involved (SN 2013cq).

Meanwhile, the orbiting observatories like SWIFT and Fermi continued to track the event as it occurred. The data they gathered showed that GRB 130427A was an impressive event. At lower energies, it showed a characteristic initial burst followed by a pause of several seconds. The pause ended with a long and complex series of emissions that lasted for roughly 10 seconds, after which there was a gradual tailing off of activity. At the highest energies, however, there was a steady buzz of activity from five seconds out to at least 30, and gamma rays continued to be detected out to 20 hours, setting a record for these events

The record-setting photon, at 95GeV, was actually detected several minutes after the initial outburst. Due to the amount of redshifting that occurred during its billions of years of travel, it actually arrived at a much lower energy than when it was first produced; calculations suggest it was initially 128 GeV.

And that causes a problem for our model of how a gamma-ray burst operates. The model posits that the first burst of radiation signaling that the supernova has started is caused by the matter being rocketed out of the explosion at nearly the speed of light. The larger burst that follows is caused by that matter slamming into material that the star had shed prior to its death, creating a massive shock wave. This slowly fades as the shockwave propagates into the material and the remains of the star start to lose energy through these interactions.

In general, it's a nice explanation for the overall pattern of emissions and what we see at optical wave lengths and on the lower side of the energy spectrum. But things start to go wrong in the details. One paper notes that the initial pulse of photons, thought to come from the material shot out from the explosion, appears to come from a region larger than the shock radius of the explosion, decays in an unexpected way, and is distributed in an unusual manner. It's possible to get the models to handle one of these oddities, but "it is a challenge to explain all these behaviors simultaneously."

Meanwhile, there are also issues with the extended high-energy emissions, which are thought to be generated by synchrotron radiation, photons released as charged particles that lose energy while traveling along a curved path. (synchrotron radiation is so named because the photons were first noticed in early particle accelerators called synchrotrons.) The interactions between the ejected material and the surrounding medium are thought to provide charged particles that travel a curved path through the turbulent environment, which neatly explains the high energy photons.

Except it doesn't. According to the model, there should be a relatively narrow window in which the energies are high enough to produce the sorts of hard gamma rays seen in this explosion. Instead, the burst continued to produce high-energy photons (including the record setter) well after that window should have been shut. The authors point out that other processes could still produce these photons (they mention magnetic reconnection and inverse Compton scattering), but the synchrotron radiation is probably out.

None of this means that the model described earlier is completely wrong, just that there are details it doesn't include or mis-estimates that are critical for understanding our observations.

In any case, it may take another relatively nearby explosion of this sort—one that can give us so many high-quality observations—to sort out the details. The only problem? It's estimated that we'll only see one explosion this close about every 60 years.

Science, 2013. DOIs: 10.1126/science.1242302, 10.1126/science.1242316, 10.1126/science.1242353, 10.1126/science.1242279  (About DOIs).

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