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GRB 101225A, a puzzling Christmas present

Ten years ago, on Christmas day, satellites detected a gamma-ray burst. The event was not like anything seen before. Was it even a gamma-ray burst? It took three years, several theories, and many observations before astronomers nailed down the event’s origin.

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“When you’ve seen one gamma ray burst, you’ve seen … only one gamma ray burst.”
Andrew Levan

Phones all over the world started beeping, ringing, and vibrating. It was an annoying yet not unexpected disturbance on a Christmas day1Morning, day, or night, depending on your coordinates on Earth. A minor detail.. Astronomers are used to similar holiday interruptions; after all, the Universe never rests. And so it happened that on the Christmas day of 2010 the Swift satellite detected a gamma-ray burst GRB 101225A2The name reflects the date on which the burst was detected. The appended “A” tells us that it was the first burst detected on that day. Sometimes, as many as four bursts in a single day are detected.. Which by itself is not a big deal, because we detect several of these cosmic explosions per week. But as astronomers soon realized, it was a rather peculiar event. And if there’s anything astronomers like, it’s the peculiar.

The Christmas burst, as the event became to be known, was unusually long. It lasted for 28 minutes, while the typical burst duration rarely exceeds a few minutes. Gamma-ray bursts are often followed by an afterglow, an emission that can last for a long time—for days or even weeks—and spans the electromagnetic spectrum from radio waves to gamma-rays. The afterglow of the Christmas burst wasn’t behaving as expected. When astronomers scrutinized the light at different frequencies, they began to suspect something strange was going on. Which raised the question: was this a gamma-ray burst at all?

Artist impression of a gamma-ray burst—the festive version. Credit: NASA

The Universe might be vast and splendorous with wonders beyond our wildest imagination, but astronomers’ curiosity and theoretical insight should not be underestimated. Soon, astronomers proposed two completely different theories to explain the puzzling observations.

A team lead by Christina Thöne suggested that we were witnessing a rare case of cosmic cannibalism. The story begins with two massive stars closely orbiting each other. The more massive one evolves faster and explodes as a supernova. Such explosion propels the star’s gaseous envelope into space, leaving behind a small but compact neutron star3Depending on the star’s initial mass, the compact left-over is either a neutron star or a black hole. Let’s stick to the former in this article.. It is quite common for a binary system to survive the explosion. The second star continues to evolve. After a while, the star puffs up and engulfs the neutron star, whose orbit rapidly decays: the two stars merge. The merger gives rise to the gamma-ray burst and the afterglow. Note that this scenario supports the gamma-ray burst premise, but invokes a peculiar scenario about its origin.

Sergio Campana had a different idea. Forget about gamma-ray bursts! What we had seen was an asteroid being ripped apart by a neutron star somewhere in the Milky Way. The mechanism was not something new. For example, it had been known for a while that the supermassive black hole in the Milky Way center occasionally tears apart an asteroid or a comet, giving rise to observable flares.

Each theory had its merits. Which one was correct? It wasn’t easy to say. The heart of the problem was the distance to the Christmas burst. Or more precisely, the fact that the distance was unknown. If a distance to an object is unknown, one cannot compute how much light it really emits4Remember that the light flux drops with distance squared.; the object’s nature usually remains vague or unidentified. The Christmas burst situation was strongly reminiscent of the early gamma-ray burst research. It took thirty years between the first burst was detected and the first burst for which its distance was measured; only then did the scientists understand their origin5I mean, generally understood. Many details still elude us..

One can measure the distance to a gamma-ray burst in two ways. The first one is by analyzing the afterglow spectrum. As the afterglow travels through the host galaxy of the burst, it encounters gas and dust clouds. The material absorbs part of the light, resulting in distinguishing absorption signatures in the afterglow spectrum, their exact positions depending on the distance. As you might suspect at this point, the Christmas burst wasn’t very cooperative—the afterglow turned out to be pretty much featureless.

Another way to get the distance is to observe the burst’s host galaxy itself. The distance is written in the position of the galaxy’s emission lines in the spectrum. This method is pretty straightforward, but it comes at a cost: it takes a lot of observation time on the best telescopes in the world to collect enough light from the faint galaxies to be able to measure their distance. However, the unusual properties made the Christmas burst appealing enough to search for its host galaxy. And find it they did.

Blue supergiant stars are likely sources of ultra-long gamma-ray bursts. Credit: NASA’s Goddard Space Flight Center/S. Wiessinger

A study lead by Andrew Levan reported that the burst exploded in a galaxy seven billion light-years away. Both previous theories were thus wrong, because both assumed the burst to occur much closer. Not only that, but the study also reported the discovery of other similar very long duration bursts. It turned out that Christmas burst, while rare, wasn’t a unique event. The paper established a new class of gamma-ray bursts called ultra-long gamma-ray bursts6The naming convention in astronomy is often… simple.. These bursts likely occur at the time of death of huge stars, whose size equals Jupiter’s orbit around the Sun!

A common problem in astronomy is incomplete information on the object of study. Let’s face it, we are observing faint blobs unimaginably far away and, using the light alone, trying to understand what emits it. It’s amazing what we can learn from a few photons. Anyway, whenever the conclusions are inconsistent, and an astronomical object has a flair of mystery, be assured that additional observations will eventually solve the problem.

The mystery of the Christmas burst: solved!

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