Scientific Method / Science & Exploration

Gamma ray bursts and sorta-supernovas

Astronomers look at what may be the most distant object ever visible from …

Anyone who was watching the right spot in the sky on March 19th got to witness what may have been the furtherst object ever visible from Earth. The event that produced the light took place roughly 3 billion years before the Sun and Earth coalesced; at 7.4 billion years, the light has been in transit for about half the age of the universe. Despite the age and distance, the object was briefly as bright as Supernova 1987A, which took place in our galactic back yard about 160,000 light years away.

Nobody's sure whether anyone was looking, but NASA's robotic Swift observatory did what it's supposed to when it detects a gamma-ray burst, and swung into action (it was actually imaging a nearby burst that cooked off a half-hour earlier). Telescopes around the globe followed, and the fading afterglow was observed across wavelengths that spanned 11.5 orders of magnitude. The results of the observations of the event, GRB 080319B, will be appearing today in Nature

The long reach of the optical emissions is the product of relativistic jets that result from the formation of a black hole from the core of an exploded star. Those jets crash into returning shock waves of the explosion, then the outwardly expanding shell, triggering further optical emissions. These observations captured and separated all three components, providing key information.

For example, the first optical output originates from the same location as the gamma-rays, indicating that the initiation of the jets themselves produces optical emissions, which eliminates some models for the production of this optical output. It also suggests that jet formation involves a lot more energy than indicated previously by the models. The picture generated using the data suggests a two-component jet, consisting of a narrow (0.4°), ultrarelativistic core moving within a few ten-millionths of the speed of light, surrounded by a broader jet of material moving somewhat more slowly. The gamma ray bursts are triggered by internal interactions in the core jet, while the remaining emissions come from the wider jet interacting with the various shockwaves.

Rendering of the gamma-ray burster.
NASA/Swift/Hrybyk-Keith & Jones

Unfortunately, the authors calculate that the extremely narrow beam of the inner jet means that we'll only wind up having one point in earth's line of sight every three to ten years, meaning it may take a while to confirm this model.

Also in this issue is a paper on the star voted "most likely to go supernova nearby," Eta Carinae, if your definition of nearby includes 8,000 light years. Back in the 1800s, people watched as the star burped a whopping 12 solar masses of matter in an eruption that was visible as a supernova-like outburst. The new work identifies a dim, fast moving shell of material that has sped away from most of the rest of that mass.

The new material suggests that the energy released in this earlier event was far higher than estimated earlier, and actually falls within the range of what's commonly expected from a supernova, and probably resulted from an event deep in the star's core. And yet, somehow, the star still exists (or, if it doesn't, the light from its demise hasn't reached us yet). The author of the paper suggests that supernova events may not be the all-or-nothing explosions we tend to view them as, and may occur across a spectrum of energies, some of which are compatible with the star's survival.

Nature, 2008. DOI: 10.1038/nature07270
Nature, 2008. DOI: 10.1038/nature07269

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