Bad astronomy | A long burst of gamma rays looked like a short

Gamma-ray bursts, or GRBs, are pretty much the ultimate explosions: catastrophic releases of energy that can be very numerous. Billions times brighter than the Sun. They explode with such power that we can clearly see them through the observable Universe!

They can be separated into two categories: short GRBs, which last from a few milliseconds to a few seconds, and long GRBs, which can last several minutes. Over the years, we’ve learned that short GRBs are caused when two super-dense neutron stars collide and collapse into a black hole, sending out a huge but extremely brief pulse of high-energy radiation. A long GRB occurs when, at the end of its life, the core of a massive star collapses and forms a black hole; under certain conditions, the collapse launches a pair of death rays, crushing beams of energy that blast their way out of the star and create the high-energy gamma rays we see. Shortly after, the star itself explodes, creating a supernova.

There’s a joke among GRB researchers: if you’ve seen a GRB, you’ve seen a GRB. They are all so different that it can be difficult to categorize them and understand the details of the explosion. But, generally speaking, the split between longs and shorts is quite good.

Where he has been, through August 26, 2020. On that day, NASA’s orbiting Fermi gamma-ray observatory detected a GRB that lasted about half a second. So it’s short, right? Yeah, not so fast – literally. Although it was short lived, all other characteristics indicated that it was a long GRB!

It is designated GRB 200826A, and it occurred in a galaxy more than six billion light-years from ours: halfway to the observable Universe. Fermi gamma ray detectors showed it to last less than a second as part of the galaxy*. An automatic alert was sent and other telescopes came into action, observing the explosion. And right away they started finding weird stuff about it [link to paper].

When neutron stars collide and form a short GRB, the gamma rays are very high energy – what we call hard gamma rays. This one was sending out gamma rays but they were less energetic, softer, than usual for a short burst, more like what you would expect from a long burst. Most short GRBs fade very quickly, but over time astronomers measured the light coming from GRB 200826A and saw the light fade much more like a long burst, and actually saw a bump in the emission, an increase in light emitted, about 12 days after the explosion, again much more like a supernova event than a neutron star merger.

So what gives?

The intensity of a gamma-ray burst comes from the formation of the jet of matter and energy expelled by the formation of the black hole. If this happens inside a collapsing stellar core, the jets must struggle against the incoming matter, which may have a mass of several octillion tons. So many. In many cases there is such an amount of power tied up in the beams that they can work their way through and create the long GRB.

What astronomers think happened in this case is that the scales tipped more in favor of core collapse. This turned the jets off so much that by the time they ate their way there was almost no punch left in them, and they squeaked a bit instead of roared, for a fraction of their usual duration.

If this is true, and it really does appear to be so, then it’s likely that some of the GRBs classified as short are actually long, masquerading as short. In this case, despite being 6.6 billion light-years away from us, GRB 200826A is actually considered quite close; most are much further away, so they’re even weaker and more difficult to study. This makes it harder to know how many are in disguise. They don’t last very long, so it’s hard to get really deep and repeated sightings.

But this all fits more with GRB’s general attitude of being a pain to understand. It took decades to even figure out what they were – it’s a great story and you should watch this video I made about it – and even now they pose more riddles than answers. To find them even more complicated and jerky is not surprising at all.

But they are fascinating and terrifying events, well worth the study. Many thousands of them have been sighted since the first was seen in 1967, and honestly the fact that we still have an incomplete understanding of them, sometimes at a basic level, is exciting. That means there’s a lot more to understand about them, and it’s one of the most fun things about science.

* The Universe is expanding, so for us a galaxy more than six billion light years away from us so quickly that relativistic effects come into play; we see time flow more slowly in this galaxy. The GRB actually lasted about 2 seconds as we see, but taking relativity into account, it actually lasted less than a second in the GRB’s host galaxy frame of reference.

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