bad astronomy | The long gamma ray burst sounded like a short burst

Gamma ray bursts, or GRBs, are pretty much the final blasts: catastrophic releases of energy that can be many, billions Times brighter than the sun. They explode so powerfully that we can see some clearly across the visible universe!

They can be divided into two categories: short GRBs, which last from a few thousandths of a second to a few seconds, and long GRBs, which can be several minutes in duration. Over the years, we’ve learned that short GRBs occur when two super-dense neutron stars collide and collapse into a black hole, sending out a massive but extremely short pulse of high-energy radiation. The long GRB occurs when the core of a massive star collapses at the end of its life and forms a black hole. Under some circumstances, the avalanche releases a pair of death rays, mind-crushing energy beams that shoot out from the star and produce the high-energy gamma rays we see. Not long after the star itself exploded, 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 classify them and understand the details of the explosion. But in general, the division between long and short is very good.

or is I was, until August 26, 2020. On that day, the Fermi Gamma Ray Observatory orbiting NASA detected a GRB that lasted about half a second. So it’s short, isn’t it? Yes, not so fast – literally. Although it was of short duration, all of its other characteristics indicated that it was a long-term GRB!

Designated GRB 200826A, it occurred in a galaxy more than six billion light-years away from our own: halfway through the observable universe. Gamma-ray detectors on Fermi showed that it lasted less than a second within the galaxy*. An automatic alert was sent, and other telescopes intervened to monitor the explosion. And immediately they started to find strange things about him [link to paper].

When neutron stars collide and form a short GRB, the gamma rays are of very high energy – what we call solid gamma rays. This sent out gamma rays but they were less energetic, and softer than usual in a short burst, more like what would be expected from a long burst. Most of the short GRBs fade very quickly, but over time astronomers measured the light coming from GRB 200826A and saw that the light faded like a long burst, and in fact they saw an emission bump, an increase in the light emitted, about 12 days after the explosion, again more like a super event A nova is more than a neutron star merger.

So what does it give?

The intensity of the gamma-ray burst comes from the formation of the jet of matter and the energy released from the formation of the black hole. If this happens inside a collapsing stellar core, the jets have to battle the flowing matter, which can contain a mass of several octillion tons. Lots. In many cases, there is so much energy restricted in the beams that they can make their way out and create the long GRB.

What astronomers believe happened in this case is that the scales tilted more in favor of the collapsed core. The jets quenched so much that by the time they ate their way through there was no gravity left in them, and they kind of squeaked instead of roaring, and they lasted for a fraction of their usual duration.

If this is true, and it really sounds like it, it is possible that some parts of GRBs categorized as short are actually long parts, only disguised as short. In this case, although GRB 200826A is 6.6 billion light-years away from us, it is considered fairly close; Most are much further away, so they are fainter and harder to study. This makes knowing the number of undercovers even more difficult. They don’t last very long, so getting deep and frequent notes is tricky.

But it all fits in with the general attitude of the GRB to being painful to understand. It took decades to get a basic understanding of what they were – it’s a great story and you should watch this video I made about it – and so far they offer more mysteries than answers. Finding out that they are more complex and convulsive is not at all surprising.

But they are wonderful and terrifying events that are worth studying. Several thousand of them have been observed since their debut in 1967, and frankly the fact that we still have an imperfect understanding of them, sometimes at the most basic level, is exciting. This means that there is a lot to understand, and this is one of the most interesting aspects of science.


* The universe is expanding, so for us a galaxy more than six billion light-years away is receding from us so fast that relativistic influences come into play; We see time flowing more slowly in that galaxy. In fact, the GRB lasted about two seconds as we saw, but given relativity, it actually lasted less than one second in the reference frame of the host GRB galaxy.

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