Last year, on October 9, we on Earth witnessed the most luminous celestial event ever recorded, by far. It was a gamma ray burst (GRB) at least 10x as bright as any other example ever seen, and it lasted 70x longer than any other known GRB. This was basically the largest explosion we are aware of in the history of our Universe. It’s estimated that this was a once-every-10,000-year event from our viewpoint here on Earth.
The first clue of something unusual came from NASA’s Earth-orbiting THEMIS probes, from the GRB-monitoring Swift Observatory, and from ESA/NASA’s Solar Orbiter. Other satellites and ground telescopes picked it up easily as well. It wasn’t long before astronomers knew this was far more powerful than anything they’d ever seen. This event was named GRB221009A (for Gamma Ray Burst 2022-10-09, first one).
Laura Hayes of ESA, upon hearing of the disturbance, wanted to know if the atmosphere had been affected by this unusual event, so she decided to check data from SuperSID, a ground-based receiver in Ireland that monitors very-low-frequency (VLF) radio signals. VLF signals are emitted intentionally by various stations on Earth, but they’re also emitted naturally by lightning and other phenomena.
VLF signals bounce between Earth’s surface and the bottom of the ionosphere (about 40 miles up off Earth’s surface), and if the electrical conductivity or effective depth of the ionosphere changes, that will be reflected in changes in the VLF signal as received by a recorder some distance from the transmitter.
The whole reason we have an ionosphere in the first place is because the Sun’s radiation (X-ray and ultraviolet) knocks electrons off their atoms, so we get ions (charged particles). Atoms that start out as neutral take on a positive charge, and negatively charged electrons are now also freely floating around. At night, many of these charged particles get back together, so on the night side you have fewer ions than on the day side. The day-night cycle is very stable, of course, so we pretty much know what to expect from the ionosphere.
But sometimes disturbances happen that upset this normal state. Solar flares are the most common cause of this. The Sun belches out some extra ionizing radiation, and so you get more free electrons in the ionosphere, and some of those may dip down below their usual altitude. A VLF detector can see that fairly readily. There are a number of VLF emitters and detectors set up here on Earth, so you can map out approximately where a disturbance happens.
GRBs from outer space can occasionally also cause these disturbances. GRBs can originate when stars collapse into neutron stars or supernovae, or from the merging of two stars, but the most-powerful ones are still a bit of a mystery. How do the extreme ones get so extreme? Most GRBs are quite weak compared to a solar flare once they arrive at Earth, just because they’re so much farther away from us than the Sun, and so the vast majority of them don’t have a measurable effect on the atmosphere.
But when Hayes looked at the VLF data from SuperSID, she located a significant disturbance over northern Europe that corresponded nicely with the appearance of the GRB. She concluded that despite its origin being 2 billion light-years away, it had affected Earth’s lower ionosphere to about the level of a respectable solar flare. That must have been one hell of an explosion!
Since then, more studies have been done, and two informative papers just came out on November 15. One, in Nature Communications, by a large team of Italian researchers, describes more fully the significant effects of this very distant blast on Earth’s atmosphere as a whole. The other, in Science Advances, by researchers at the Large High-Altitude Air Shower Observatory (LHAASO) in China, quantifies just how immensely energetic this explosion must have been.
The Italian researchers showed that the gamma-ray illumination here on Earth was centered over India’s west coast, but that it had lit up essentially all of Europe, Africa, Asia, and Australia.
They also describe the first-ever measurement of a GRB affecting the top part of the ionosphere (about 300 miles up), in part because no one had ever been able to measure that sort of thing before. A Chinese satellite called CSES, which studies the top part of the ionosphere but was only designed to last until the end of 2023 (whew!), picked up the disturbance while it was over Europe, because it just so happened to be in the right place at the right time:
The CSES satellite measured obvious changes in the electric field and total electron content around its position right at the time the burst hit the Earth, so it is extremely unlikely the upper-ionosphere disturbance was caused by anything else. So now we can infer that the entire ionosphere took a hit from this event.
The LHAASO group picked up about 5,000 high-energy gamma rays within the half-hour duration of the disturbance. LHAASO has three detectors, one of which, the 1.3-km2 Kilometer Squared Array (KM2A), is dedicated to detection of very high-energy gamma rays, those above 3 teraelectron-volts (TeV).
The number of gamma rays reaching Earth from something so immensely far away will be small, because Earth occupies such a microscopically tiny part of the sky where that event took place, and so not much of the event was pointed directly at us. But each individual gamma ray, as long as it doesn’t collide with anything, will maintain all of its energy (except the little bit it loses due to the expansion of space), even over the 2 billion years it took to get here.
So the gamma rays that struck LHAASO’s KM2A detector were nearly as energetic as when they got blasted out of whatever exceptionally crazy event caused them. The most energetic gamma rays detected by LHAASO carried up to an amazing 13 TeV. Now, it’s true that in 2021, LHAASO picked up some photons with 100 to 1000 TeV — the most energetic radiation ever detected anywhere — but those were thought to be from weird natural-particle-accelerator situations, probably not the direct result of an explosion.
What does it mean to have gamma rays with an energy of 13 TeV? The “tera-” prefix means “trillion”, so 13 trillion electron-volts certainly sounds like a big number. But how strong is that really?
Let’s say we have a flashlight that puts out 10 watts:
Looks pretty normal. Let’s turn it on for one second and then turn it off. We’ll aim it at a 5-million-gallon tank full of water at room temperature:
What will happen? Well, nothing. Nothing at all.
But now let’s replace the visible photons coming out of our flashlight with 13-TeV gamma rays. Then we’ll shine it at our tank for one second again. We should probably bend our knees a little, because assuming the water absorbs it all, it will be enough energy to boil off the entire tank and leave it empty. That’s … a lot of energy. Now, instead of a teeny flashlight, imagine a whole star exploding with energy like that. Pity on anybody or anything in that galaxy.
Part of why I am writing this is because we seem to be in a sudden proliferation of Weird High-Energy Events In Space, or at least we’re becoming a lot more aware of them. Just last month, 20-TeV gamma rays were measured coming from a pulsar within the constellation Vela, the highest-energy pulsar radiation ever detected. And also on November 15, an additional layer of weirdness got reported in Nature: an explosion nicknamed ‘Tasmanian devil’ that was 100 times brighter than a typical supernova and kept reaching peak brightness over and over for months after it happened. What is going on out there?!
Anyway, the Italian researchers point out that if anything like GRB221009A were to happen within the Milky Way, it would blast off our entire ozone layer and subject all the plants and animals on Earth to the full brunt of UV rays from the Sun, most likely killing them all, and us with them.
Fortunately this seems to be a very rare event, but it makes you wonder if this could be part of the reason we haven’t come across any transmissions from intelligent life. Maybe these kinds of events squelch life around them often enough to counter the comparably rare development of advanced lifeforms.
Then again, if some sort of life is hiding in oceans beneath the surface of planets and moons, such as within Enceladus, Europa, or even Pluto, it could be protected from such radiative assaults. If we’re ever going to find life like that — and we will try — it’ll probably have to be right here in our own Solar System.
But the task in front of us now — and to be sure, many suitors bearing theories are certainly already lining up out there — is to fully explain what sort of event could have led to an explosion with such unbelievably intense power. The Universe is getting kind of hectic.
It occurred to me to use this song because I went to high school with frontman Durron Butler (the guy with the Raiders hat here), who was a couple years older than me. I’d see him rapping in the hallways between classes, and I thought he was an all-right guy. I don’t know what he’s up to these days, but I hope he’s doing well.