Supernovae are some of the most energetic events in the universe. A subset of these includes gamma-ray bursts, where much of the energy emitted comes from extremely high-energy photons. We think we know why this generally happens – the black hole left over from the explosion expels jets of material at nearly the speed of light. But the details of how and where these jets produce photons are nowhere near fully worked out.
Unfortunately, these events happen too fast and too far away, so it’s not easy to get detailed notes about them. However, a recent gamma-ray burst dubbed BOAT (Brightest Ever Recorded) may provide us with new information about events within a few days of the supernova explosion. A new paper describes data from a telescope that was pointing in the right direction and sensitive to the extremely high-energy radiation from the event.
I need to take a shower
The aforementioned “telescope” is Large air shower observatory at high altitude (LHAASO). Located three miles (4,400 meters) above sea level, the observatory is a suite of instruments that is not a telescope in the traditional sense. Instead, it’s supposed to capture airshowers – an intricate chain of debris and photons that are produced when high-energy particles from outer space collide with the atmosphere.
While limited compared to conventional telescopes, air shower detectors do have some advantages with regard to events such as BOAT. They have a very wide field of view because they don’t really need to focus on an event so much as they need to reconstruct it based on the photons and particles that reach the Earth’s surface. They are only sensitive to high-energy events, which means daylight is too low-energy to interfere, so they can operate around the clock.
Because LHAASO was taking data when the BOAT supernova erupted, its detectors not only picked up the onset of the event, but were able to track its evolution for several days afterward. While there was poor spatial resolution, there was a massive amount of data, all separated by wavelength. The first 100 minutes saw the detection of more than 64,000 photons at energies in excess of 200 GeV. For context, converting the entire mass of a proton into energy yields just under one GeV.
One of the first things that was evident was that there was a huge difference between photons at lower (but still very high!) energies and those at the more extreme ends of the electromagnetic spectrum. Data from photons that were above the TeV changed smoothly over time, while those in the megaelectronvolt range fluctuated up and down.
Understanding the data
The researchers suggest that this data is consistent with the proposal that low-energy events are caused by the jets interacting with the turbulent debris of the supernova. Because this debris would be complex and close to the source of the jets, it would limit the amount of space particles in the jets that would have to speed up, thus putting a cap on their energy.
By contrast, higher-energy photons are produced in regions where the jets have scraped away the supernova’s debris and begun interacting with the matter that made up the star’s surroundings—particles likely bombarded by the stellar equivalent of the solar wind. It is a more sparse and uniform environment, allowing the jets a less turbulent path to accelerate particles to the extreme energies needed to produce photons of higher-than-TeV energies.
While getting past the supernova debris seems difficult, the process happens very quickly because the jets accelerate the particles to nearly the speed of light. Therefore, it only takes about five seconds to see the rapid rise of TeV photons in the data.
From there, it’s a more gentle downhill that lasts for about 13 seconds. The research team behind the work suggests that this involves the jets interacting with and accelerating particles in the environment beyond the star’s remnant. This increases the number of high-energy photons but at the same time drains some of the energy away from the jets as they are pushed up against a larger pile of material as they advance through the environment.
Eventually, this accumulation of material draws in enough energy that the number of high-energy photons begins to gradually decrease. This drop is slow enough that it lasts about 11 minutes or so.
In the case of the BOAT supernova, this was followed by a sharp drop in high-energy photons. This is thought to be caused by the jets getting wider as they move away from their source, which means the boat was as bright as we observed it because the central core of its jet was pointed directly at the ground. The timing of this descent also provides some information about how wide the aircraft is at this time.
There’s still a lot to learn about these events — we’re still not sure how black holes release jets of material in the first place, for example. But these kinds of detailed observations can give us a better idea of the timing and dynamics of jet formation, which will ultimately help provide models of what happens during black hole formation and jet formation.
Science, 2023. DOI: 10.1126/science.adg9328 (about DOIs).
