[NOTE ADDED: A reader forwarded a message that IceCube did not see any neutrinos with energies above 1 TeV = 1000 GeV from this GRB. Maybe this is not quite the final word (there would still be sensitivity, with some effort, to neutrinos in the 100 GeV - 1000 GeV range) but clearly the neutrino signal isn't striking, and it is probably not there at all. But as I've suggested below, even a non-observation might have significant implications for the science; the question is, how many neutrinos would the standard speculations about how GRB's work have led you to expect at IceCube? If a reader can provide that info, I'd appreciate that.]
The very recent report of a powerful and long-lived gamma-ray burst (GRB), and questions and remarks by my readers (thank you!), have motivated me, both as a scientist and a blogger, to try to understand whether we should have observed neutrinos from this GRB. This is forcing me to catch up on the related subjects of GRB’s, searches for high-energy neutrinos, and the highest-energy cosmic rays. I’m certainly not caught up yet; there are decades of research out there, and I’m quite far behind on developments over the past three or four years. But here are some of the basics that I believe I understand. Still, be cautious with the content of this post, both because I’m not an expert and because this is a very active area of research in which some fraction of the more speculative stuff will surely turn out to be wrong. I will try to refine this post with a more detailed and corrected article sometime later, perhaps once we know whether neutrinos from this GRB were or were not observed.
GRBs that last more than a few seconds are widely believed to be associated with an exceptional form of Type II (or “core-collapse”) supernova, though this is not known for certain. In these types of GRBs, there are (at least) two sources of photons (everything from gamma-rays to visible light to radio waves) and two sources of neutrinos. It is important not to confuse the different sources!
Neutrino Source 1 (pretty well understood): These are the neutrinos from the main supernova explosion. In a Type II supernova, the core of a a star is converted from a dense and extremely hot “gas” (really “plasma”) of ordinary atomic nuclei and electrons into a ball of neutrons, called a neutron star. (This may further collapse into a black hole.) The basic process by which this occurs is proton + electron → neutron + neutrino. As a result, a vast number of neutrinos with energies in the 0.01-0.03 GeV range are emitted. How vast is vast? Somewhere in the vicinity of
i.e. 1057 or more. Yeah, these are big numbers. (Actually the numbers are even bigger because additional neutrinos and anti-neutrinos can be made by other processes in the explosion.) These neutrinos carry off the vast majority of the energy of the supernova! Photons emitted in a supernova carries only a small fraction of the energy released.
But these neutrinos are emitted in all directions (Figure 1), so only a tiny, tiny fraction arrive at Earth. And the farther away is the supernova, the smaller this fraction becomes, falling like the square of the distance to the supernova. Moreover, because these neutrinos are of only moderate energy (relative to the second neutrino source mentioned below), their interactions with matter are very rare. So only a tiny, tiny fraction of these neutrinos interact with anything in the Earth. And only a tiny, tiny fraction of those go inside the detectors that scientists have built to detect neutrinos of these energies. Consequently, neutrinos from Source 1 can only be detected if the supernova that makes them is inside our galaxy (the Milky Way) or in one of its satellites, such as the Large Magellanic Cloud (LMC). In fact the first (and so far only) confirmed detection of neutrinos from a supernova occurred in 1987, when Supernova 1987a exploded in the LMC. Only about 10 neutrinos were observed. Today detectors are larger and better, but still the numbers wouldn’t be much larger.
The LMC is about 170,000 light years away. The big GRB observed very recently was about 3.6 billion (3,600,000,000) light years away. So there’s no chance we’d observe neutrinos from Source 1; the supernova is much, much too distant.
Photon Source 1 (quite well understood): These photons provide the visible-light sign of a supernova. It’s by this light that historically most supernovas have been discovered, typically a week or two after the explosion. The main explosion mentioned above occurs within the dense environment of the star. It’s so dense that even the neutrinos can’t escape it for a full second. It takes much longer for the explosion’s debris, and its glow in visible light and in somewhat higher-energy photons, to start emerging from the exploding star and the shells of gas that the star emitted in its old age (Figure 2). Supernova 1987a didn’t start to glow in visible light for about two hours after the neutrinos emerged. As the hot debris from the explosion moves outward, making a growing ball of heated gas around the exploded star, such supernovas become brighter and brighter for two or three weeks. Most supernovas are discovered only as they approach peak brightness; only a few have been observed very soon after they began.
Most of the resulting photons have energies far below the energies of the Source 1 neutrinos; instead of energies in the 1 MeV = 0.001 GeV = 1,000,000 eV range, they have energies in the 1 – 1,000 eV range. [Visible-light photons have energy of a few eV; photons from a decaying Higgs particle have energy of about 60 GeV.] For a supernova as far away as the one that produced the recent gamma-ray burst [GRB], the supernova won’t be bright enough to see in even a powerful telescope for a week or two after it occurs. We are expecting it to be seen soon, if our understanding of GRBs is correct and the recent GRB really came from a supernova.
Now those photons and neutrinos are made in every Type II supernova. By contrast, the next two sources are (presumably) special to GRBs.
Photon Source 2 (very poorly understood): These photons are ones that give the GRB its name; they have a wide range of energies, but are often observed up to many GeV. That’s much more energetic than the Source 1 neutrinos and vastly more energetic than the Source 1 photons. It is widely believed that only rare Type II supernovas make these photons. Also, it is widely believed the photons are produced within directed beams consisting of a variety of high-energy particles, called “jets” (Figure 3). (These astronomical jets are not to be confused with what Large Hadron Collider physicists call “jets”.) These GRB photons can only be observed when the jets happen to point more or less at the Earth, which makes it even more rare for us to see them.
Jets of material are quite common in astrophysical contexts, occurring in everything from supernovas to giant black holes in the centers of galaxies. They are very poorly understood, however. And I understand them even less well. So I’m just going to report that yes, such things do occur, and are probably due to very complicated effects of electric and magnetic fields outside of rapidly spinning objects.
In any case, what supposedly is happening is that shock waves in the explosion, and powerful electric and magnetic fields within the jets, are serving as a natural particle accelerator. Charged particles in the jets (electrons, protons, and atomic nuclei) are accelerated to very high energy, perhaps even to energies billions of times larger than what the Large Hadron Collider can do.
In collisions of the high-energy electrons with lower-energy photons that are present around the exploding star, energy can be transferred from the electrons to the photons. There might be other sources for the photons too. So along with the high-energy electrons come some high-energy photons. Since photons are not bent by magnetic fields, these gamma-rays, unlike their parent electrons, travel straight from the supernova to Earth, without delay and without changing direction (Figure 4). So even though we can’t observe the electrons that are accelerated, the photons produced by them go straight across the universe, and a few of them arrive at Earth, if the jet is favorably pointed.
The jets are apparently only present for a relatively short time (minutes? hours?) after the explosion, and so the same is true of these photons; they are observed only for minutes or a few hours, and their average energy decreases over time. The GRB observed recently had a record duration (many hours), as well as having a single photon which is the highest one observed so far (nearly 100 GeV).
Lots and lots and lots of lower energy photons are generated as well, including visible-light photons (around 1 eV = 0.000,000,001 GeV) and even radio waves (another billion times smaller in energy). The recent GRB’s photons were observed in visible-light telescopes and in radio telescopes. The diversity and length of these observations will probably allow us to learn a great deal about this particular GRB, and about GRBs in general.
Neutrino Source 2 (but note these neutrinos haven’t been observed, so this is very speculative at this point): Not only photons but also neutrinos and anti-neutrinos will be generated in these jets. But can they be observed? We have to know “How many?” and “How energetic?” before we can say whether the neutrinos that were produced in the recent GRB, or in any GRB, can be detected with modern technology.
Without going into too many details (which would probably be partly erroneous anyway, given how little is known about a GRB’s jets), the predominant process making neutrinos is expected to be this one. Protons are accelerated inside the jets to ultra-high-energies, perhaps billions of times higher energies than those found at the Large Hadron Collider. Some of these protons may be among the most energetic cosmic rays that we observe arriving from outer space. These protons will sometimes collide with other particles (perhaps mainly with the abundant moderate-energy photons) in the material around the exploding star, and from these collisions, pions and other hadrons will sometimes emerge. A positively-charged pion immediately decays to an anti-muon and a neutrino (while a negatively-charged pion decays a muon and anti-neutrino.) Since the protons are extremely energetic, the neutrinos will be very energetic as well. And since (presumably) the protons were produced within a jet and are thus all moving in roughly the same direction, these neutrinos will also be produced moving in that direction. The best guess is that these neutrinos have energies typically of hundreds of TeV, though this is clearly dependent on the details of the speculation. [1 TeV = 1000 GeV.]
Just like the GRB’s photons, these neutrinos are produced at the time of, or soon after, the explosion, travel straight across the universe without much delay, and may have much higher energy than the neutrinos produced in Source 1 [making each neutrino easier to detect]. Unlike the Source 1 neutrinos and the Source 1 photons, but like the Source 2 photons, they can only be observed when a jet of material happens to be aimed at the Earth. Or so goes the general speculation. See Figure 4.
Meanwhile the high-energy protons that produced those neutrinos aren’t so easy to observe, because they are bent by magnetic fields and take a circuitous route (Figure 4). As a result they are much delayed in time and arrive at Earth (if they do) from a random direction; we observe them but can’t be sure where they came from. As I mentioned, they may include the highest-energy cosmic rays that we have ever detected.
Neutrinos from this GRB?
Again, Source 2 of the neutrinos is still highly speculative in its details. In some versions of this basic speculation, it has been predicted that these neutrinos are common enough and energetic enough that we should see them in detectors such as IceCube. IceCube consists of a large number of photon detectors that have been embedded into a huge volume (roughly a cubic kilometer) of ice at the south pole. The idea of IceCube (leaving out a number of important details for now) is to detect the rare high-energy neutrino that happens to hit an atomic nucleus and is converted into a high-energy muon; the muon then, in turn, emits Cerenkov light that can traverse the ice and be observed by the photon detectors. IceCube can tell you when the neutrinos arrived and more or less what direction they came from. Since GRBs occur only a couple of times a day, and we measure where each one is on the sky, IceCube can say pretty reliably if a given neutrino is likely to have come from a particular GRB.
(By the way, neutrinos that IceCube can detect have such high energy, tens of GeV or more, that even though neutrinos have small masses, 1 eV or less, their velocities will be so close to the speed of light that there will be no measurable delay of their arrival time relative to photons from the GRB. Any delays observed in the photons and/or neutrinos will therefore be due to processes intrinsic to the GRB itself.)
According to IceCube’s most recent results, just released a couple of weeks ago (thanks to reader Dan for pointing this out), IceCube has seen no such neutrinos, implying the number of such neutrinos is at least a factor of four smaller than predicted by the most common speculations for neutrino Source 2. This may mean that something significant about these speculations needs to be revised… though to my eye, these speculations seem complicated enough that the revisions required might just be in the details, rather than in the overall basic idea.
Note, however, that the number of observable neutrinos that the speculations predict is small. IceCube had to look for neutrinos from several hundred GRBs before it could say firmly that it’s not seeing as many as predicted. That means the typical GRB wouldn’t produce an observable neutrino on average; few would produce more than one or two neutrinos in IceCube, even if the current theories about GRBs and neutrinos were correct.
The exciting thing about the most recent GRB, to my admittedly naive eye, is that it was so bright and so close and so long-lived that the number of neutrinos expected at IceCube is probably significantly larger than for any previous GRB. I say “probably” because I myself haven’t done a calculation, so I’m just making an educated guess; and I’m not sure any calculation would be very reliable anyway. [If anyone sees a calculation published somewhere, please alert me!] But because this most recent GRB is several times closer than the average GRB, the number of photons and neutrinos that will pass through a detector is several-squared times larger than the average GRB. [Would an expert help me get these numbers right, please?] Also, although the redshift from the expansion of the universe reduces the energies of photons and neutrinos in the average GRB by a factor of two or so, making them a lot harder to detect, that effect doesn’t much apply to this one, for which the redshift is only 20%. Finally, since it produced remarkably high-energy photons, this GRB may have been unusually energetic overall. So this new GRB perhaps represents an exceptionally favorable case for having produced detectable neutrinos, even if the standard speculative ideas are all wrong about how the neutrinos are produced, how many there are, and how energetic they are!
[Note: I believe that the ANTARES experiment, similar in concept to IceCube but in water instead of ice, may not be as useful for this particular GRB, because of the GRB's location.]
So I do think we should be looking forward to news from IceCube… as well as more details from the various satellites and telescopes that already observed this GRB. And I’m sure theorists are busily calculating what IceCube might expect to observe, and what we might learn if it does observe it, or if it doesn’t.
Ok, that’s my best effort so far — though I remind you one last time that I’m not an expert, and am still learning things. If any experts read this, please help me fix my mistakes and misconceptions!
(Thanks again to readers JollyJoker and Dan for pushing me to think about this, and to Peter Kurczynski at Rutgers for helping me on some of these details and providing me with some articles to read.)