Observing a supernova as it happens is a very tough thing to do. If you blink, you’ll miss it. Astronomers are constantly trying to find ways to look in the direction of a massive star just before it blows, but supernova prediction is a very young science. Now, combining the sensitivity of neutrino detectors and attempting to make the data as “real time” as possible, the SuperNova Early Warning System (SNEWS) is born, sending you a neutrino weather forecast direct to your inbox hours before a star explodes.
In the moments before a massive star explodes as a supernova (one of the most energetic events we can observe in the Universe), the core of the star will collapse. At the event of a core collapse, huge quantities of energy are released. Much of this energy is converted into neutrinos which are blasted from the core and pass through the overlying matter of the destructing star as if it wasn’t there.
Neutrinos, you see, are “ghosts” of particles – very, very weakly interacting and travel very close to the speed of light. In this situation, neutrinos will even outrun photons of electromagnetic waves when escaping the star. Therefore, neutrinos have become the ultimate tool to measure the conditions of a star’s core immediately after they are generated by fusion reactions. The EM radiation will take much longer to reach the surface of the star; in the case of our Sun, photons emitted from the core can take millions of years to reach the surface, they are constantly absorbed and re-emitted by the opaque matter. Neutrinos will do the same trip in a matter of minutes!
This poses an interesting question. As a supernova will happen very quickly, is it possible that neutrinos created as the massive star’s core collapses can be “seen” before the bright EM radiation? The answer to this is “yes”, and the method has already been proven… but only after the event. Neutrinos are detected before the supernova light is observed, but only after vast amounts of data is analysed. A “real time” system is required so the neutrino signal can be detected, and telescopes turned toward the star with a collapsing core.
We already have a method of detecting weakly interacting neutrinos. Neutrino detectors are strange things. They sit in deep mines, away from the interference of cosmic rays and use the Earth itself as a shield. Composed from vast volumes of water (with some variations containing chlorine or gallium), neutrino detectors wait for a few interactions between the water molecules and neutrinos. When an interaction occurs, a flash of Cherenkov radiation can be measured. Although there are likely to be billions of neutrinos passing through the detector at any given second, only a tiny number will hit the water molecules “head on”. Should there be a “spike” in neutrino flashes, then statistically speaking, there are more neutrinos passing through the detector than usual. In this case, there has either been a sudden increase in core production of neutrinos in our Sun or there has been another event, such as a supernova in our galaxy. An approximate direction to the source of neutrinos can also be deduced.
It is worth noting that the size of a neutrino detector is critical, the bigger it is (like any telescope), the further it “sees” (the number density of neutrinos diminish with distance from the source). The current detectors in operation can see up to a few hundred kiloparsecs (a few tens of thousands of light years), encompassing the whole of the Milky Way. The next generation of detectors are going to be so large that their reach will extend to other galaxies such as Andromeda.
So, we can detect the neutrino spikes by sifting through the data after the supernova has happened, but we still cannot detect the neutrino signal and react in time to actually see the supernova happen.
Now help is at hand. Kate Scholberg from Duke University, Durham (NC) has an elegant solution. Each neutrino observatory around the planet has its own built-in software that identifies an increase in neutrino flux. In a recent publication, she describes a method of collecting all this data from each observatory and routing it through a “coincidence server”. Put very simply, the coincidence server monitors the input from the neutrino flux from each observatory, should any signal coincide with another within ten seconds of each other, an automated warning is issued that an “event” is about to occur.
Called the SuperNova Early Warning System (SNEWS), the network has been working in “automatic mode” since 2004. By signing up for the system, astronomers are able to receive a warning newsletter of an impending supernova within our galaxy. It is predicted that the system will be able to give a minimum of a 2 hour warning, allowing astronomers to direct their telescopes toward the predicted location of a pre-supernova event with time to spare.
In summary, Scholberg highlights the main attributes to the system:
- The neutrino signal for a core collapse event precedes its electromagnetic fireworks by hours, or perhaps tens of hours.
- The burst of neutrinos itself lasts tens of seconds.
- The pointing from the neutrinos will be a few degrees in an optimistic case. There may be no pointing information at all, or the pointing information may be not be available immediately.
- Currently running experiments are sensitive to a core collapse in theMilkyWay, or just beyond. The next generation of detectors may reach to Mpc range.
- A few Galactic supernovae are expected per century.
- SNEWS is online, and can provide an alert within minutes of a Galactic core collapse.
Information from arXiv eprint: “The SuperNova Early Warning System“