The Supernova Early Warning System

Studying supernovae is a very tricky business. They occur so rarely in our galaxy that nobody knows when and where they will happen. The current strategy involves patiently waiting for a “new star” to appear in the sky. The problem with this strategy is that the entire sky cannot be monitored constantly, so astronomers don’t get to see the supernova as it happens. Instead, they stumble upon it hours or days later, after the initial explosion has happened. This leaves a major gap in our understanding of supernova processes that astronomers are desperate to close. But what are we to do without the telescopic manpower to monitor every arc-second of the sky all the time?

Enter SNEWS—the SuperNova Early Warning System—a world-wide network of observatories that looks for the sign that a galactic supernova is about to become visible and alerts the astronomical community so they can point their telescopes towards the supposed location and watch the show. The telltale sign SNEWS is looking for is a shower of neutrinos that are produced when an unfortunate star’s core takes its proverbial last breath and collapses.

Neutrinos Production in Supernovae

Neutrinos have no charge and are incredibly light-weight. In fact, they were believed to have no mass at all for quite some time. For this reason, neutrinos barely interact with anything at all. They just go. Photons, on the other hand, get held up in traffic despite having no mass. The material within a star is dense enough so that light is continuously absorbed and reemitted in a random direction. This causes the light to take a random walk on its way from the core to the surface. So although light moves faster than neutrinos do, it gets caught up within the stellar envelope and neutrinos are the first ones out the door.

Neutrino Detectors Worldwide

Neutrinos are infamously non-interactive, but it is still possible to detect them. The most common method involves detecting not the neutrinos themselves, but the products of their interactions with matter. A very common reaction involving neutrinos is inverse beta decay wherein an electron antineutrino interacts with a proton to create a neutron and a positron ( $\bar{\nu}_e + p \rightarrow n + e^+$ ). The trick is then to detect the neutrino-induced positron. For this reason, most detectors contain an ample amount of protons to maximize positron production.

Figure 1: The Sudbury Neutrino Observatory (SNO) is a heavy-water Cherenkov detector.

Scintillation Detectors — These detectors are large volumes of organic hydrocarbons that produce light when charged particles (like positrons) within them lose energy. The light produced then enters the photomultiplier tubes (PMTs), which are extremely sensitive photon detectors. The PMTs generate an amplified current that is sent to computers for analysis. Unfortunately, scintillators emit light in random directions, so there is no directional information contained in the created photons. Examples include: Large Volume Detector (LVD) in Italy, Mini-BooNE in the US, KamLAND in Japan, and Borexino in Italy.

Water Cherenkov Detectors — When neutrinos interact with water, the resultant charged particles are travelling faster than the speed of light for water and emit Cherenkov radiation. This radiation is again detected by PMTs. The light emitted by this process is directional and allow us to know which direction the neutrinos are coming from. Example: Super-Kamiokande (Super-K) in Japan.

Long String Water Cherenkov Detectors — These detectors are very similar to water detectors. The difference is that they’re arranged on long strings that are embedded within water or ice. They are designed to detect high-energy neutrinos, but can still be used to look for the signs of inverse beta decay. Example: IceCube in Antarctica.

High-Z / Neutron Detectors — Lead and Iron are commonly used in these detectors. They make use of another interactions between neutrinos of all flavors (there are three types of neutrinos, the electron, tau, and muon) and the nuclei of the detector material. This interaction scatters neutrons that are then detected and sent for analysis. Example: The Helium and Lead Observatory (HALO) in Canada.

Detectors ASSEMBLE!

Some of these detectors are part of the SuperNova Early Warning System. Like Marvel’s Avengers, these detectors form a team that quietly wait for the signal that a supernova is about to become visible, and then they save the day by alerting the world, telling them where to look.

Figure 2: Locations of SNEWS-affiliated neutrino detectors. Mini-BooNE–Chicago (Orange), SNOLAB–Sudbury (Pink), LVD/Borexino–Gran Sasso (Blue), Super-K–Hida (Red), IceCube–South Pole (Green).

The network of detectors reports to a centralized computer (referred to as the “coincidence server”) located at the Brookhaven National Laboratory. This server listens for triggers from any of the SNEWS detectors. False alarms are an issue, however, so no single detector can issue an alert. A coincident signal between multiple detectors must be provided before the coincidence server issues an alert to the community. Even so, the possibility of a false alarm still exists and SNEWS has designed its coincidence algorithms so that the rate of false alarms is approximately 1 per century.

Once a coincident signal has been received by the server, it designates the alert as either GOLD or SILVER depending on the confidence level. A GOLD alert is automatically sent to the public. A SILVER alert is sent to the experiments and requires human checking before more widespread dissemination. After all, it is important to prevent false alerts from being issued; we don’t want a ‘cry wolf’ sort of situation to arise and end up missing out on some serious science. To ensure the accuracy and utility of SNEWS, the system was designed with “the three P’s” in mind.

Figure 3: A flow chart describing how the coincidence server handles triggers and issues alerts.

The Three P’s:

1. Prompt — The alert must be quick. Once the neutrinos reach the detectors, it is only a matter of hours before the early stages of the shock are visible. The automated process that produces a GOLD warning takes approximately 5 minutes. However, a SILVER warning needs human input to determine whether it’s worth alerting astronomers. This checking process could take 20 minutes or more.
2. Pointing — The usefulness of SNEWS is severely diminished if we don’t know which direction in the sky we should look. It’s important to have some directional information provided by the detectors. The only detector capable of pointing is the Super-K in Japan. Technically a triangulation technique is possible by using timing information from multiple detectors, but it is not very accurate at this time.
3. Positive — “There must be no false supernova alerts to the astronomical community.” No single experiment can decrease the false alarm rate to zero. However, requiring a coincidence between experiments effectively can.
Since SNEWS has been developed, there have been no galactic supernovae and thus, no alerts sent out. Supernovae occur in the Milky Way approximately once in a 30-year period, so it’s only a matter of time before SNEWS gets put to the test. And when it does, you can be part of the action. Anyone can sign up to be alerted by SNEWS when a supernova is detected. Amateur astronomers are an integral part the world-wide observing network. Sign up to receive alerts here.