We haven’t seen a supernova in our galaxy since 1604. “We’re really due for one,” says Duke physicist Kate Scholberg. To make sure that scientists get the most out of the next event, Scholberg started the SuperNova Early Warning System (SNEWS), a program that will notify thousands of professional and amateur astronomers as the supernova begins.
Scholberg studies neutrinos and she is particularly interested in those that are produced in supernovae. When a star explodes, neutrinos escape the conflagration first, even before visible light, because they interact with matter only through the weak force. Neutrinos zip right through things, whether supernovae or solid rock or human bodies, only rarely interacting with other particles as they do so. In other words, neutrinos are slippery little devils, which makes them tricky for physicists to detect. (See below a video simulation of a neutrino burst from a supernova in our galaxy. From the SNEWS.bnl.gov website.)
There are five large-scale neutrino detectors worldwide. Scholberg does experiments at Super-Kamiokande (Super-K) in Japan, which holds 50 kilotons of water in an enormous tank lined with phototubes. The tank is underground to shield it from interference from cosmic radiation at the surface of the Earth.
Of the trillions of neutrinos that pass through the water tank every day, a few dozen interact with particles in the water, producing charged particles that light up the phototubes. Most of these neutrinos are from the sun, and the rest are produced when cosmic rays collide in the Earth’s atmosphere. “If a supernova happened at the center of our galaxy, we’d see about 8,000 neutrinos in about 20 seconds,” says Scholberg.
A central SNEWS computer at Brookhaven National Laboratory keeps track of bursts of activity at Super-K and three other detectors in real-time. If more than one detector reports periods of high neutrino activity within 10 seconds of each other, the computer automatically sends out a supernova alert to astronomers. Scholberg says the neutrino burst will arrive on Earth anywhere from half an hour to a day before the visible light arrives. Physicists may even be able to glean information from the neutrino burst indicating the location of the supernova.
SNEWS will benefit astronomers by giving them a chance to observe a supernova from its very beginnings. SNEWS will also benefit particle physicists. “The neutrinos from a supernova are very, very useful for understanding neutrino properties,” Scholberg says. “The more we understand the progenitor of the supernova, the better off we are in neutrino physics.”
Neutrinos come in three different generations or “flavors”—electron neutrinos, muon neutrinos, and tau neutrinos. Scholberg would like to know more about how neutrinos oscillate between flavors as they travel. Figuring out neutrinos will help physicists untangle bigger questions, such as why the universe is made of matter instead of antimatter.
SNEWS has existed in its present form since 2005, but has sent out no alerts other than test runs. Astronomers estimate that a supernova probably occurs in our galaxy every couple of decades. Any that have occurred since 1604 have been missed, or gone unrecorded, or were obscured by a cosmic cloud of dust, or perhaps collapsed into a black hole rather than exploding. Scholberg says if any had occurred in our galaxy in the past 20 or 30 years, we’d know about it because the detectors would have picked up their neutrino bursts.
In 1987, a supernova occurred in a nearby galaxy, the Large Magellanic Cloud, and was visible from Earth. Nineteen neutrinos from “1987A” were detected in two earlier-generation detectors that were much smaller than today’s models.
Scholberg is ready for the next nearby supernova, and thanks to SNEWS, a whole host of other scientists are too.
Scholberg's Active Research Projects
- Super-Kamiokande and T2K in Japan
- CLEAR experiment at Spallation Neutron Source at Oak Ridge National Laboratory
- LBNE water Cherenkov project at DUSEL in South Dakota
This animation shows how the 50-kiloton Cherenkov detector at Super-Kamiokande in Japan might look in the event of a neutrino burst from a supernova in our galaxy. When the animation first begins, you will see a few individual phototubes lighting up in no apparent pattern (“noise”). After the onscreen countdown, phototubes light up in circular patterns, each indicating a shock wave produced by a particle that has been hit by a neutrino.