Sky watchers both amateur and professional will soon have a new system to alert them to the spectacular death throes of stars in our galaxy. A revamped version of the SuperNova Early Warning System (SNEWS), a software program hosted on servers at Brookhaven National Laboratory on Long Island, New York, will soon provide more reliable, precise and timely notice of any star going supernova in the Milky Way—an event that promises to be a once-in-a-lifetime display.
Depending on the properties of an exploding star, it can take anywhere from seconds to hours for the light associated with a supernova to make its way out of the fireball. Tiny neutrino particles, however, barely interact with matter at all, making them excellent harbingers of a supernova. When a stellar core collapses just before a supernova explodes, it releases a blast of neutrinos that escapes almost instantly at very nearly the speed of light. SNEWS collects potential signs of supernovae from neutrino observatories around the world to give warning as soon as possible.
Most supernovae that astronomers observe are in far-off galaxies, where they briefly outshine their galactic surroundings. But catching one closer to home would allow scientists to gather much more data. Stars are nuclear factories where hydrogen atoms fuse to create heavier elements, making planets like Earth—and us—possible. Studying them up close will help researchers understand the processes that create the elements, and the types of stars that explode and scatter them across the universe. Many scientists note, however, that we can’t really be sure what we’ll discover by watching a supernova from the very beginning. We missed the only opportunity we’ve had in the modern age by realizing hours too late that a supernova erupted in the southern sky on February 23, 1987. Scientists got a lot out of supernova SN1987A, including an expansion of their ideas about what sort of stars can go supernova, and a clear view of the debris that supernovae cast into space. Sadly, the precious early moments of the explosion and the secrets they could have revealed were lost. SNEWS 2.0 will be crucial to making the most out of the next opportunity. “We’ve discovered more than 5,000 supernovae in galaxies far, far away,” says Mansi Kasliwal, an astronomy professor at the California Institute of Technology. “To miss one that’s at home would be very, very disturbing.”
“These are relatively rare events on human lifetime timescales,” says Kate Scholberg, a professor of physics at Duke University who cofounded SNEWS. “The frequency that we expect is something like a few per century … and that makes it especially important to capture every possible, last piece of information we can get.” Early light from a supernova, Scholberg explains, is crucial to understanding the star that exploded and the nearby environment that existed prior to a supernova event. And monitoring neutrinos is the best way to catch one in time.
Observing galactic supernovae could help resolve challenging, fundamental questions in physics, Kasliwal says. “Take something as basic as, when does a supernova form a neutron star, and when does it form a black hole,” says Kasliwal, who works with telescopes at Palomar Observatory in California and is not involved in SNEWS. “One would think that is a very simple question, and that the more massive stars form black holes and that the less massive star should form neutron stars…. It turns out it’s much, much more complicated than that.”
In preparation for the advent of SNEWS 2.0, Kasliwal led the development of an infrared telescope survey called Palomar Gattini-IR to collect detailed information on the stars in the galaxy that are likely to go supernova sometime soon. “We will have a long baseline of monitoring stars before they explode to be able to understand what the structure of the star is just prior to explosion,” says Kasliwal. “Once it explodes, we will be able to do high resolution spectroscopy and understand details in the explosion physics that so far we just have good guesses for.”
Early supernova warnings can also lead to insights into neutrinos themselves. Studying neutrinos erupting from stellar explosions could help scientists measure the particles’ mass—a current mystery—as well as understand how they shift from one flavor to another. And because the density of neutrinos is so high in supernovae, they offer a rare opportunity to study neutrinos interacting directly each other.
The SNEWS 2.0 collaboration plans to extend alerts to catch the rise in neutrino emission hours or days before a star makes the transition from burning to collapsing core. Presupernova neutrinos are much lower in intensity than those coming from the core collapse neutrino burst, which currently limits detection to only the few, closest supernova-ready stars, such as Betelgeuse and Antares. By increasing detector sensitivity and combining information from multiple detectors, the system’s presupernova neutrino signal range could soon extend as far as the center of our galaxy. A paper describing SNEWS 2.0 will soon appear in the New Journal of Physics. A preliminary draft of the paper is currently available online.
When Scholberg and her SNEWS cofounder Alec Habig, a professor at University of Minnesota Duluth, first developed the warning system in the late 1990s, one of the overriding design principles was keeping errant notifications to around one per century. “Now, because there has been so much good stuff happening from people chasing ghosts,” as Habig calls errant alerts and test runs sent from other observatories, “SNEWS subscribers have said, ‘Hey, why aren’t you sending us ghosts? We want to practice.’”
“We plan to have streams with different alert levels,” says Habig, “so you with your telescope can select, ‘I only want to chase things that are reasonably sure’ or ‘I wanna roll the dice and go after everything.’ People will be able to choose to act on things based on the confidence level.” Although higher sensitivity means a higher false-positive rate, it allows SNEWS 2.0 to catch supernovae that would have slipped under the prior signal threshold, as well as leading to speedier alerts and a reduced likelihood of missing the early moments of a supernova that has a shorter lag time between a core collapse and the subsequent light show.
No group is more eager to chase phantom alerts than amateur astronomers, in hope of eventually catching the real thing, according to Stella Kafka, CEO of the American Association of Variable Star Observers. Many AAVSO members, who are primarily amateur observers, welcome the practice that comes with responding to false positives with SNEWS 2.0. “Our volunteers are very dedicated,” Kafka says. “They really want to leave a legacy.” As an added benefit, all astronomers, including amateurs, who contribute to a supernova observation will have their efforts etched in the scientific record. “My observers will be co-authors on the paper,” Kafka says. “The SNEWS team has guaranteed that.”