On September 22, 2017, four billion years into its journey through space, a ghostly particle hit the ice under Earth’s South Pole. This rare event was picked up by IceCube, the largest neutrino detector on the planet, triggering a worldwide alert. In response, telescopes on the ground and in orbit turned toward the region of the sky that had produced the particle to collect other particles and waves coming from the same source. These diverse tools allowed physicists to work out where this cosmic messenger came from—and the answer took everyone by surprise (1).

In July 2018, using measurements from Fermi, VERITAS in southern Arizona (pictured), and other telescopes, researchers revealed for the first time the origins of a high-energy neutrino, tracing it to a supermassive black hole at the center of a distant galaxy. Image credit: NSF/VERITAS.

It was a shining example of multimessenger astronomy (MMA)—the use of different types of cosmic “messengers” to study deep-space phenomena. Those messengers include electromagnetic waves (i.e., light, radio, and others), particles (e.g., neutrinos and cosmic rays), and the ripples in spacetime called gravitational waves. MMA builds on multiwavelength studies, which began in the mid-20th century and combine observations of different swaths of the electromagnetic spectrum. The idea driving MMA is to merge the strengths of individual tools, offering a synergistic approach.

MMA also brings together researchers who approach astronomy in different ways, says astrophysicist Teddy Cheung at the Naval Research Laboratory in Washington, DC, who has searched for the sources of neutrinos, such as those in the 2017 event, using gamma ray data collected by the Fermi orbiting telescope. “Every type of researcher, including theorists, experimentalists, and observers is really excited about talking to each other,” he says. MMA is now helping astronomers test theories about deep space events, make serendipitous discoveries, and test ideas about some of the most exotic objects in the universe.

Ways of Seeing Astronomers have long tried to study the same event in different ways. For the past few decades, that has meant observing objects at several wavelengths across the electromagnetic spectrum, from radio waves to gamma rays. Radio telescopes measure low-energy emissions from stars, galaxies, and other sources; gamma ray telescopes, such as the orbiting Fermi telescope that surveys the entire sky, measure high-energy waves. The spectrum in between includes infrared and ultraviolet radiation, as well as optical light—the visible glow of a star. In recent years, new observatories have been able to add neutrinos and gravitational waves to the list of cosmic messengers. Neutrinos are particularly revealing. A neutrino races through the universe almost unaffected by any matter or forces it meets, so its trajectory traces directly back to its origin. For decades, astronomers have monitored relatively low-energy neutrinos, mainly from the Sun, but since IceCube was completed in 2010, they have begun to detect much higher energy neutrinos coming from unknown sources in the cosmos. IceCube is buried in the Antarctic ice and points downward, using the whole of Earth as a shield against other forms of radiation. The most recent arrival is the gravitational wave: a disruption in the fabric of space that can travel at the speed of light. When massive objects accelerate violently, they create ripples detectable by observatories on Earth. Since 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo detectors have recorded waves from colliding black holes and colliding neutron stars. (High-energy cosmic rays are the fourth known messenger. These are charged particles, mostly protons, traveling near the speed of light. Tracking their source is tricky because their trajectories can be bent by magnetic fields, but theory predicts the same events that produce neutrinos also produce cosmic rays.) MMA investigations use data from at least two of these messengers. In recent years, the studies have taken a big jump, with observations that can verify or even challenge existing theory, says Cheung. The 2017 neutrino revealed that blazers can produce the high-energy particles, despite prevailing theory suggesting they wouldn’t. MMA observations gave astronomers evidence that heavy elements such as gold and platinum form when neutron stars collide. Data from gravitational waves and electromagnetic radiation rule out some alternative theories of gravity that challenge general relativity and at the same time provide theorists with new constraints on future models. Recent MMA studies, says Cheung, “have demonstrated how the scientific potential that once only existed in pen and paper form has actually been borne out by these observations.” In 2017, the Virgo interferometer near Pisa, Italy, helped identify a gravitational wave produced by the collision of two neutron stars. Researchers were able to combine this gravitational wave data with electromagnetic observations for the first time. Image credit: Wikimedia Commons/The Virgo Collaboration.

Rapid Response Astronomers are optimistic about MMA’s potential to reveal a more complete picture of transient events—for example, supernovas or gravitational waves—with signals that fade over time. But capturing those diverse observations only works when the first detection of the event is rapidly shared with the world so that other instruments can watch the right spot in the sky. “It’s very important to get the word out,” says astrophysicist Reshmi Mukherjee, at Barnard College of Columbia University in New York City. “We don’t want to lose any time.” Mukherjee works on a ground-based array of gamma ray detectors called VERITAS, for Very Energetic Radiation Imaging Telescope Array System. In the case of the neutrino that arrived in 2017, rapid communication paid off. IceCube sent out an alert that prompted other instruments to gather data in the same swath of sky. VERITAS, for example, observed the location of the neutrino through February 2018. In July 2018, using measurements from Fermi, VERITAS, and other telescopes, researchers revealed the provenance of the neutrino. Its trajectory pointed directly back to a supermassive black hole at the center of a distant galaxy (2). Some supermassive black holes create energetic jets, and sometimes those jets point directly at us. These “blazars” are among the brightest objects in space. The survey confirmed that this particular blazar, TXS 0506+056, was producing an unusually high quantity of gamma rays at the time of the neutrino detection. Astronomers hypothesize that high-energy neutrinos could form when high-energy protons collide with low-energy photons. Years ago, astronomers suggested blazars might be good sites for this process, but in recent years, that idea had been all but abandoned because studies hadn’t found a correlation between the arrival directions of IceCube cosmic neutrinos and known Fermi blazars. “It was a surprise that blazars showed up,” says physicist Francis Halzen at the University of Wisconsin-Madison, principal investigator for IceCube. The researches wondered if they’d missed others. And they had: When Halzen and his collaborators combed through archived IceCube data, they discovered a burst of more than a dozen neutrinos in late 2014 and early 2015 that likely came from the same blazar. This is leading astronomers to rethink how such neutrinos form. If only 5% of known blazars emit a burst like the one from 2014, Halzen says, they would account for the all-sky neutrino flux observed by IceCube. However, although the Fermi telescope has identified 1,000 blazars, so far only one has ejected neutrinos that IceCube detected. Halzen says it’s still possible that the September 22, 2017, neutrino was “a kind of freak event.” Perhaps, he adds, a real flaring jet accidentally produced one neutrino, meaning, Halzen says, that “Nature has really not done us a favor.”

Collision Alert The mystery of the neutrino’s birthplace was solved thanks to rapid, worldwide alert systems unthinkable 30 years ago, says astrophysicist Patrick Brady at the University of Wisconsin–Milwaukee. He points to the case of Supernova1987a, which was first seen by astronomers in a mountaintop observatory in Chile. “They noticed a star that wasn’t there before,” he says. To get confirmation from other telescopes, the astronomers had to drive down the mountain. “They had to get to a place where a telegram can be sent to notify the rest of the world about a supernova explosion.” Eventually, astronomers determined that it was an exploded star in the Large Magellanic Cloud, and in the spring of 1987, it shone with the light of 100 million suns. Brady thinks a lot about how alert systems have shaped MMA studies—mainly because of the one he knows best. On August 17, 2017, the LIGO and Virgo detectors identified a gravitational wave produced by the collision of two neutron stars (3, 4). Within seconds, LIGO sent out an alert: a text message with a link to a webpage displaying automated data analyses. At the time, Brady was walking down a street in Amsterdam. “Suddenly I’m on my phone, looking for a restaurant to sit in, and join a phone call,” he says. The event made history because scientists could combine gravitational wave data with electromagnetic observations for the first time to untangle what happens when neutron stars merge. “It’s a beautiful, rich, messy, and complicated process,” says physicist Peter Shawhan at the University of Maryland, College Park, who studies gravitational waves in LIGO data. “We all hope someday we’re going to get a notification through our system that says: ’Pay attention to me, this is weird’.“ —Patrick Brady The stars circled each other closer and closer until they smashed together into a dense single object, an event recorded by the LIGO and Virgo detectors as a kind of “chirp” that lasted 100 seconds. (For comparison, in the black hole collisions seen so far the chirp lasts only a fraction of a second.) At the same time, the collision released high-energy gamma rays. The gamma rays arrived at Earth just under 2 seconds after the end of the gravitational wave chirp, suggesting that the spacetime ripples travel at the speed of light. Afterward, the event produced a flare of optical and ultraviolet emission called a kilonova, thought to be fueled by the radioactive decay of newly formed heavy elements, which faded over a matter of days. This was followed by an afterglow in X-rays and radio waves. These electromagnetic observations provided evidence that heavy elements, including gold and lead, are formed in neutron star collisions. Continued analyses of the data led one group of astronomers to suggest in the spring of 2018 that the merger left behind a black hole (5). But the case isn’t settled: A follow-up analysis, published in January, offers evidence that the remnant was not a black hole but a magnetar—a supermassive neutron star with a formidable magnetic field (6).

Messengers of the Future Shawhan says astronomers don’t know when they’ll see gravitational waves from neutron star mergers again after LIGO starts its next run in 2019. “We don’t know how lucky we got with this first event,” he says. “Is it once in a decade, or what?” Changes are afoot that astrophysicists hope will bring answers. LIGO, Virgo, and IceCube will soon all be upgraded, and sophisticated new telescopes are scheduled to launch into space in the near future. “Three years from now, we’ll be inundated with 10 times more events than we see now,” predicts Cheung. The upgrades increase the possibility of seeing exotic events through three messengers at once—neutrinos, electromagnetic radiation, and gravitational waves. Cheung says the MMA community will be watching, in particular, for the next logical mashup: black holes merging with neutron stars. “The black hole–neutron star case is unique because the gravitational tear-up of the neutron star material by the black hole could produce detectable electromagnetic radiation,” he explains, “and could also produce detectable neutrinos.” Brady, in Milwaukee, is excited at the prospect of MMA studies finding something completely unexpected in the cosmos. “We all hope someday we’re going to get a notification through our system that says: ‘Pay attention to me, this is weird’.”