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Multimessenger probes deep- events with an arsenal of lenses Stephen Ornes, Science Writer

On September 22, 2017, four billion years into its MMA also brings together researchers who ap- journey through space, a ghostly particle hit the ice proach astronomy in different ways, says astrophysicist under Earth’s South Pole. This rare event was picked Teddy Cheung at the Naval Research Laboratory in up by IceCube, the largest detector on the Washington, DC, who has searched for the sources of planet, triggering a worldwide alert. In response, tele- , such as those in the 2017 event, using scopes on the ground and in orbit turned toward the data collected by the Fermi orbiting region of the sky that had produced the particle to telescope. “Every type of researcher, including theo- collect other particles and waves coming from the rists, experimentalists, and observers is really excited same source. These diverse tools allowed physicists about talking to each other,” he says. MMA is now to work out where this cosmic messenger came from— helping astronomers test theories about deep space and the answer took everyone by surprise (1). events, make serendipitous discoveries, and test ideas It was a shining example of multimessenger as- about some of the most exotic objects in the . tronomy (MMA)—the use of different types of cosmic “messengers” to study deep-space phenomena. Ways of Seeing Those messengers include electromagnetic waves Astronomers have long tried to study the same event (i.e., , radio, and others), particles (e.g., neutrinos in different ways. For the past few decades, that has and cosmic rays), and the ripples in called meant observing objects at several wavelengths across gravitational waves. MMA builds on multiwave- the , from radio waves to length studies, which began in the mid-20th century gamma rays. Radio telescopes measure low- and combine observations of different swaths of the emissions from stars, , and other sources; electromagnetic spectrum. The idea driving MMA is gamma ray telescopes, such as the orbiting Fermi to merge the strengths of individual tools, offering telescope that surveys the entire sky, measure high- a synergistic approach. energy waves. The spectrum in between includes

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 at the center of a distant . Image credit: NSF/VERITAS.

Published under the PNAS license.

3354–3357 | PNAS | February 26, 2019 | vol. 116 | no. 9 www.pnas.org/cgi/doi/10.1073/pnas.1900974116 Downloaded by guest on October 3, 2021 In 2017, the interferometer near Pisa, , helped identify a 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.

and radiation, as well as optical MMA investigations use data from at least two of light—the visible glow of a star. these messengers. In recent years, the studies have In recent years, new observatories have been able to taken a big jump, with observations that can verify or add neutrinos and gravitational waves to the list of cos- even challenge existing theory, says Cheung. The mic messengers. Neutrinos are particularly revealing. A 2017 neutrino revealed that blazers can produce the neutrino races through the universe almost unaffected by high-energy particles, despite prevailing theory sug- anymatterorforcesitmeets,soitstrajectorytracesdi- gesting they wouldn’t. MMA observations gave as- rectly back to its origin. For decades, astronomers have tronomers evidence that heavy elements such as monitored relatively low-energy neutrinos, mainly from and platinum form when neutron stars collide. Data the Sun, but since IceCube was completed in 2010, they from gravitational waves and electromagnetic radia- have begun to detect much higher energy neutrinos tion rule out some alternative theories of that coming from unknown sources in the cosmos. IceCube is challenge and at the same time buried in the Antarctic ice and points downward, using provide theorists with new constraints on future the whole of Earth as a shield against other forms models. Recent MMA studies, says Cheung, “have of radiation. demonstrated how the scientific potential that once The most recent arrival is the gravitational wave: a only existed in pen and paper form has actually been disruption in the fabric of space that can travel at the borne out by these observations.” . When massive objects accelerate vio- lently, they create ripples detectable by observato- Rapid Response ries on Earth. Since 2015, the Interferometer Astronomers are optimistic about MMA’s potential to Gravitational-Wave Observatory (LIGO) and Virgo reveal a more complete picture of transient events— detectors have recorded waves from colliding black for example, or gravitational waves—with holes and colliding neutron stars. (High-energy cosmic signals that fade over time. But capturing those di- rays are the fourth known messenger. These are verse observations only works when the first detection charged particles, mostly protons, traveling near the of the event is rapidly shared with the world so that speed of light. Tracking their source is tricky because other instruments can watch the right spot in the sky. their trajectories can be bent by magnetic fields, but “It’s very important to get the word out,” says as- theory predicts the same events that produce neutri- trophysicist Reshmi Mukherjee, at Barnard College of nos also produce cosmic rays.) Columbia University in New York City. “We don’t want

Ornes PNAS | February 26, 2019 | vol. 116 | no. 9 | 3355 Downloaded by guest on October 3, 2021 to lose any time.” Mukherjee works on a ground- University of Wisconsin–Milwaukee. He points to the based array of gamma ray detectors called VERITAS, case of Supernova1987a, which was first seen by as- for Very Energetic Radiation Imaging Telescope tronomers in a mountaintop observatory in Chile. Array System. “They noticed a star that wasn’t there before,” he In the case of the neutrino that arrived in 2017, says. To get confirmation from other telescopes, the rapid communication paid off. IceCube sent out an astronomers had to drive down the mountain. “They alert that prompted other instruments to gather data had to get to a place where a telegram can be sent to in the same swath of sky. VERITAS, for example, ob- notify the rest of the world about a explo- served the location of the neutrino through February sion.” Eventually, astronomers determined that it was 2018. In July 2018, using measurements from Fermi, an exploded star in the Large Magellanic Cloud, and VERITAS, and other telescopes, researchers revealed in the spring of 1987, it shone with the light of 100 the provenance of the neutrino. Its trajectory pointed million suns. directly back to a at the Brady thinks a lot about how alert systems have center of a distant galaxy (2). Some supermassive shaped MMA studies—mainly because of the one he black holes create energetic jets, and sometimes knows best. On August 17, 2017, the LIGO and Virgo those jets point directly at us. These “blazars” are detectors identified a gravitational wave produced by among the brightest objects in space. The survey the collision of two neutron stars (3, 4). Within sec- confirmed that this particular blazar, TXS 0506+056, onds, LIGO sent out an alert: a text message with a link was producing an unusually high quantity of gamma to a webpage displaying automated data analyses. At rays at the time of the neutrino detection. 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. “We all hope someday we’re going to get a notification The event made history because scientists could through our system that says: ’Pay attention to me, this combine gravitational wave data with electromagnetic is weird’.“ observations for the first time to untangle what hap- pens when neutron stars merge. “It’s a beautiful, rich, —Patrick Brady messy, and complicated process,” says physicist Peter Shawhan at the University of Maryland, College Park, who studies gravitational waves in LIGO data. Astronomers hypothesize that high-energy neutri- The stars circled each other closer and closer until nos could form when high-energy protons collide with they smashed together into a dense single object, an low-energy . Years ago, astronomers sug- event recorded by the LIGO and Virgo detectors as a gested blazars might be good sites for this process, kind of “chirp” that lasted 100 seconds. (For com- but in recent years, that idea had been all but aban- parison, in the black hole collisions seen so far the ’ doned because studies hadn t found a correlation chirp lasts only a fraction of a second.) At the same between the arrival directions of IceCube cosmic time, the collision released high-energy gamma rays. “ neutrinos and known Fermi blazars. It was a surprise The gamma rays arrived at Earth just under 2 seconds ” that blazars showed up, says physicist Francis Halzen after the end of the gravitational wave chirp, sug- at the University of Wisconsin-Madison, principal in- gesting that the spacetime ripples travel at the speed vestigator for IceCube. of light. ’ The researches wondered if they d missed others. Afterward, the event produced a flare of optical And they had: When Halzen and his collaborators and ultraviolet emission called a , thought to combed through archived IceCube data, they dis- be fueled by the radioactive decay of newly formed covered a burst of more than a dozen neutrinos in late heavy elements, which faded over a matter of days. 2014 and early 2015 that likely came from the same This was followed by an afterglow in X-rays and radio blazar. This is leading astronomers to rethink how such waves. These electromagnetic observations provided neutrinos form. If only 5% of known blazars emit a evidence that heavy elements, including gold and burst like the one from 2014, Halzen says, they would lead, are formed in collisions. Continued account for the all-sky neutrino flux observed by analyses of the data led one group of astronomers to IceCube. suggest in the spring of 2018 that the merger left However, although the Fermi telescope has iden- behind a black hole (5). But the case isn’t settled: A tified 1,000 blazars, so far only one has ejected neu- follow-up analysis, published in January, offers evi- trinos that IceCube detected. Halzen says it’s still dence that the remnant was not a black hole but a possible that the September 22, 2017, neutrino was “a magnetar—a supermassive neutron star with a formi- kind of freak event.” Perhaps, he adds, a real flaring jet dable magnetic field (6). accidentally produced one neutrino, meaning, Halzen says, that “Nature has really not done us a favor.” Messengers of the Future Shawhan says astronomers don’t know when they’ll Collision Alert see gravitational waves from neutron star mergers The mystery of the neutrino’s birthplace was solved again after LIGO starts its next run in 2019. “We don’t thanks to rapid, worldwide alert systems unthinkable know how lucky we got with this first event,” he says. 30 years ago, says astrophysicist Patrick Brady at the “Is it once in a decade, or what?”

3356 | www.pnas.org/cgi/doi/10.1073/pnas.1900974116 Ornes Downloaded by guest on October 3, 2021 Changes are afoot that astrophysicists hope will holes merging with neutron stars. “The black hole–neutron bring answers. LIGO, Virgo, and IceCube will soon all star case is unique because the gravitational tear- be upgraded, and sophisticated new telescopes are up of the neutron star material by the black hole scheduled to launch into space in the near future. “Three could produce detectable electromagnetic radiation,” years from now, we’ll be inundated with 10 times he explains, “and could also produce detectable more events than we see now,” predicts Cheung. The neutrinos.” upgrades increase the possibility of seeing exotic Brady, in Milwaukee, is excited at the prospect of events through three messengers at once—neutrinos, MMA studies finding something completely unex- electromagnetic radiation, and gravitational waves. pected in the cosmos. “We all hope someday we’re Cheung says the MMA community will be watch- going to get a notification through our system that ing, in particular, for the next logical mashup: black says: ‘Pay attention to me, this is weird’.”

1 Redd NT (2018) Inner workings: After years of listening with detectors buried in Antarctic ice, IceCube researchers trace neutrino source. Proc Natl Acad Sci USA 115:8463–8465. 2 IceCube Collaboration (2018) Neutrino emission from the direction of the blazar TXS 0506+056 prior to the IceCube-170922A alert. Science 361:147–151. 3 Abbott BP, et al.; LIGO Scientific Collaboration and Virgo Collaboration (2017) GW170817: Observation of gravitational waves from a binary neutron star inspiral. Phys Rev Lett 119:161101. 4 Kasliwal MM, et al. (2017) Illuminating gravitational waves: A concordant picture of photons from a . Science 358:1559–1565. 5 Pooley D, Kumar P, Wheeler JC, Grossan B (2018) GW170817 most likely made a black hole. Astrophys J Lett 859:L23. 6 van Putten MHPM, Della Valle M (2019) Observational evidence for extended emission to GW170817. Mon Not R Astron Soc Lett 482:L46–L49.

Ornes PNAS | February 26, 2019 | vol. 116 | no. 9 | 3357 Downloaded by guest on October 3, 2021