The cosmos is speaking to us—bearing news of phenomenal events spanning billions of years and astronomical distances, from ancient, far-away galaxies all the way to the here and now. Its messengers—all wavelengths of light, undulating ripples of gravity stretching the fabric of space-time, and streaming particles of nearly unfathomably high energy—tell the stories of stars and galaxies, how the universe came to be and how it continues to evolve.
In the past, we heard only bits and snippets of those stories, relayed in a stream of photons or perhaps a burst of gamma rays; we heard a lone voice where, in fact, there was a chorus. From observations of the sun starting in the late 1960s, we learned that cosmic particles called neutrinos were produced in the fusion furnaces of stars. Then, in February 1987, astronomers saw an explosion of light from a dying star—a supernova in the nearby Large Magellanic Cloud—preceded by a burst of neutrinos; the light and the neutrino emissions were correlated to the supernova event, and multi-messenger astronomy was born.
Named in reference to the “cosmic messengers”—gravitational waves, photons, cosmic rays, and neutrinos, which are manifestations of the four fundamental forces of nature (respectively, the gravitational, electromagnetic, strong nuclear, and weak nuclear forces)—multi-messenger astronomy aims to gain a more detailed and complete understanding of the universe through the coordinated observation and correlative interpretation of these “messengers” of cosmic phenomena such as black holes and active galactic nuclei, neutron star mergers, and supernovae.
Following that first multi-messenger observation in 1987, the field took nearly 30 years more to identify another multi-messenger source. As it happened, two such sources were identified in the same year, and Penn State scientists were integral to both discoveries: a binary neutron star merger, via a gravitational wave detected by the LIGO Scientific Collaboration; and a flaring supermassive black hole, or “blazar,” via a neutrino detected by the IceCube Collaboration; both in the final months of 2017. Such breakthroughs would not have been possible without the knowledge, instrumentation, and global coordination that came with multi-messenger astronomy’s maturing into a truly viable, mainstream scientific domain. And Penn State scientists were instrumental in developing the foundational theories, building the Earth- and space-based instrumentation, and leading initiatives within the globe-spanning collaborations that have brought about multi-messenger astronomy’s ascendance and are now yielding some of the most promising astronomical discoveries of the modern era.
In fact, long before LIGO’s and IceCube’s incredible breakthroughs, a group of scientists at Penn State was already anticipating such discoveries and the significance of the multi-messenger approach to the future of astronomy and astrophysics. Nearly twenty years earlier, in 1993, Abhay Ashtekar and a number of other physicists, astronomers, and mathematicians at Penn State formed the Institute for Gravitational Physics and Geometry—which would later become the Institute for Gravitation and the Cosmos (IGC) in 2007 and, ultimately, the genesis of multi-messenger astrophysics at Penn State.
ILLUSTRATION OF TWO MERGING NEUTRON STARS. The rippling space-time grid represents gravitational waves that travel out from the collision, while the narrow beams show the burst of gamma rays that are shot out just seconds after the gravitational waves. Swirling clouds of material ejected from the merging stars are also depicted—these clouds glow with visible and other wavelengths of light. Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet
“This institute has always been an evolving entity,” says Ashtekar, IGC’s founding director. “We change depending on the intellectual opportunities that are available in science and people that happen to be around us. So in the early 2000s, we extended the institute to include particle astrophysics, whereas before it was primarily focused on gravitational physics and geometry. Our Center for Gravitational Wave Physics had really served to create the field of gravitational science in this country, because it brought together people who had never talked before—diverse communities of astronomers, general relativity people, computational scientists, data analysis people, and astrophysicists. And so with the addition of particle astrophysics, IGC really became a kind of a crucible in which something new, intellectually, was born.”
Forging ahead into this new realm, Penn State scientists at IGC produced seminal methods and theories that would be foundational to the LIGO collaboration’s discovery of gravitational waves—which garnered the 2017 Nobel Prize in Physics—as well as to the entire field of multi-messenger astronomy. Over several decades, B.S. Sathyaprakash developed methods to extract and analyze gravitational wave signals from the raw data and to determine source parameters. And Peter Mészáros, with his colleague Martin Rees, had predicted that gravitational waves and gamma ray bursts would result from neutron star mergers—theories which would be confirmed in subsequent gravitational wave detection by the LIGO collaboration in 2017.
A Penn State team led by Chad Hanna was instrumental in developing the LIGO data analysis that detected the binary neutron star merger in August 2017; and, in fact, Hanna’s graduate student Cody Messick was the person doing that analysis when the gravitational wave signal came in. On the morning of August 17, LIGO was experiencing a variety of unexpected technical problems: Data from LIGO’s Italian partner observatory, Virgo, had suddenly stopped being transferred; and at the time of the gravitational wave detection, a sudden burst of noise swamped the signal at the LIGO detector in Livingston, Louisiana, which meant that there was only one clear signal—at the detector in Hanford, Washington. But that signal was so incredibly loud that it still rose well above the collaboration’s established significance level for candidate events; LIGO’s automated analysis identified it in real time, uploaded it to a database of gravitational wave candidates, and an alert was sent to a small group within the collaboration, including Messick.
“I got a text on my phone,” Messick says, “and I saw there was a single-detector event uploaded to our database. At first I thought it was probably nothing. Single-detector events, at that point, had always been noise, but then I started to really look at it. We measure significance with a ‘false-alarm rate,’ where the lower the false-alarm rate, the more significant the event. And this thing had a really, really low false-alarm rate. I actually misread it the first time—the real number was even lower—but even misreading it, I thought ‘Wow!’ That was the lowest I’d ever seen for a single-detector event.”
Prior observations of LIGO-detected events had been limited to the gravitational wave signals picked up by the advanced LIGO detectors in Hanford, Washington, and Livingston, Louisiana. This time, there was a coincident gamma ray burst detected by NASA’s Fermi space telescope within seconds of the gravitational wave created by the neutron star merger, and the satellite sent an alert to LIGO’s automated system.
“Under normal circumstances, the gamma ray burst would have basically been entirely ignored,” Hanna says, “because they’re reasonably common, and this one was kind of uneventful. However, our real-time processing software had noted that the timing with the binary neutron star merger was essentially perfect: The gamma ray burst happened within about two seconds of the merger that we detected from the gravitational waves, and that was basically a ‘smoking gun’ association.”
A FLARING SUPERMASSIVE BLACK HOLE 3.7 billion light-years from Earth in the constellation Orion is the likely source of a super-high-energy subatomic particle, a neutrino, that has launched a new era of space research. Credit: Nate Follmer, Penn State
Messick quickly sent an alert to the rest of the LIGO collaboration, which then notified a worldwide network of observatories, and within hours dozens Storyof ground-based observatories as well as NASA’s Neil Gehrels Swift Observatory, Chandra X-Ray Observatory, and Spitzer and Hubble Space Telescopes were all gathering follow-up data from across the electromagnetic spectrum. The end result was the first multi-messenger source discovery in nearly 30 years, only the second in history—and Penn State scientists were the ones to raise the flag.
“Not exaggerating,” Hanna says, “I was more or less incapacitated. It was entirely surreal—something that I, personally, had been looking forward to for nearly ten years.”
It was a phenomenal discovery, and one of the very sort that Penn State’s IGC scientists had foreseen the importance of the multi-messenger approach to when they announced several years earlier, in a March 2013 paper published in the journal Astroparticle Physics, the development of the Astrophysical Multimessenger Observatory Network (AMON) at University Park. AMON, they explained, would connect observatories around the world via a high-performance computer network—enhancing their combined sensitivity and enabling near real-time follow-up observations of newly identified candidates as well as archival data analysis to produce new candidates for observation.
“It’s very hard for any one observatory to make out anything from a single event,” says AMON’s principal investigator, Miguel Mostafá. “But if you put that together with an observation of another messenger at the same time and you get a correlation—in the same time window and the same spatial window in the sky—now it is interesting, and we can claim a discovery where the individual observatories can’t by themselves. That’s where the idea of AMON makes a lot of sense. By putting together the data, combining the different messengers, we can enhance the scientific output of all these observatories. And we were the pioneers of that.”
THE UNUSUALLY HIGH ENERGY OF THE DETECTED NEUTRINO (see previous illustration, pg. 10) triggered immediate alerts by IceCube to astronomical observatories through Penn State’s Astrophysical Multimessenger Observatory Network (AMON), enabling the Neil Gehrels Swift Observatory and many others worldwide to observe the neutrino’s likely birthplace. Credit: Nate Follmer, Penn State
Around the same time AMON was launched at Penn State, one of its signatory participants—the IceCube Neutrino Observatory in Antarctica—had achieved the first-ever detection of high-energy cosmic neutrinos. Several years later, in April 2016, IceCube distributed its first neutrino-detection alert via AMON; and over the next 16 months, 11 IceCube-AMON alerts would be sent to observatories worldwide for follow-up observations, but none of those observations revealed a specific astrophysical source for the neutrinos.
Then, in September 2017—a little more than a month after the LIGO discovery—IceCube scientists detected a high-energy neutrino whose trajectory they were able to trace back to a small patch of sky in the constellation Orion. Within seconds, an alert was sent via AMON that triggered rapid follow-up observations by NASA’s Swift Observatory and subsequent studies by NASA’s Fermi Gamma-Ray Space Telescope and 13 other observatories around the world. The result was the discovery of a flaring supermassive black hole, or “blazar,” 3.7 billion light years from Earth—the first identified deep-space source of high-energy neutrinos, the third multi-messenger source ever discovered, and the second such breakthrough in the span of a year to hinge on the work of Penn State scientists.
“Without that follow-up, the single neutrino that IceCube had seen would have been relatively unremarkable,” says IceCube founding member and AMON co-PI Doug Cowen. “It had a high probability of being astrophysical—and therefore it would have still been interesting to us—but we’d already seen a number of these neutrinos before, and, in short, we didn’t have any evidence for astrophysical sources until this follow-up, which saw something else coming from the same place, at the same time.”
Swift’s unique ability to rapidly reorient itself to new targets has made it a premier follow-up instrument in multi-messenger observations worldwide, and the satellite was a critical component of both IceCube’s and LIGO’s discoveries in 2017. Responding to LIGO’s August 2017 gravitational wave alert, Swift was retargeted and searching for electromagnetic counterparts in roughly 16 minutes. Following up on the September 2017 IceCube-AMON alert, Swift was in fact the first facility to identify the blazar as a potential counterpart to the neutrino event.
“The identification of an electromagnetic counterpart for a non-electromagnetic discovery is absolutely critical, and we are the world’s fastest-response instrument,” says John Nousek, director of Swift’s Mission Operations Center. “In terms of getting a fast identification, we are the people to use.”
“It’s exciting that we got both of those results—with LIGO and with IceCube—in the same year,” says Swift Science Operations Team lead Jamie Kennea. “2017 was a big year for multi-messenger astronomy, and Swift was on the front lines of both of those discoveries.”
AN ARTIST’S RENDERING of the Neil Gehrels Swift Observatory. Credit: NASA E/PO, Sonoma State University/A. Simonnet
Since its launch in 2004, Swift has held a special place at Penn State: Its Mission Operations Center and Science Operations Team are both based at University Park and led by Nousek and Kennea, respectively; and two of the satellite’s three instruments—the Ultraviolet/Optical Telescope (UVOT) and X-Ray Telescope (XRT)—are led by Penn State teams, headed by Mike Siegel and David Burrows, respectively. Another Penn State scientist—Gordon Garmire, the co-discoverer of high-energy gamma rays—led the team that conceived and designed, for NASA’s Chandra X-Ray Observatory, one of its primary instruments: the ACIS X-ray camera. Like Swift, Chandra also is a key follow-up facility in global multi-messenger efforts, as are a number of ground-based observatories in whose research Penn State’s IGC scientists figure prominently.
Stephane Coutu and Miguel Mostafá lead Penn State research teams at the Pierre Auger Cosmic Ray Observatory—one of the first AMON signatory members—which in 2017 discovered ultra-high-energy cosmic rays originating far outside our own Milky Way Galaxy; and Mostafá also leads a team of Penn State scientists at the High-Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory, another early AMON signatory member. Penn State’s Abe Falcone is one of the investigators utilizing yet another AMON signatory observatory: VERITAS (the Very Energetic Radiation Imaging Telescope Array System), a ground-based gamma-ray instrument operating at the Fred Lawrence Whipple Observatory in southern Arizona. Niel Brandt is an investigator with the XMM-Newton X-Ray Observatory, which he and an international team of scientists used to discover the most-distant X-rays ever observed, from the farthest known quasar in the universe. And Penn State Abington physicists Carl and Ann Schmiedekamp are among the investigators with the North American Nanohertz Observatory for Gravitational Waves (NANOGrav)—a complementary effort to LIGO that aims to detect long-period gravitational waves by timing the travel of light pulses from rotating neutron stars. There is even a major theoretical effort currently underway at Penn State, led by Kohta Murase, who has developed a first-of-itskind model explaining the common origin of very high-energy neutrinos, ultrahigh-energy cosmic rays, and high-energy gamma rays—one of the biggest mysteries in astroparticle physics.
As increasingly more scientists, collaborations, and observatories adopt the multi-messenger approach, the next wave of astronomical discovery promises to be even bigger than the last— potentially by an order of magnitude or more— and the volume of data from these combined observations is nearly unfathomable, not to mention the challenge of processing and analyzing those data in real time. AMON, however, is up to that challenge—with its funding recently renewed in full by the National Science Foundation, and buttressed by the world-class cyber-infrastructure of Penn State’s Institute for CyberScience (ICS).
“In the ICS Advanced CyberInfrastructure, we have an impressive amount of computing power,” says ICS assistant director Wayne Figurelle. “We have more than 26,000 cores running in aggregate and around 20 petabytes—or 20 million gigabytes—of storage, and our systems operate on an InfiniBand network, which allows us to rapidly process and transfer massive amounts of data. AMON is unique in having very high availability and a high level of redundancy in its infrastructure to ensure that it stays up and running, and it’s able to use all of our cores to do its real-time data analysis.”
Equipped with extraordinary tools and expertise, their merit borne out by monumental discoveries, Penn State’s scientists in IGC and AMON are undoubtedly among the global scientific leaders charting the frontiers of cosmic insight.
“In the past year, we have seen the dawn of a new era of multi-messenger astronomy, which is opening our vision to the universe in fundamentally new ways,” says AMON co-PI Derek Fox. “We are confident that we’ll see more multi-messenger sources and more types of sources in the years coming up shortly. And, while it’s hard to make promises, I highly anticipate that there will also be entirely new phenomena that we identify in taking this more expansive view of the skies.”
Penn State’s IGC scientists gratefully acknowledge the Office of the Senior Vice President for Research at Penn State and the Office of the Dean of the Eberly College of Science for funding the initial development of AMON, as well as the National Science Foundation for their current funding of AMON since 2014.
Join us at University Park in June 2019 for an international conference of global leaders in multi-messenger astrophysics—IGC@25: The Multi-Messenger Universe—hosted by the Institute for Gravitation and the Cosmos in celebration of their silver jubilee.
More information is available at gravity.psu.edu/events/igc25/index_igc25.shtml