Robotic Astronomer Finds New Home

When it comes to telescopes, bigger is almost always better. Larger telescopes collect more light, enabling them to see the farthest and faintest objects in the sky. But older and smaller telescopes can still do important science—and, when a less-in-demand, smaller telescope is equipped with an autonomous, robotic, state-of-the-art laser adaptive optics system to erase the atmosphere's blurring effect, the data that can be obtained give us a unique window on the universe around us.

For the last three years, the world's only robotic adaptive optics system, Robo-AO, has been making new discoveries with the 60-inch telescope at Caltech's Palomar Observatory north of San Diego. One of the major projects that Robo-AO undertook was a comprehensive study of stars that were identified by NASA's Kepler space telescope as likely to host planets. Now, the instrument, developed at Caltech, is moving to a new home: the 2.1-meter telescope at Kitt Peak National Observatory in Arizona.

The hallmark of Robo-AO is its efficiency. Whereas other adaptive optics systems must be monitored and set up by several people, Robo-AO is fully robotic, which makes it 10 times more efficient than any other adaptive optics system in the world. Robo-AO is a joint project of the Caltech Optical Observatories (COO) and the Inter-University Center for Astronomy and Astrophysics (ICUAA) in Pune, India, guided by principal investigator Christoph Baranec (BS '01), a former Caltech postdoc (now a professor and Sloan Research Fellow at the University of Hawaii), and co-principal investigator Professor A. N. Ramaprakash of the IUCAA.

Budget cuts forced the National Optical Astronomy Observatory, which operates the Kitt Peak Observatory for the National Science Foundation, to seek new operators for the observatory's 2.1-meter telescope. "We immediately sent a proposal," says Shri Kulkarni, Caltech's John D. and Catherine T. MacArthur Professor of Astronomy and Planetary Science and director of the COO. "The larger size of the telescope and the excellent site made for a perfect match between Robo-AO and the 2.1-m telescope." The proposal won. Funding for three years of operation was provided through a $1 million dollar donation from Rohan Murty—an entrepreneur based in Boston and Bangalore—and his family. "Automating the discovery of the universe impacts one of mankind's oldest activities: to look up at the heavens and wonder about our place in the universe," says Murty. "This activity has been the source of inspiration for religion, poetry, literature, science, and culture in general. Hence, I am pleased to support a dedicated telescopic facility for Robo-AO, which will serve as a new frontier for what computer science can do for astronomy and for mankind, in general."

"What Robo-AO is best in the world at doing is large surveys at high angular resolution," says Reed Riddle, the project scientist for Robo-AO's Kitt Peak operation. This makes it ideal for follow-up observations of the candidate planetary systems that Kepler continues to find. Kepler detects exoplanets when they pass in front of, or transit, their stars, causing a slight dip in starlight. This allows astronomers to both discover and measure the sizes of thousands of exoplanets. "However, if another star is close to the target star, it dilutes the depth of the transit dip, which means we measure incorrect sizes for the planet candidates," says Nicholas Law, a former Caltech postdoc who is now a professor at the University of North Carolina, Chapel Hill, and a Robo-AO team member. Astronomers must study these planets in more detail to confirm that they are real and not a spurious reading caused by nearby stars. To date, Robo-AO has evaluated about 90 percent of Kepler's targets—some 3,300 potential planetary systems. In comparison, astronomers using all other adaptive optics systems in the world put together have investigated far fewer of the Kepler planet candidates.

But Robo-AO can do much more than evaluate exoplanets, Kulkarni notes. "As a dedicated facility, Robo-AO on the 2.1-m telescope can undertake other long-term studies, such as monitoring weather on Neptune, and large surveys, such as tracking and resolving asteroids. Furthermore, it is an excellent opportunity for young people to have hands-on experience with a state-of-the-art facility," he says. Indeed, Caltech graduate student Rebecca Jensen-Clem will be using Robo-AO for her thesis project to study weather on brown dwarfs, and postdoctoral fellow Dmitry Duev, though trained in radio astronomy, plans to use the facility for an ambitious study of binarity in asteroids—whether an asteroid is part of a single or double system (observing a binary orbit allows for a measurement of the masses of both members of the pair).

Over the past two decades, adaptive optics has revolutionized astronomy, giving ground-based telescopes the ability to obtain pristine views that previously had been possible only from space. Adaptive systems, now found on all large telescopes, such as the Keck Observatory in Hawaii, send a laser beam into the sky, where the light scatters off air molecules and returns to the telescope, traveling through the same atmospheric turbulence that blurs the telescope's images. Once a detector measures this blur, a deformable mirror can then warp the light in such a way that the distortion is precisely canceled out.

"Don't get me wrong, the adaptive optics on big telescopes offer us the best angular resolution," says Baranec, who also serves as the Robo-AO instrument scientist.  Robo-AO, with its high on-sky efficiency, excels in its ability to observe a large sample rapidly. Using its data, astronomers are working to determine how the prevalence of giant gas planets close to their stars depends on the presence of a companion star. Robo-AO also has discovered one of only two known quadruple star systems containing planets. Twenty science papers have been published since the system was commissioned three years ago, and several more are in progress.

In this same vein, Robo-AO has surveyed 3,000 of the nearest stars searching for multiple star systems (our solar system has only one star but many others contain stars two, three, even more stellar companions). Robo-AO will continue this program, investigating tens of thousands of stars in a survey Kulkarni describes as undertaking "galactic geography."

And there will be more extrasolar planets discovered. The Kepler mission is winding down, but its successor, the Transiting Exoplanet Survey Satellite, scheduled for launch in 2017, is expected to find tens of thousands of new worlds—which Robo-AO will be uniquely suited to study. "The Robo-AO team is making one-sixth of its observing time available to the U.S. astronomical community. In addition we have had discussions with several colleagues at astronomy departments in the U.S., India, and Europe with the view of expanding the range of astronomical research that can be undertaken by Robo-AO," Kulkarni says.

So while it may not be the biggest system in use, Kulkarni says, Robo-AO does its job very well. "Not everything has to be big telescopes or big space missions," he says. "It's a little celebration of less-is-more."

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The world's only robotic adaptive optics system, Robo-AO, is moving from Palomar Observatory to Kitt Peak National Observatory.
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Patrick Rall: A Passion for Entanglement

SURF participant Patrick Rall and a summer of quantum information science.

For senior Patrick Rall, a native of Munich, Germany, the summer offers one of the year's few chances to visit home. But for the last two summers, Rall, a Caltech physics major, has been spending his summers on campus, drawn by another opportunity—the chance to conduct cutting-edge research while being mentored by John Preskill, the Richard P. Feynman Professor of Theoretical Physics, as part of the Institute's Summer Undergraduate Research Fellowships (SURF) program. Last year, Rall worked in the laser lab of Assistant Professor of Physics David Hsieh on a condensed matter physics experiment. This summer, he switched his attention to quantum information science, a new field that seeks to exploit quantum mechanical effects to create next-generation computers that will be faster and more secure than those currently available.

A key idea in quantum mechanics is superposition of states. Subatomic particles like electrons can be described as having multiple positions, or more than one speed or energy level. This is illustrated by the thought experiment developed in 1935 by Austrian physicist Edwin Schrödinger. In it, a cat is placed into an imaginary box containing a bottle of poison, radioactive material, and a radiation detector. If a radioactive particle decays and radiation is detected inside the box, the poison is released and the cat is killed. But according to quantum mechanics, the cat could be simultaneously alive and dead. Yet if one were to open the lid of the box, the cat would become alive or dead. By opening the box, we have destroyed the quantum nature of the state; that is to say, the observation itself affects the outcome, and yet that outcome is randomly determined.

"Where this gets really interesting is when more than one cat gets involved," Rall says. "Then we can have states where looking at one cat determines the outcome of looking at the other, even if they are on different continents or even different planets. For example, I cannot know if I will see a live or a dead cat upon opening either box, but I can know that the cats are either both alive or both dead."

This "spooky action at a distance"—as Einstein phrased it—is called entanglement, and an entangled state, physicists say, can store information. "When looking at systems with many cats, the amount of entanglement information is much larger than what I can obtain by looking at the cats individually," Rall says. "To harness the sheer quantity of information stored in these so-called many-body systems, we must better understand the structure of these spooky correlations. This is what I worked on this summer."

Quantum many-body systems are difficult to simulate on a computer, but by looking at small-enough systems and using mathematical tools, researchers can study complex entangled quantum states. Physicists have been studying many-body entanglement for a long time because of its importance in understanding certain semiconductors.

"This summer, I had the privilege to work under Professor Preskill, and that was an incredible experience," Rall says. A central interest of Preskill's lab is to design schemes for quantum computation. Modern computers use classical bits—ones and zeroes—to store data. A quantum computer would use quantum bits—or qubits—and use their superposition and entanglement to perform computation. Quantum computers, while still in the experimental stage (with heavy investment from companies like IBM, Microsoft, and Google), have been touted for their potential to generate unbreakable codes and to efficiently simulate many complex systems, with implications for computational chemistry and biology.

"The most interesting thing about the quantum computer is that we have no idea what it could be capable of," says Rall. "We know some quantum algorithms that are faster than the best-known classical algorithms. But what are the limits? Nobody knows."

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Alumnus Arthur McDonald Wins 2015 Nobel Prize in Physics

Arthur B. McDonald (PhD '70), director of the Sudbury Neutrino Observatory (SNO) in Ontario, Canada, and Takaaki Kajita, at the University of Tokyo, Kashiwa, Japan, have shared the 2015 Nobel Prize in Physics for the discovery that neutrinos can change their identities as they travel through space.

McDonald and Kajita lead two large research teams whose work has upended the standard model of particle physics and settled a debate that has raged since 1930, when the neutrino's existence was first proposed by physicist Wolfgang Pauli. Pauli initially devised the neutrino as a bookkeeping device—one to carry away surplus energy from nuclear reactions in stars and from radioactive decay processes on Earth. In order to make the math work, he gave it no charge, almost no mass, and only the weakest of interactions with ordinary matter. Billions of them are coursing through our bodies every second, and we are entirely unaware of them.

There are three types of neutrinos—electron, muon, and tau—and they were, for many years, assumed to be massless and immutable. The technology to detect electron neutrinos emerged in the 1950s, and it slowly became apparent that as few as one-third of the neutrinos the theorists said the sun should be emitting were actually being observed. Various theories were proposed to explain the deficit, including the possibility that the detectable electron neutrinos were somehow transmuting into their undetectable kin en route to Earth.

Solving the mystery of the missing neutrinos would require extremely large detectors in order to catch enough of the elusive particles to get accurate statistics. Such sensitive detectors also require enormous amounts of shielding to avoid false readings.

The University of Tokyo's Super-Kamiokande neutrino detector, which came online in 1996, was built 1,000 meters underground in a zinc mine. Its detector, which counts muon neutrinos and records their direction of travel, found fewer cosmic-ray neutrinos coming up through the Earth than from any other direction. Since they should not be affected in any way by traveling through the 12,742-kilometer diameter of our planet, Kajita and his colleagues concluded that the extra distance had given them a little extra time to change their identities.

McDonald's SNO, built 2,100 meters deep in a nickel mine, began taking data in 1999. It has two counting systems. One is exclusively sensitive to electron neutrinos, which are the type emitted by the sun; the other records all neutrinos but does not identify their types. The SNO also recorded only about one-third of the predicted number of solar electron-type neutrinos—but the aggregate of all three types measured by the other counting systems matched the theory.

The conclusion, for which McDonald and Kajita were awarded the Nobel Prize, was that neutrinos must have a nonzero mass. Quantum mechanics treats particles as waves, and the potentially differing masses associated with muons and taus gives them different wavelengths. The probability waves of the three particle types are aligned when the particle is formed, but as they propagate they get out of synch. Therefore, there is a one-third chance of seeing any particular neutrino in its electron form. Because these particles have this nonzero mass, their gravitational effects on the large-scale behavior of the universe must be taken into account—a profound implication for cosmology.

McDonald came to Caltech in 1965 to pursue a PhD in physics in the Kellogg Radiation Laboratory under the mentorship of the late Charles A. Barnes, professor of physics, emeritus, who passed away in August 2015. "Charlie Barnes was a great mentor who was very proud of his students," says Bradley W. Filippone, professor of physics and a postdoctoral researcher under Barnes. "It is a shame that Charlie didn't get to see Art receive this tremendous honor."

A native of Sydney, Canada, McDonald received his bachelor of science and master's degrees, both in physics, from Dalhousie University in Halifax, Nova Scotia, in 1964 and 1965, respectively. After receiving his doctorate, he worked for the Chalk River Laboratories in Ontario until 1982, when he became a professor of physics at Princeton University. He left Princeton in 1989 and became a professor at Queen's University in Kingston, Canada; the same year, he became the director of the SNO. In 2006, he became the holder of the Gordon and Patricia Gray Chair in Particle Astrophysics, a position he held until his retirement in 2013.

Among many other awards and honors, McDonald is a fellow of the American Physical Society, the Royal Society of Canada, and of Great Britain's Royal Society. He is the recipient of the Killam Prize in the Natural Sciences; the Henry Marshall Tory Medal from the Royal Society of Canada, its highest award for scientific achievement; and the European Physics Society HEP Division Giuseppe and Vanna Cocconi Prize for Particle Astrophysics.

To date, 34 Caltech alumni and faculty have won a total of 35 Nobel Prizes. Last year, alumnus Eric Betzig (BS '83) received the Nobel Prize in Chemistry.

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LIGO's SURF Students Look for the Perfect Wave

As the Advanced LIGO Project geared up last summer, 27 undergraduates from around the world became full partners in one of the biggest, most complex physics experiment ever. Their contributions ranged from creating hardware and software for current use to helping design next-generation detectors.

LIGO, the Laser Interferometer Gravitational-Wave Observatory, is designed to detect the ripples in the fabric of space and time produced by such violent events as supernova explosions or the mergers of pairs of black holes trapped in a death spiral. Such waves were proposed by Einstein as a consequence of general relativity. His theory turns 100 this November, and all of its other predictions have been confirmed; LIGO, a joint project of Caltech and MIT, entered the search with its first science run in 2002.

Gravitational waves are so subtle that the hunt requires ingenuity on a grand scale—each of LIGO's twin observatories, located in Hanford, Washington, and Livingston, Louisiana, consists of a four-kilometer-long, L-shaped interferometer containing hanging mirrors designed to bob on such a wave as it passes through. Lasers measure the mirrors' motions down to one-thousandth the diameter of a proton. Advanced LIGO, a package of upgrades that became operational on September 18, 2015, is increasing that sensitivity by a factor of 10.

Thanks to Caltech's Summer Undergraduate Research Fellowships (SURF) program, students have been part of LIGO since the 1990s—more than 350 of them. Some have gone on to careers in LIGO, and some are now mentoring students themselves. Anamaria Effler SURFed with LIGO in 2004 and 2005 before graduating from Caltech with a bachelor's degree in physics in 2006. But, she says, "I wasn't sure I wanted to go to grad school, so I worked as an operator at LIGO Hanford for three years." The experience moved her to enroll at Louisiana State, "the closest school to a LIGO site." Effler is now a Caltech postdoc at LIGO Livingston, and last summer she mentored an undergraduate from the University of Michigan–Ann Arbor in a project to track down noise sources generated within the interferometer itself.

Noise comes from everywhere, at all frequencies—from stray high-energy photons jostling the mirrors to low-pitched rumblings from within the earth itself. LIGO Hanford feels waves lapping at the beach, for example, even though the Pacific Ocean is more than 250 miles to the west. Elaborate electromechanical systems and sophisticated mathematical filters compensate for such things, but battling noise is like peeling an onion. Removing a layer reveals a fresh one.

Advanced LIGO will have to conquer "Newtonian noise," a slow vibration caused by fluctuations in the earth's density. Explains Professor of Physics Alan Weinstein, head of Caltech's astrophysical analysis group for LIGO, "When the ground shakes, it shakes the suspensions, which shake the mirrors. A perfect suspension keeps that shaking from reaching the mirrors, but that shaking also changes the local gravitational field. You can screen out mechanical motion, but you can't screen out gravity." He expects LIGO will encounter Newtonian noise at frequencies below 10 Hz, or cycles per second, where it will mask the gravitational waves radiated by doomed black-hole pairs up until their last few seconds of life. Weinstein's colleague, Professor of Physics Rana Adhikari, is trying to develop what are essentially high-performance noise-cancelling headphones—feed-forward systems that use inputs from seismometers and geophones to compute the noise's ever-shifting waveforms and manipulate the mirror suspensions to negate the noise's effects in real time. Student contributions included working on a prototype seismometer and testing noise-filtering software.

Other students used Einstein's equations to simulate the waves that such a merger would generate. Says postdoctoral researcher Tjonnie Li, who mentored four students, "General relativity allows us to predict the waves' shapes quite precisely." But many physicists believe that, just as general relativity goes beyond Newton's laws, so will another theory go beyond relativity. "LIGO will be a good test, because every detail of the signal we see should match the predictions," Li adds. "One of my students was modeling how long it should take a pair of black holes to merge under various scenarios. If they merge faster than expected, it would mean that some energy is escaping in some way that general relativity did not predict. This might not point you to the right explanation, but it does tell you that there's something missing." Furthermore, Li says, by comparing their simulations to LIGO's actual data, researchers "can infer something about how the fundamental forces interact with one another. They're all equally strong when matter gets that dense."

Just as Advanced LIGO marks the next generation of detectors, LIGO's undergraduates embody the next generation of researchers. In the 2014–15 academic year, two of LIGO's three incoming graduate students had SURFed here while earning their bachelor's degrees elsewhere. Says Li, "It takes a lot of effort from all of the mentors, and in particular Alan for organizing all of it, but the benefits are immediately visible. If their interest has been piqued, they often do their PhDs in this field. They either come here, or go on to other institutions that do LIGO."

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The Topolariton, a New Half-Matter, Half-Light Particle

Caltech scientist theorizes a new quasiparticle with unique characteristics

A new type of "quasiparticle" theorized by Caltech's Gil Refael, a professor of theoretical physics and condensed matter theory, could help improve the efficiency of a wide range of photonic devices—technologies, such as optical amplifiers, solar photovoltaic cells, and even barcode scanners, which create, manipulate, or detect light.

Electrons traveling through the semiconductors used in modern computers lose energy via heat because of resistance. This is not the case with light signals, but there can be other causes of signal loss in light transmission, such as unwanted reflection and scattering of photons, or light particles. Refael says that a type of quasiparticle called the "topolariton" could reduce such signal degradation and enhance the stability of the photons as they move along the edges of semiconductors. He described topolaritons and their properties in a paper published in the July 2015 issue of the journal Physical Review X.

Refael's work at Caltech concentrates on quantum aspects of matter, including quantum entanglement (in which quantum particles share behaviors regardless of distance), quantum computing, and the emergence and control of new quantum states.

Quasiparticles such as the topolariton are entities that exhibit some, but not all, of the characteristics of elementary particles like the electrons and quarks that make up atoms. They are classified as emergent phenomena—those that arise from the dynamical behavior of a system—and they exist only inside solid materials. Some examples of quasiparticles include phonons ("packets" of atoms or molecules vibrating collectively), solitons (solitary wave packets or pulses that retain their shape while moving at a constant velocity), and excitons (formed when an electron binds with an electron hole, or the void left when an electron departs a valence orbital). The interaction of photons with excitons can form another quasiparticle called the polariton.

During a workshop at the National Science Foundation (NSF)-sponsored Institute for Quantum Information (now the Institute for Quantum Information and Matter) in 2010, Refael and two colleagues came up with the idea for the topolariton, a type of polariton with the ability to flow in one direction along the edges of semiconducting quantum wells embedded in optical cavities. The quantum well and the cavity confine electrons and photons to motion within a single plane. "We suggested that you could take a simple semiconductor and a regular quantum well, and give rise to a special excitation, which is a hybrid of a photon and an electron-hole pair," that is, of a polariton. "Shining light at the frequency can kick an electron out of the balance and initiate a polariton that travels exclusively on the edge of the system. The light–matter interaction, in this case, produces so-called topological quantum states that are not there in each of the components."

Because topolaritons are part matter and part light, they could be guided and controlled with reflectors or with photonic band-gaps—regions in the optical medium through which photons cannot travel. Furthermore, the direction of a topolariton's motion could be reversed by the application of a magnetic field. "This would be like a one-way filter for light, providing a directional communication with minimum losses of energy," he says.

Photonic devices are currently in widespread use, and are expected to eventually replace traditional semiconductors in many applications. They are more efficient and accurate, work better over long distances, and save energy. They are also less prone to interference from outside influences such as electromagnetic fields. Exploiting toploaritons into such devices should lead to further improvements in their performance.

Evolving from theory to practical applications, however, may be a rather long process, Refael admits. "We'll need to create some new interfaces between the photonic world and the electronic world," he says. "One challenge is making one-way photon wave-guides for visible light. The topolaritons provide a route to such devices using standard semi-conductor technology, and can also act as an intermediary between photonic and electron-based devices—a necessary step for any optoelectronic device."

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Summer Interns Return with a World of Experiences

Caltech undergraduate students returned to campus this week, many after spending the summer working at companies in biotechnology, technology, and finance, among other fields. These students have had the opportunity to learn firsthand about the career opportunities and paths that may be available to them after graduation. They also had the chance to put Caltech's rigorous academic and problem-solving training to the test.

In the summer of 2015, nearly a third of returning sophomores, juniors, and seniors were placed in an internship position through Caltech's Summer Undergraduate Internship Program (SUIP). The program, run through the Institute's Career Development Center (CDC), helps connect current undergraduate students with a wide range of companies and businesses that can provide practical skills and work experiences that give the students an edge in the future job market.

Many undergraduates find paid summer internships through the CDC, says Lauren Stolper, the director of fellowships, advising, study abroad, and the CDC. The center organizes fall and winter career fairs and offers workshops related to finding internships; provides individual advising on internship options and conducting a job hunt for an internship; organizes interviews for students through its on-campus recruiting program; and provides web-based internship listings and company information through Techerlink, its online job-posting system.

Through the formal establishment of SUIP two years ago—thanks, in part, to the initiative of Craig SanPietro (BS '68, engineering; MS '69, mechanical engineering) and with seed money provided by him and three of his alumni friends and former Dabney House roommates, Peter Cross (BS '68, engineering), Eric Garen (BS '68, engineering), and Charles Zeller (BS '68, engineering)—the CDC has been able to dedicate even more time and attention to helping undergraduates secure these important positions, Stolper says.

"Through internships, students have the opportunity to learn more about the practical applications of their knowledge by contributing to ongoing projects under the guidance of professionals," says Aneesha Akram, a career counselor for internship development/advising, who oversees SUIP.

"Completing summer internships help undergraduates become competitive candidates for full-time positions," says Akram. "When it comes to recruiting for full-time positions, companies seek out candidates with previous internship experience. We have found that many large companies extend return offers and full-time conversions to students who previously interned with them."

The infographic at the above right provides a snapshot of Caltech undergraduate internships over this past summer. Students seeking internships for next summer can contact Akram or look at the CDC website for more information.

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NSF Supports Caltech-Led Global Project to Study Cosmic Flashes

GROWTH network aims to keep astronomers and telescopes unbeaten by sunrise

An international project led by Caltech astrophysicist Mansi M. Kasliwal has been selected to receive $4.5 million over five years by the National Science Foundation through its Partnership for International Research and Education (PIRE) program. The project aims to improve our understanding of cosmic transients—extremely bright flashes of light that suddenly appear in the night sky, shining like new stars, a million to a billion times brighter than the sun, and then quickly fade away.

The project, dubbed Global Relay of Observatories Watching Transients Happen (GROWTH), is a collaboration among six universities in the United States and six abroad. It formally establishes a network of telescopes at longitudes throughout North America, Asia, and Europe (see illustration at right) to extend the hours of night-time observing, enabling researchers to continue monitoring cosmic transients that might otherwise disappear before the next night in a single location.

"There are many questions in astrophysics that depend on these hour timescales, where all the action happens in that first night," says Kasliwal, a new assistant professor of astronomy at Caltech. "So we aim to stay unbeaten by sunrise."

This is the fifth round of PIRE funding since the program started in 2005 with the goal of supporting innovative, high-quality projects in which advances in research and education could not occur without international collaboration.

"In astronomy, where international collaboration is the norm, the PIRE award provides the resources for students and postdocs to interact closely with international partners through extended visits and internships. Such experiences will be invaluable in their future scientific careers," says Tom Prince, an investigator on the GROWTH project, professor of physics and director of the W.M. Keck Institute for Space Studies at Caltech, and senior research scientist at JPL.

Many GROWTH observations will begin with a transient candidate discovered in the data stream generated every night by the Palomar Transient Factory (PTF), a fully automated, wide-field survey systematically searching for such flashes using a camera mounted on the 48-inch Samuel Oschin Telescope at Palomar Observatory.

"GROWTH is extremely timely. By 2017, we will have commissioned the Zwicky Transient Facility, or ZTF, at Palomar, which will be an order of magnitude more sensitive than PTF. GROWTH follow-up of these transients should result in spectacular science," says Shri Kulkarni, an investigator on the GROWTH project, principal investigator on PTF and ZTF, the John D. and Catherine T. MacArthur Professor of Astronomy and Planetary Science, and director of the Caltech Optical Observatories.

If a transient candidate looks promising, Kasliwal might trigger larger optical telescopes like the Gemini North telescope or the W. M. Keck Observatory, both in Hawaii, gaining three additional hours of darkness. After collecting data there, the team might contact members of the network in Japan or Taiwan, then India, Israel, Sweden, and Germany. "We just go around the globe and keep passing the baton so that the sky remains dark," says Kasliwal.

While supernovae—explosions related to the collapse of massive stars—typically fade over months, some cosmic transients, especially rarer events such as the merger of two extremely dense stellar remnants called neutron stars, or the merger of a neutron star and a black hole, are believed to disappear in a matter of hours—a day at most. Some of these more exotic transients are thought to be the source of heavy elements, such as gold and platinum. But no one has seen, in action, the process that creates them. "None of the explosions that we've found so far has been extreme enough to actually synthesize enough heavy elements," says Kasliwal. "As you try to find these rarer and rarer events, you need to be able to respond quickly because the flash of light they produce is even more short-lived, and that's where GROWTH becomes necessary."

Even being able to observe supernovae within a few hours of the blast yields important information that is lost if observations are completed later. That is because after the initial burst flash-ionizes everything, a shock wave follows, sweeping up all the surrounding material. And with that material goes a lot of data about the progenitor, the star that exploded.

"If you respond quickly enough to a young supernova, you can get direct clues about the progenitor system," explains Kasliwal. "With our network, the most common events that we will follow up are very young supernovae that are within a few hours of explosion. For the first time, we're seeing certain ionization lines, which tell us what sort of star it was that just died and gave rise to this particular supernova."

Beyond rapid response, the GROWTH network is also able to use ground- and space-based telescopes to observe cosmic transients at X-ray, ultraviolet, infrared, optical, and radio wavelengths. "Within minutes to hours, we will be able to get a panchromatic picture," says Kasliwal.

That will be key for the network's coordination with Advanced LIGO (aLIGO). That project, led by Caltech and MIT, is searching for gravitational waves—ripples in the fabric of space and time that are predicted to accompany violent events in the universe such as the merger of a neutron star with a black hole. PTF will receive notifications of events that aLIGO identifies as possible sources of gravitational waves. But aLIGO can only narrow the source down to a swath of the sky that is hundreds of square degrees (the full moon on the sky covers about half a degree). PTF will image that swath of sky and pinpoint new transients within it, most likely identifying tens of thousands of events. Then, using new software, the researchers will be able to narrow those down to just a few promising candidates, which will then be fully investigated by the GROWTH network.

"Then we will have data to actually inform us that this one is just a blip, this one is a supernova that has nothing to do with an Advanced LIGO event, and this one is the real counterpart," says Kasliwal. "You need the full picture to be able to get to the heart of the physics."

In addition to its work with transients, the GROWTH network will also help detect and characterize small near-Earth asteroids—those with a diameter smaller than about 140 meters, which can still do significant damage.

"I am very excited about GROWTH's capability for following up detections of small near-Earth asteroids," says Prince. "Determination of the orbits of such objects is critical, and the GROWTH network of telescopes will allow us to make such determinations much more effectively."

Additional participants in the consortium include Lin Yan, a staff scientist at Caltech; Bryan Penprase of Pomona College; Robert Quimby of San Diego State University; Przemek Wozniak of Los Alamos National Laboratory; Stuart Vogel of University of Maryland College Park; David Kaplan of University of Wisconsin–Milwaukee; Nobuyuki Kawai of the Tokyo Institute of Technology; Chow-Choong Ngeow of National Central University in Taiwan; G. C. Anupama of Indian Institute of Astrophysics in Bengalore, India; Varun Bhalerao of The Inter-University Centre for Astronomy and Astrophysics in Pune, India; Eran Ofek of Weizmann Institute of Science in Israel; Ariel Goobar of Stockholm University in Sweden; and Marek Kowalski of Humboldt University in Germany. 

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Kimm Fesenmaier
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Global Project to Study Cosmic Flashes
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