The equivalence principle is confirmed

Galileo’s 400-year-old theory of free-falling objects passes space test

By Edwin Cartlidge |

A key tenet of Albert Einstein’s general theory of relativity has passed yet another test with flying colors—and for the first time in space. A French satellite experiment has shown that objects with different masses fall at exactly the same rate under gravity, just as relativity dictates. The result is the most precise confirmation yet of the equivalence principle, first tested more than 400 years ago by Galileo Galilei. “The mission appears to have performed fantastically,” says Clifford Will, a theoretical physicist at the University of Florida in Gainesville.

Physicists scrutinize the equivalence principle because any violation could point to new forces of nature that might resolve a long-standing impasse between general relativity and quantum theory. The satellite, called MICROSCOPE, found no discrepancy in the acceleration of two small test masses to about one part in 100 trillion (1014).That’s more than 10 times better than the most sensitive ground-based experiments, which look for disparities in the response of weights to Earth’s spin.

The €200 million MICROSCOPE, launched by France’s space agency CNES in April 2016, benefits from avoiding terrestrial vibrations. It relies on a pair of concentric cylindrical shells a few centimeters long. The outer cylinder is made of a titanium and aluminum alloy, whereas the inner one is composed of much denser platinum and rhodium. As the spacecraft orbits Earth, the cylinders are in continuous free fall. Electrodes monitor their position and keep them centered by applying tiny voltages to nudge them electrostatically whenever they stray. As the satellite traces out a 1.5-hour-long orbit, a characteristic rise and fall in the difference between the two applied voltages would indicate that one of the cylinders is falling slightly faster than the other—and signal a violation of the equivalence principle.

After more than 1500 orbits by the satellite, the MICROSCOPE team—with researchers from France, Germany, the Netherlands, and the United Kingdom—found no such signal, they report in an accepted paper at Physical Review Letters. With another 900 research orbits before the mission ends next year, the team may reach its goal of confirming the equivalence principle to one part in a quadrillion (1015).

Will says that so far, the measurements don’t rule out any specific alternatives to relativity that predict a violation of equivalence. Nevertheless, he argues it is important to keep raising sensitivities in case new physics lurks. “Until we get there we don’t know,” he says.

A proposed Italian satellite, aptly named Galileo Galilei, would test equivalence to a precision of one part in 1017, partly by spinning rapidly and isolating any signal from more slowly varying systematic effects. Researchers at Stanford University in Palo Alto, California, have proposed a satellite that aims to reach one part in 1018 using noise-reducing cryogenics. Still other researchers hope to use Bose-Einstein condensates—clouds of cold atoms that behave as a single quantum wave—to reach tight limits.

Anna Nobili, a physicist at the University of Pisa in Italy and Galileo Galilei principal investigator, admits that finding the money for another space mission will not be easy. But the latest result “demonstrates that these tests are easy in space,” she says.


Rocky planet rotation

Rocky planet rotation, thermal tide resonances, and the influence of biological activity

ABSTRACT: It has been established theoretically that atmospheric thermal tides on rocky planets can lead to significant modifications of rotational evolution, both close to synchronous rotation and at faster rotations if certain resonant conditions are met. Here it is demonstrated that the normally considered dissipative gravitational tidal evolution of rocky planet rotation could, in principle, be ‘stalled’ by thermal tide resonances for Earth-analog worlds in the liquid water orbital zone of stars more massive than ~0.3 Msolar. The possibility of feedback effects between a planetary biosphere and the planetary rotational evolution are examined. Building on earlier studies, it is suggested that atmospheric oxygenation, and ozone production could play a key role in planetary rotation evolution, and therefore represents a surprising but potent form of biological imprint on astronomically accessible planetary characteristics.


Subsurface Exolife

Subsurface Exolife

ABSTRACT: We study the prospects for life on planets with subsurface oceans, and find that a wide range of planets can exist in diverse habitats with relatively thin ice envelopes. We quantify the energy sources available to these worlds, the rate of production of prebiotic compounds, and assess their potential for hosting biospheres. Life on these planets is likely to face challenges, which could be overcome through a combination of different mechanisms. We quantify the number of such worlds, and find that they may outnumber rocky planets in the habitable zone of stars by a few orders of magnitude.


Plate Tectonics on Exoplanets

Initiation of Plate Tectonics on Exoplanets with Significant Tidal Stress

ABSTRACT: Plate tectonics is a geophysical process currently unique to Earth, has an important role in regulating the Earth’s climate, and may be better understood by identifying rocky planets outside our solar system with tectonic activity. The key criterion for whether or not plate tectonics may occur on a terrestrial planet is if the stress on a planet’s lithosphere from mantle convection may overcome the lithosphere’s yield stress. Although many rocky exoplanets closely orbiting their host stars have been detected, all studies to date of plate tectonics on exoplanets have neglected tidal stresses in the planet’s lithosphere. Modeling a rocky exoplanet as a constant density, homogeneous, incompressible sphere, we show the tidal stress from the host star acting on close-in planets may become comparable to the stress on the lithosphere from mantle convection. We also show that tidal stresses from planet-planet interactions are unlikely to be significant for plate tectonics, but may be strong enough to trigger Earthquakes. Our work may imply planets orbiting close to their host stars are more likely to experience plate tectonics, with implications for exoplanetary geophysics and habitability. We produce a list of detected rocky exoplanets under the most intense stresses. Atmospheric and topographic observations may confirm our predictions in the near future. Investigations of planets with significant tidal stress can not only lead to observable parameters linked to the presence of active plate tectonics, but may also be used as a tool to test theories on the main driving force behind tectonic activity.


‘Oumuamua & planetesimal formation

Implications of the interstellar object 1I/’Oumuamua for planetary dynamics and planetesimal formation

ABSTRACT: The first bona-fide interstellar planetesimal — the ~100 m-sized 1I/’Oumuamua — was discovered passing through our Solar System on a hyperbolic orbit. This object was likely ejected from a distant star system and provides constraints that on average ~1 Earth mass of planetesimals are ejected per Solar mass of Galactic stars. Using simulations of giant planet dynamics that include rocky and icy planetesimals, we find that this average mass ejection efficiency is consistent with known exoplanet populations if ‘Oumuamua was an icy planetesimal in a population dominated (by mass) by similar small bodies. An asteroidal composition is dynamically disfavoured, and would require both high masses for typical asteroid belts and a low ratio by number of icy to rocky planetesimals. Regardless of the composition, it is unlikely that we would have found an object like ‘Oumuamua if it samples a broad planetesimal mass function that is dominated by massive objects. Further detections may therefore place strong constraints on predictions for the planetesimal mass function from streaming instability-induced collapse, or point to unexpected collisional or dynamical evolution taking place at large radii in planet-forming discs around young stars.


Origin of interstellar small bodies

A general method for assessing the origin of interstellar small bodies: the case of 1I/2017 U1 (Oumuamua)

ABSTRACT: With the advent of more and deeper sky surveys, the discovery of interstellar small objects entering into the Solar System has been finally possible. In October 19, 2017, using observations of the PANSTARRS survey, a fast moving object, now officially named 1I/2017 U1 (Oumuamua), was discovered in a heliocentric unbound trajectory suggesting an interstellar origin. Assessing the provenance of interstellar small objects is key for understanding their distribution, spatial density and the processes responsible for their ejection from planetary system. However, their peculiar trajectories place a limit on the number of observations available to determine a precise orbit. As a result, when its position is propagated ∼ 105-106 years backward in time, small errors in orbital elements become large uncertainties in position in the interstellar space. In this paper we present a general method for assigning probabilities to nearby stars of being the parent system of an observed interstellar object. We describe the method in detail and apply it for assessing the origin of 1I/2017 U1. A preliminary list of potential progenitors and their corresponding probabilities is provided. In the future, when further information about the object and/or the nearby stars be refined, the probabilities computed with our method can be updated. We provide all the data and codes we developed for this purpose in the form of an open source C++/Python package, iWander is publicly available at this http URL

A flyby anomaly for Juno?

A possible flyby anomaly for Juno at Jupiter

ABSTRACT: In the last decades there have been an increasing interest in improving the accuracy of spacecraft navigation and trajectory data. In the course of this plan some anomalies have been found that cannot, in principle, be explained in the context of the most accurate orbital models including all known effects from classical dynamics and general relativity. Of particular interest for its puzzling nature, and the lack of any accepted explanation for the moment, is the flyby anomaly discovered in some spacecraft flybys of the Earth over the course of twenty years. This anomaly manifest itself as the impossibility of matching the pre and post-encounter Doppler tracking and ranging data within a single orbit but, on the contrary, a difference of a few mm/s in the asymptotic velocities is required to perform the fitting.
Nevertheless, no dedicated missions have been carried out to elucidate the origin of this phenomenon with the objective either of revising our understanding of gravity or to improve the accuracy of spacecraft Doppler tracking by revealing a conventional origin.
With the occasion of the Juno mission arrival at Jupiter and the close flybys of this planet, that are currently been performed, we have developed an orbital model suited to the time window close to the perijove. This model shows that an anomalous acceleration of a few mm/s2 is also present in this case. The chance for overlooked conventional or possible unconventional explanations is discussed.

Ready to return to the hell of Venus

Armed with tough computer chips, scientists are ready to return to the hell of Venus

By Paul Voosen |

CLEVELAND, OHIO—In an underdog city, at an underdog NASA lab, researchers are thinking hard about an undeservedly neglected planet. Venus is Earth’s cousin, closest in composition and size, but for decades it has remained veiled. NASA hasn’t sent a mission there since 1989; more recent European and Japanese orbiters have made halting progress that stops largely at the planet’s thick sulfur clouds. No craft has touched down since 1985, when the last of a series of advanced Soviet landers clad in armored pressure vessels endured a couple hours before succumbing to the deep-ocean pressure and furnacelike temperature of the planet’s surface. The baleful conditions and lack of funding have made Venus, Earth’s closest neighbor, feel more distant than ever. That is, except here.

In September, Phil Neudeck, an electrical engineer at NASA’s Glenn Research Center, a complex abutting the main airport in this Rust Belt city, sat watching purple and turquoise waveforms on a display. It was his window into the Venus next door. Behind sealed doors stood a 14-ton stainless steel tank, its massive ports sealed to hold pressures so high that the screws to secure its nuts have their own nuts. For 33 days, the Glenn Extreme Environments Rig (GEER) had run nonstop, simulating an atmosphere at 460°C and flooded with carbon dioxide at pressures that render it supercritical, both liquid and gas. Inside sat two microchips, pulsing with metronomic accuracy. Neudeck was running a clock on Venus, and it was keeping perfect time.

Neudeck and his Glenn colleagues are helping drive a technological leap that could transform the exploration of Venus, making it almost as accessible as Mars. Rather than barricading electronics within pressure vessels, by early next decade NASA may be able to land simple unprotected robots on Venus that can measure wind, temperature, chemistry, pressure, and seismic waves. And instead of running for a few hours, the landers could last for months. “We don’t have the world’s fastest chips,” Neudeck says. “We don’t have the world’s most complex chips. But in terms of Venus environment durability—that’s what we got.”

If the chips live up to their potential, scientists’ elusive dream of extended stays on Venus may at last be within reach. “The paradigm has been that long-term surface stuff is way down the road,” says Tibor Kremic, the scientist who has launched a push toward Venus at Glenn, a little-known NASA lab that has specialized in aviation. But early this decade, engineers here began to build heat-resistant electronics out of a new type of semiconductor, with an eye to placing sensors inside jet engines. Neudeck kept adding transistors to build more complex circuits. Meanwhile, at meetings, Russian researchers told Kremic they were seeking U.S. help in creating a pressure-vessel probe for a possible return-to-Venus mission called Venera-D. Kremic recalled Neudeck’s work and thought, “Maybe there’s another way to do this?”

Glenn’s campaign, if successful, could help revive interest in the planet. Once a primary target of planetary exploration, thanks especially to the brute-force Soviet campaign to land on its surface, Venus has long been overshadowed by missions to Mars, asteroids, and the outer planets and their moons. Early this year, missions to orbit Venus or dive into its atmosphere made up two of the five finalists for NASA’s latest Discovery mission—its line of $500 million planetary probes. The odds seemed good, but neither made the final cut.

Later this year, the agency will announce finalists in the competition for its next billion-dollar New Frontiers mission; among the dozen candidates, three target Venus. But they face stiff competition—including a return to the saturnian moons Enceladus and Titan, which the Cassini mission showed have the potential for harboring life.

If Venus loses out again, Glenn’s innovations could be the best route back—not just to venusian orbit, but all the way to the surface. “The pie is finite,” says Bob Grimm, a geophysicist at the Southwest Research Institute in Boulder, Colorado, and chairman of NASA’s Venus Exploration Analysis Group. “If we want to improve Venus’s share, we have to have some kind of initial mission to get people excited again.” Ralph Harvey, a planetary scientist at Case Western Reserve University here, agrees: “This is the kind of technology development that could take a flagship kind of planetary mission and suddenly allow it to deliver a hell of a lot more.”

The scientific case for Venus is strong. No planet has more to say about how Earth came to be. Mars is tiny and frozen, its heat and atmosphere largely lost to space long ago. “In terms of Earth-sized terrestrial planets, it’s really Venus,” says Colin Wilson, a planetary scientist at the University of Oxford in the United Kingdom. “Venus is all we got.” The planet could host active volcanoes, and it may have once featured oceans and continents, which are critical to the evolution of life. Plate tectonics roughly like Earth’s might have held sway there, or might be starting today, hidden under the clouds. Venus also proves by example that orbiting within a star’s “habitable zone” doesn’t guarantee that a planet is suitable for life. Understanding how Venus’s atmosphere went bad and turned into a runaway greenhouse, boiling away any oceans and baking the surface, could help astronomers studying other solar systems distinguish truly Earth-like exoplanets from our evil twins.

When Neudeck joined Glenn in the early 1990s, Venus was far from his mind. He was chasing an earthly quarry: new semiconductors, materials that can deliver finely controlled doses of electric current. Under extreme heat, silicon, the backbone of modern electronics, becomes a pure conductor. That makes it useless for computing, because stopping and starting the flow of electricity is how zeroes turn into ones.

Neudeck had his eye on silicon carbide, a hybrid of silicon and carbon commonly used as an abrasive in sandpaper and for growing fake diamonds. Silicon carbide has a bigger bandgap than silicon, which means its electrons can absorb much more energy before it becomes a conductor. As a result, it functions as a semiconductor at much higher temperatures. But it is difficult to work with. Because silicon carbide doesn’t melt, the techniques used to produce large silicon wafers break down. When researchers managed to grow wafers by vaporizing the material and depositing it on a seed crystal, the resulting films were riddled with impurities that made them unreliable.

The allure of high-temperature electronics was too great to ignore, however. Slowly, with the support of NASA and the Office of Naval Research, researchers, led especially by Cree, an upstart electronics company, devised ways to grow usable silicon carbide crystals more than 150 millimeters in diameter. The power industry is now harnessing the material to build smaller transformers and more efficient power plants, Neudeck says.

He and his colleagues set out to turn the material into full-fledged computer circuits, assembling more and more complex chips in their clean room. The biggest breakthrough came 4 years ago, when they figured out how to create layered chips that allow electrical signals to crisscross, rapidly increasing potential complexity. “We’re really trying to recreate Moore’s law, but to do it for high temperature,” Neudeck says. In 1 year, they increased the number of transistors on their silicon carbide chips 10-fold.

Pentiums these are not. A modern silicon chip can contain 7 billion transistors; each of the chips running in the Venus chamber has 175. Neudeck also uses an old-school transistor design, long since abandoned in conventional microelectronics. It’s basically a hyperexpensive, obtuse pocket calculator. But a pocket calculator running on Venus could be valuable indeed. “This is already the complexity of many of the early scientific missions flown back in the ’60s and ’70s,” Neudeck says, and more powerful than the chips on Apollo flight computers. “You really can do science.”

Still, Neudeck’s team’s work would have been a sideshow unless he had a way to prove his chips’ endurance. Last year, that’s exactly what the team did.

You could be forgiven for not knowing that Glenn even exists. It has never become a scientific hotbed like the Jet Propulsion Laboratory (JPL) in Pasadena, California, the leader in robotic space exploration. Several times, past NASA administrators or Congress have considered closing it.

The Venus chamber started as another failed dream. NASA planned to test efficient nuclear-fueled engines called Stirling generators that could have driven refrigerators to keep traditional silicon-based electronics cool. Glenn’s experience in simulating the extreme environments inside jet and rocket engines made it a natural home for the chamber. “The mantra of our branch is small, smart, and rugged,” says Glenn engineer Gary Hunter, who had developed basic chemical sensors for the Venus environment. But the agency canceled the program in 2013, leaving the stainless steel vessel and its gas-mixing apparatus gathering dust.

Kremic, who had spent some time at NASA headquarters in Washington, D.C., in planetary science before returning to Glenn, saw an opportunity. In fits and starts, he cobbled together money to create the largest and most advanced facility for simulating the surface of Venus. The minivan-size chamber was rebuilt and upgraded last year. Now, besides simply running large volumes of gas at high temperature and pressures of 90 bars or more, the GEER can mix eight different gases to create a Venus-like atmosphere, and it can inject water and other liquids into the cauldron. “The end result is we’re much more likely to understand more fully what Venus will be like when we get there,” Harvey says.

The chamber quickly proved its worth both for stress tests and for basic science. “It was kind of heaven,” says Harvey, who has pushed the GEER to run for a record 80 days in two “cook and look” experiments to see how different volcanic rocks would react with the venusian atmosphere—long a matter of debate among planetary scientists.

The chamber’s marquee moment came last year, when the previous, 24-transitor generation of Neudeck’s chip survived 21 days in the GEER—an ordeal that may have changed the course of Venus exploration. Since then, NASA has funded Kremic’s team to explore three different concepts for longlived landers. Across the country, a team at JPL, led by mechatronics engineer Jonathan Sauder, had been exploring a clockwork rover, virtually free of electronics, that could explore the surface of Venus. When they caught wind of the developments at Glenn, they began to think about how their mechanical designs could supplement the high-temperature chips. “We’re starting to get into a different realm,” says Lori Glaze, who is based at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and is leading one of the proposed billion-dollar New Frontiers missions to Venus (all of which rely on conventional silicon chips). “I definitely think that being able to rove or explore the surface of Venus is within the future horizon.”

That surface could prove a lot more active than planetary scientists thought just a few years ago. In the 1990s, cloud-penetrating radar aboard NASA’s Magellan orbiter showed relatively few craters, distributed seemingly at random. Some 500 million years ago, researchers theorized, a catastrophic event—perhaps a vast flood of magma—wiped the surface clean, like a planet-size slate, smothering any possibility of volcanoes or plate tectonics beneath a thick, cold crust. And Venus has been pretty much dead ever since.

In the late 2000s, however, the European Venus Express orbiter began to sketch a much livelier picture. Tracking the atmosphere, it saw what appeared to be a fourfold spike in sulfur dioxide that lasted about a year—perhaps the sign of a large, Mount Pinatubo-style volcanic eruption. Peering through the clouds in specific wavelengths of light, Venus Express seemed to discern unusually dark terrain near volcanic features—what fresh lava might look like on Earth. And near the end of its mission, in a rift on the side of a volcano, it saw what seemed to be a spike in temperatures of several hundred degrees. “This really makes us think Venus should be active,” Wilson says.

Scientists would love to find out. Missions such as NASA’s three New Frontiers candidates, however, focus more on the planet’s distant past. Two of them—one led by Glaze, the other by Larry Esposito, a planetary scientist at the University of Colorado in Boulder—would be short-lived. Each would drop a pressure vessel into the atmosphere, which would measure atmospheric chemistry on the way down and spend its few hours of life on the surface sampling rocks with either lasers or a drill. Analyzing isotopes of nonreactive noble gases in the atmosphere could give scientists a window into whether Venus started with as much water as Earth did—and whether it might still be hiding water, the lubricant of plate tectonics, deep in its interior. Probing the rock composition could reveal whether, as some researchers suspect, the slightly elevated regions called tesserae are remnants of continents.

The third New Frontiers mission, proposed by JPL research scientist Suzanne Smrekar, would take a more unconventional approach: using orbiting radar and spectrometers to probe the surface’s composition while a small probe swoops in and out of the atmosphere to capture air for isotope analysis. The high-resolution radar could reveal surface features lost in the noise of old measurements, Smrekar says: perhaps chasms that resemble Earth’s midocean ridges, or the details of mysterious oval-shaped features called coronae, which could mark where plumes of hot material from Venus’s mantle are causing parts of the crust to sink under others. Smrekar suspects Venus is a good analog for the time when plate tectonics began on Earth. Its greenhouse-heated surface is cooling much more slowly than Earth’s, and may only now be starting to crack into plates. “We may be seeing evidence for the process of subduction starting on Venus today,” she says.

To know for sure, however, researchers need to measure what’s happening in Venus’s interior today. That information can come only with sustained listening—exactly what Glenn’s landers propose to provide.

Each of them, Kremic says, was designed to be small enough to hitch a ride on other missions—either one of the New Frontiers spacecraft, Venera-D, or spacecraft that could swoop by Venus en route to other destinations. The first proposal, called the Long-Life In-Situ Solar System Explorer (LLISSE), was modest: a glorified cube the size of a car battery that would drop from a balloon or larger probe and record temperature, pressure, wind speed, and a few specific chemicals for 60 Earth days. Because silicon carbide isn’t good for storing data, the LLISSE would stream its observations either up to an orbiter or straight to Earth. The readings would provide ground truth for circulation models of the planet’s atmosphere, and they would help researchers estimate how mass is distributed throughout the planet—one fundamental mystery that a short-term mission could not answer.

A slightly larger design, the Seismic and Atmospheric Exploration of Venus, unveiled this month at a Venus meeting at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, could add a seismometer, a heat flux gauge, possibly even a camera, all for $100 million. Such a seismometer would be the killer app for long-lived landers. “The ultimate goal is to have seismometers on Venus,” Smrekar says. Nothing can tell you more about the interior structure of a planet. By listening to the ground over a long span of time, Wilson says, you might hear the crust stretching or cracking from tectonic processes, including “Venusquakes,” or hear the rumbling of distant volcanoes. Such a mission could quickly answer the fundamental question: Is Venus dead or alive?

Another test of Glenn’s silicon carbide electronics could potentially come quite soon: a proposal called Venus Bridge Orbiter and Surface Science (V-BOSS), one of two candidates for a quick-to-fly, low-cost (less than $200 million) “Venus Bridge” mission that NASA’s associate administrator for science, Thomas Zurbuchen, asked Venus scientists to prepare in the wake of the failed Discovery round. While details of the V-BOSS won’t be set until early next year, it would build off of the LLISSE and add an orbiter to relay lander data back to Earth.

Some researchers, blindsided by the brisk progress in high-temperature electronics, worry that the Glenn and Venus Bridge landers could outcompete more conventional missions such as New Frontiers. That would be a loss to science, Esposito says, because the cut-rate landers can’t match sophisticated sensors, such as mass spectrometers and radar, for answering key questions. “There’s not a cheap way to find out the dominant mineral on the surface of Venus,” he says. But Harvey says Glenn-style electronics could make even more ambitious future Venus probes—such as a long-delayed potential multibillion-dollar flagship mission—vastly more productive.

Meanwhile, back here in Cleveland, the latest endurance test has wound up. Neudeck reports that his microchips worked the whole way through, and could have run longer. One day, he is confident, devices like these will brave the hellish surface of Venus. Until they are ready, he will keep putting them through their paces, marking time in the little hell next door.


Update on 1I/2017 ‘Oumuamua

Updated: For the first time, astronomers are tracking a distant visitor streaking through our solar system

By Paul Voosen |

*Update, 20 November, 12:50 p.m.: During the brief visit by this interstellar visitor over the past month, many of the world’s most powerful telescopes swung to take a look. What they saw, reported today in Nature, was both familiar and extraordinary. The asteroid, now dubbed ‘Oumuamua (scout or messenger in the Hawaiian language), is dark red in color, similar to objects from the far reaches of our own solar system. The authors conclude it’s most likely a lump of metal-rich rock without much water or ice that has been reddened by millions of years of bombardment by cosmic rays as it crossed interstellar space. The surprise was that the light coming from it pulsed in brightness by a factor of 10 every 7.3 hours, suggesting both that it is spinning rapidly and is 10 times longer than it is wide—more elongated than anything known among our planets. They estimate that its mean radius is about 100 meters and its length is about 800 meters. Researchers believe that such visitors may zip through our neighborhood about once a year, but they’re usually too small to see. They’ll be watching extra keenly now that they’ve bagged their first one. Astronomers continue to monitor ‘Oumuamua as it fades into the distance, trying to figure out where it might have come from and where it will go next. 


New FRB discoveries and follow-up

The SUrvey for Pulsars and Extragalactic Radio Bursts II: New FRB discoveries and their follow-up

ABSTRACT: We report the discovery of four Fast Radio Bursts (FRBs) in the ongoing SUrvey for Pulsars and Extragalactic Radio Bursts (SUPERB) at the Parkes Radio Telescope: FRBs 150610, 151206, 151230 and 160102. Our real-time discoveries have enabled us to conduct extensive, rapid multi-messenger follow-up at 12 major facilities sensitive to radio, optical, X-ray, gamma-ray photons and neutrinos on time scales ranging from an hour to a few months post-burst. No counterparts to the FRBs were found and we provide upper limits on afterglow luminosities. None of the FRBs were seen to repeat. Formal fits to all FRBs show hints of scattering while their intrinsic widths are unresolved in time. FRB 151206 is at low Galactic latitude, FRB 151230 shows a sharp spectral cutoff, and FRB 160102 has the highest dispersion measure (DM = 2596.1±0.3 pc/cm3) detected to date. Three of the FRBs have high dispersion measures (DM > 1500 pc/cm3), favouring a scenario where the DM is dominated by contributions from the Intergalactic Medium. The slope of the Parkes FRB source counts distribution with fluences >2 Jy⋅ms is α=-2.2+0.6-1.2 and still consistent with a Euclidean distribution (α=-3/2). We also find that the all-sky rate is 1.7+1.5-0.9×1000 FRBs/(4π sr)/day above ∼2 Jy⋅ms and there is currently no strong evidence for a latitude-dependent FRB sky-rate.