Month: November 2016

More Hints of Exotic Cosmic-Ray Origin

Observing the constant rain of cosmic rays hitting Earth can provide information on the “magnetic weather” in other parts of the Galaxy. A new high-precision measurement of two cosmic-ray elements, boron and carbon, supports a specific model of the magnetic turbulence that deflects cosmic rays on their journey through the Galaxy.

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The data, which come from the Alpha Magnetic Spectrometer (AMS) aboard the International Space Station, appear to rule out alternative models for cosmic-ray propagation. By ruling out these models, the AMS results support the alternative explanation – a new primary cosmic ray source that emits positrons. Candidates include pulsars and dark matter, but a lot of mystery still surrounds the unexplained positron data.

The majority of cosmic rays are particles or nuclei produced in supernovae or other astrophysical sources. However, as these so-called primary cosmic rays travel through the Galaxy to Earth, they collide with gas atoms in the interstellar medium. The collisions produce a secondary class of cosmic rays with masses and energies that differ from primary cosmic rays.

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To investigate the relationship of the two classes, astrophysicists often look at the ratio of the number of detection’s of two nuclei, such as boron and carbon. For the most part, carbon cosmic rays have a primary origin, whereas boron is almost exclusively created in secondary processes. A relatively high boron-to-carbon (B/C) ratio in a certain energy range implies that the relevant cosmic rays are traversing a lot of gas before reaching us. “The B/C ratio tells you how cosmic rays propagate through space,” says AMS principal investigator Samuel Ting of MIT.

Previous measurements of the B/C ratio have had large errors of 15% or more, especially at high energy, mainly because of the brief data collection time available for balloon-based detectors. But the AMS has been operating on the Space Station for five years, and over this time it has collected more than 80 billion cosmic rays. The AMS detectors measure the charges of these cosmic rays, allowing the elements to be identified. The collaboration has detected over ten million carbon and boron nuclei, with energies per nucleon ranging from a few hundred MeV up to a few TeV.

The B/C ratio decreases with energy because higher-energy cosmic rays tend to take a more direct path to us (and therefore experience fewer collisions producing boron). By contrast, lower-energy cosmic rays are diverted more strongly by magnetic fields, so they bounce around like pinballs among magnetic turbulence regions in the Galaxy. Several theories have been proposed to describe the size and spacing of these turbulent regions, and these theories lead to predictions for the energy dependence of the B/C ratio. However, previous B/C observations have not been precise enough to favor one theory over another. The AMS data show very clearly that the B/C ratio is proportional to the energy raised to the -1/3 power. This result matches a prediction based on a theory of magnetohydrodynamics developed in 1941 by the Russian mathematician Andrey Kolmogorov.

These results conflict with models that predict that the B/C ratio should exhibit some more complex energy dependence, such as kinks in the B/C spectrum at specific energies. Theorists proposed these models to explain anomalous observations – by AMS and other experiments – that showed an increase in the number of positrons (anti-electrons) reaching Earth relative to electrons at high energy. The idea was that these “excess” positrons are – like boron – produced in collisions between cosmic rays and interstellar gas. But such a scenario would require that cosmic rays encounter additional scattering sites, not just magnetically turbulent regions. By ruling out these models, the AMS results support the alternative explanation – a new primary cosmic ray source that emits positrons. Candidates include pulsars and dark matter, but a lot of mystery still surrounds the unexplained positron data.

Igor Moskalenko from Stanford University is very surprised at the close match between the data and the Kolmogorov model. He expected that the ratio would deviate from a single power law in a way that might provide clues to the origin of the excess positrons. “This is a dramatic result that should lead to much better understanding of interstellar magnetohydrodynamic turbulence and propagation of cosmic rays,” he says. “On the other hand, it is very much unexpected in that it makes recent discoveries in astrophysics of cosmic rays even more puzzling.”

Why Are There Volcanoes Where They Shouldn’t Be?

Easy Answer: Mantle Plumes….But here is the long winded answer served up by this latest find: Madagascar, the big island off the east coast of Africa with the lemurs and baobabs, is thought to be sitting in the middle of an old tectonic plate, and so, by the rules of plate tectonics, should be tectonically quiet: few earthquakes and no volcanoes.

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But it’s not. The island has been away from tectonic action for the past 80 million years, said Martin Pratt, research scientist in earth and planetary sciences at Washington University in St. Louis, yet it experiences about 500 earthquakes per year.

The island also has volcanoes that have been active within the recent geologic past. “Having active volcanoes in Madagascar is like having erupting volcanoes in St. Louis,” said Michael Wysession, professor of earth and planetary sciences. “You have to ask yourself, ‘What are they doing there?'”

Since this part of the world is geologically complex, there are lots of interesting possible explanations for the volcanoes. To figure it out, the geologists needed to be able to examine not just the island’s accessible surface, but also what lies beneath the rigid crust and upper mantle.

To image Earth’s interior, geologists use a technique called seismic tomography that is similar to the medical CT scan, probing the earth’s stricture with seismic waves from distant earthquakes and ambient noise.

But remote and politically unstable Madagascar was largely unexplored by seismic methods until recently. Starting in 2010, however, three groups, including one led by Washington University seismologists Wysession and Doug Wiens, began to deploy seismic arrays on Madagascar, on nearby islands in the Mozambique channel (between the island and Africa), and on the ocean floor east of Madagascar.

In an article published online Nov. 22 in Earth and Planetary Science Letters, the Washington University scientists report that they found three areas of hot rock within the mantle beneath three separate volcanic provinces on the island. They also see signs that the bottom of the lithosphere beneath the central volcanic province has peeled off. As the cold rock sank into the mantle, hotter rock flowed around it to the center and the south of the island. The crust, unburdened, bobbed higher. The northern volcanic province, meanwhile, probably taps a different heat source.

A busted-up chunk of an ancient continent

Madagascar, originally part of the ancient continent Gondwana, was formed in two steps. The island, together with India, pulled away from Africa 150 million years ago, stretching and thinning the crust on the island’s west coast before it finally snapped off. The thinned crust on the west coast sagged and the dips filled with sediments, forming deep basins of sedimentary rocks.

Then, about 90 million years ago, when the mini-continent migrated over the Marion hotspot (a mantle plume that now lies beneath the Antarctic plate to the south), brief but voluminous eruptions covered the island in lava. The blast of heat is thought to have cracked the overriding continent into two parts, Madagascar and India, which scraped past the east coast of Madagascar on its way north toward Asia, leaving a very straight coastline there.

But the volcanism in the central, northern, and southern provinces are much younger than the basaltic remains of the 90-million-year-old eruption still found around the perimeter of Madagascar. So the question was: Where did they come from?

Lead-author Pratt used three complementary methods to analyze surface waves (seismic waves trapped near Earth’s surface), which are created by distant earthquakes and from sources of seismic noise, such as ocean storms.

“His approach is clever and creative,” Wysession said. “He’s taken three really different data sets, some good at high frequencies that give you better resolution at shallow depths, and some better at low frequencies that give you better resolution at greater depths, and he’s put them all together. It’s a bit like combining an X-ray, an MRI and a CT scan to get a clearer image.”

The images show three low-velocity seismic anomalies corresponding to the upwelling of hotter mantle rock along the island’s backbone.

“We knew about the named volcanic provinces in the center and north,” Wysession said. “But we didn’t know about the one in the southwest. When we saw the third blob in the images, we checked the literature and discovered that, sure enough, there was volcanic activity there as recently as 9 million years ago.”

The cause of the three hot regions in the mantle is a mystery, however. Though there is some indication from the tomographic images that the regions might be connected, particularly the southern two, further modeling of deeper structure will be needed to confirm. One origin of the hot regions previously has been proposed to be hot rock rising through the mantle as the Comores hot spot, which has created a set of volcanic islands just west of the north end of the island.

The authors have a different idea, however, and it comes from the way that the central and southwestern provinces appear to be connected at depth. “If you look at the images that Martin has made,” Wysession said, “you can see a horseshoe shape where the central hot mantle anomaly swings west and then comes back east again, connecting the central and southern provinces.

The deflecting obstacle seems to be a slab of colder rock. “We think the lithosphere (the crust and rigid upper mantle) has delaminated, and the bottom of it fell off,” Wysession said. “As the cold, dense slab began to sink, hotter rock flowed up and in to replace it, buoying the central province and, as it tilted, blocking flow to the south.”

But what caused the bottom of the lithosphere to peel off? “We think it may have been the Marion hotspot,” Wysession said. “The underside of the plate was heated by this huge blow torch 95 million years ago, weakening the rock enough that it was able to peel off. So we’re still seeing collateral damage from this ancient event.” This idea also has the advantage of explaining the unusually high elevations of the northern half of the island. Once the heavy bottom of the plate fell off, it stopped pulling down the crust, which rebounded upward as much as a kilometer as hot rock from below took the place of the delaminated slab.

Something similar happened underneath the Great Basin of the western United States, he said, where the bottom of the lithosphere also split off, forming a large blob of cold material sinking down through the mantle below the surface of central Nevada. There, the blow torch that delaminated the plate was an ocean spreading center that was overridden by the North American plate, Wysession said.

Earth’s Largest Exposed Fault Plane Identified

Lead researcher Dr Jonathan Pownall from the Australian National University (ANU) said the find will help researchers assess dangers of future tsunamis in the area, which is part of the Ring of Fire – an area around the Pacific Ocean basin known for earthquakes and volcanic eruptions.

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“The abyss has been known for 90 years but until now no one has been able to explain how it got so deep,” Dr Pownall said.

“Our research found that a 7 km-deep abyss beneath the Banda Sea off eastern Indonesia was formed by extension along what might be Earth’s largest-identified exposed fault plane.”

By analyzing high-resolution maps of the Banda Sea floor, geologists from ANU and Royal Holloway University of London found the rocks flooring the seas are cut by hundreds of straight parallel scars.

These wounds show that a piece of crust bigger than Belgium or Tasmania must have been ripped apart by 120 km of extension along a low-angle crack, or detachment fault, to form the present-day ocean-floor depression.

Dr Pownall said this fault, the Banda Detachment, represents a rip in the ocean floor exposed over 60,000 square kilometers. “The discovery will help explain how one of the Earth’s deepest sea areas became so deep,” he said.

Professor Gordon Lister also from the ANU Research School of Earth Sciences said this was the first time the fault has been seen and documented by researchers. “We had made a good argument for the existence of this fault we named the Banda Detachment based on the bathymetry data and on knowledge of the regional geology,” said Professor Lister.

Dr Pownall said he was on a boat journey in eastern Indonesia in July when he noticed the prominent landforms consistent with surface extensions of the fault line. “I was stunned to see the hypothesized fault plane, this time not on a computer screen, but poking above the waves,” said Dr Pownall. He said rocks immediately below the fault include those brought up from the mantle.

“This demonstrates the extreme amount of extension that must have taken place as the oceanic crust was thinned, in some places to zero,” he said.

Dr Pownall also said the discovery of the Banda Detachment fault would help assesses dangers of future tsunamis and earthquakes.

“In a region of extreme tsunami risk, knowledge of major faults such as the Banda Detachment, which could make big earthquakes when they slip, is fundamental to being able to properly assess tectonic hazards,” he said.

Violent Collision with Superluminous Supernovae

In a unique study, an international team of researchers including members from the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) simulated the violent collisions between supernovae and its surrounding gas – which is ejected before a supernova explosion, thereby giving off an extreme brightness.

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Many supernovae have been discovered in the last decade with peak luminosity one-to-two orders of magnitude higher than for normal supernovae of known types. These stellar explosions are called Superluminous Supernovae (SLSNe).

Some of them have hydrogen in their spectra, while some others demonstrate a lack of hydrogen. The latter are called Type I, or hydrogen-poor, SLSNe-I. SLSNe-I challenge the theory of stellar evolution, since even normal supernovae are not yet completely understood from first principles.

Led by Sternberg Astronomical Institute researcher Elena Sorokina, who was a guest investigator at Kavli IPMU, and Kavli IPMU Principal Investigator Ken’ichi Nomoto, Scientific Associate Sergei Blinnikov, as well as Project Researcher Alexey Tolstov, the team developed a model that can explain a wide range of observed light curves of SLSNe-I in a scenario which requires much less energy than other proposed models.

The models demonstrating the events with the minimum energy budget involve multiple ejections of mass in presupernova stars. Mass loss and buildup of envelopes around massive stars are generic features of stellar evolution. Normally, those envelopes are rather diluted, and they do not change significantly the light produced in the majority of supernovae.

In some cases, large amount of mass are expelled just a few years before the final explosion. Then, the “clouds” around supernovae may be quite dense. The shockwaves produced in collisions of supernova ejecta and those dense shells may provide the required power of light to make the supernova much brighter than a “naked” supernova without pre-ejected surrounding material.

This class of the models is referred to as “interacting” supernovae. The authors show that the interacting scenario is able to explain both fast and slowly fading SLSNe-I, so the large range of these intriguingly bright objects can in reality be almost ordinary supernovae placed into extraordinary surroundings.

Another extraordinarity is the chemical composition expected for the circumstellar “clouds.” Normally, stellar wind consists of mostly hydrogen, because all thermonuclear reactions happen in the center of a star, while outer layers are hydrogenous.

In the case of SLSNe-I, the situation must be different. The progenitor star must lose its hydrogen and a large part of helium well before the explosion, so that a few months to a few years before the explosion, it ejects mostly carbon and oxygen, and then explode inside that dense CO cloud. Only this composition can explain the spectral and photometric features of observed hydrogen-poor SLSNe in the interacting scenario.

It is a challenge for the stellar evolution theory to explain the origin of such hydrogen- and helium-poor progenitors and the very intensive mass loss of CO material just before the final explosion of the star. These results have been published in a paper accepted by The Astrophysical Journal.

New Family of Stars Discovered in Milky Way

An astronomer from LJMU’s Astrophysics Research Institute has discovered a new family of stars in the core of the Milky Way Galaxy which provides new insights into the early stages of the Galaxy’s formation.

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The discovery has shed new light on the origins of globular clusters – which are concentrations of typically a million stars, formed at the beginning of the Milky Way’s history.

LJMU is a member of Sloan Digital Sky Survey – an international collaboration of scientists at numerous institutions. One of the projects of this collaboration is APOGEE (the Apache Point Observatory Galactic Evolution Experiment) which collects infrared data for hundreds of thousands of stars in the Milky Way.

It was through observing stars in the infrared towards the Galactic center that led to the discovery of a new population of stars, the likes of which had only been seen before inside globular clusters.

This intriguing new family of stars could have possibly belonged to globular clusters that were destroyed during the violent initial formation of the Galactic center, in which case there would have been about 10 times more globular clusters in the Milky Way in early life than today. This means that a substantial fraction of the old stars inhabiting the inner parts of the Galaxy today may have been initially formed in globular clusters that were later destroyed.

Ricardo Schiavon, lead researcher on the project said:

“This is a very exciting finding that helps us address fascinating questions such as what is the nature of the stars in the inner regions of the Milky Way, how globular clusters formed and what role they played in the formation of the early Milky Way—and by extension the formation of other galaxies.”

“The center of the Milky Way is poorly understood, because it is blocked from view by intervening dust. Observing in the infrared, which is less absorbed by dust than visible light, APOGEE can see the center of the Galaxy better than other teams

“From our observations we could determine the chemical compositions of thousands of stars, among which we spotted a considerable number of stars that differed from the bulk of the stars in the inner regions of the Galaxy, due to their very high abundance of nitrogen. While not certain, we suspect that these stars resulted from globular cluster destruction. They could also be the byproducts of the first episodes of star formation taking place at the beginning of the Galaxy’s history. We are conducting further observations to test these hypotheses.”

Powerful Punch of Gamma Rays Found in Mysterious Fast Radio Bursts

Penn State University astronomers have discovered that the mysterious “cosmic whistles” known as fast radio bursts can pack a serious punch, in some cases releasing a billion times more energy in gamma-rays than they do in radio waves and rivaling the stellar cataclysms known as supernovae in their explosive power. The discovery, the first-ever finding of non-radio emission from any fast radio burst, drastically raises the stakes for models of fast radio bursts and is expected to further energize efforts by astronomers to chase down and identify long-lived counterparts to fast radio bursts using X-ray, optical, and radio telescopes.

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Fast radio bursts, which astronomers refer to as FRBs, were first discovered in 2007, and in the years since radio astronomers have detected a few dozen of these events. Although they last mere milliseconds at any single frequency, their great distances from Earth — and large quantities of intervening plasma — delay their arrival at lower frequencies, spreading the signal out over a second or more and yielding a distinctive downward-swooping “whistle” across the typical radio receiver band.

“This discovery revolutionizes our picture of FRBs, some of which apparently manifest as both a whistle and a bang,” said coauthor Derek Fox, a Penn State professor of astronomy and astrophysics. The radio whistle can be detected by ground-based radio telescopes, while the gamma-ray bang can be picked up by high-energy satellites like NASA’s Swift mission. “Rate and distance estimates for FRBs suggest that, whatever they are, they are a relatively common phenomenon, occurring somewhere in the universe more than 2,000 times a day.”

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Efforts to identify FRB counterparts began soon after their discovery but have all come up empty until now. In a paper recently published in Astrophysical Journal Letters the Penn State team, led by physics graduate student James DeLaunay, reports bright gamma-ray emission from the fast radio burst FRB 131104, named after the date it occurred, 4 November 2013. “I started this search for FRB counterparts without expecting to find anything,” said DeLaunay. “This burst was the first that even had useful data to analyse. When I saw that it showed a possible gamma-ray counterpart, I couldn’t believe my luck!”

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Discovery of the gamma-ray “bang” from FRB 131104, the first non-radio counterpart to any FRB, was made possible by NASA’s Earth-orbiting Swift satellite, which was observing the exact part of the sky where FRB 131104 occurred as the burst was detected by the Parkes Observatory radio telescope in Parkes, Australia. “Swift is always watching the sky for bursts of X-rays and gamma-rays,” said Neil Gehrels, the mission’s principal investigator and chief of the Astroparticle Physics Laboratory at NASA’s Goddard Space Flight Center. “What a delight it was to catch this flash from one of the mysterious fast radio bursts.”

“Although theorists had anticipated that FRBs might be accompanied by gamma rays, the gamma-ray emission we see from FRB 131104 is surprisingly long-lasting and bright,” Fox said. The duration of the gamma-ray emission, at two to six minutes, is many times the millisecond duration of the radio emission. And the gamma-ray emission from FRB 131104 outshines its radio emissions by more than a billion times, dramatically raising estimates of the burst’s energy requirements and suggesting severe consequences for the burst’s surroundings and host galaxy.

Two common models for gamma-ray emission from FRBs exist: one invoking magnetic flare events from magnetars — highly magnetized neutron stars that are the dense remnants of collapsed stars — and another invoking the catastrophic merger of two neutron stars, colliding to form a black hole. According to coauthor Kohta Murase, a Penn State professor and theorist, “The energy release we see is challenging for the magnetar model unless the burst is relatively nearby. The long timescale of the gamma-ray emission, while unexpected in both models, might be possible in a merger event if we observe the merger from the side, in an off-axis scenario.”

“In fact, the energy and timescale of the gamma-ray emission is a better match to some types of supernovae, or to some of the supermassive black hole accretion events that Swift has seen,” Fox said. “The problem is that no existing models predict that we would see an FRB in these cases.”

The bright gamma-ray emission from FRB 131104 suggests that the burst, and others like it, might be accompanied by long-lived X-ray, optical, or radio emissions. Such counterparts are dependably seen in the wake of comparably energetic cosmic explosions, including both stellar-scale cataclysms — supernovae, magnetar flares, and gamma-ray bursts — and episodic or continuous accretion activity of the supermassive black holes that commonly lurk in the centers of galaxies.

In fact, Swift X-ray and optical observations were carried out two days after FRB 131104, thanks to prompt analysis by radio astronomers (who were not aware of the gamma-ray counterpart) and a nimble response from the Swift mission operations team, headquartered at Penn State. In spite of this relatively well-coordinated response, no long-lived X-ray, ultraviolet, or optical counterpart was seen.

The authors hope to participate in future campaigns aimed at discovering more FRB counterparts, and in this way, finally revealing the sources responsible for these ubiquitous and mysterious events. “Ideally, these campaigns would begin soon after the burst and would continue for several weeks afterward to make sure nothing gets missed. Maybe we’ll get even luckier next time,” DeLaunay said.

Rupture in Lithosphere Driving Undersea Volcanism

Scientists analyzing a volcanic eruption at a mid – ocean ridge under the Pacific have come up with a somewhat contrarian explanation for what initiated it. Many scientists say undersea volcanism is triggered mainly by upwelling magma that reaches a critical pressure in the lithosphere and forces its way up. The new study says the dominant force, at least in this case, was the seafloor itself – basically that it ripped itself open, allowing magma to spill out. The eruption took place on the East Pacific Rise, some 700 miles off Mexico.

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“Mid – ocean ridges are commonly viewed as seafloor volcanoes, operating like volcanoes on land,” said the study’s lead author, Yen Joe Tan, a graduate student at Columbia University’s Lamont – Doherty Earth Observatory. “We’re saying they should actually be viewed as tears in the crust, where magma oozes out.” The study appears in the journal Nature this week.

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The authors of the new paper took another look after a 2015 eruption at an unconnected study site, at Axial Seamount, off the coast of Oregon. Unlike the earlier East Pacific Rise eruption, this one was studied in real time with an assortment of instruments. Among the data produced were recordings of violent popping noises that appeared related to the emergence of lava on the seafloor – possibly the result of exploding gas bubbles, or implosions of hardening lava.

In light of the Axial Seamount observations, the researchers reviewed the 2005 – 2006 data from the East Pacific Rise, and came up with a newly sharpened picture. Their reanalysis suggested that most of the eruption took place rapidly, not over months. Other researchers had already identified a series of conventional earthquakes of about magnitude 2 on Jan. 22, 2006, of the kind usually associated with the rupture of a rock boundary, along a 35 – kilometer – long segment of the ridge. About 15 minutes later, the seismometers started picking up clusters of lower – frequency earthquakes, of a type usually associated with rising magma. Another hour or two on, popping sounds like those heard at Axial Seamount appeared, in four separate areas along the segment, each in an area about 5 kilometers long.

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“It’s been a kind of chicken – and – egg question,” said coauthor Maya Tolstoy, a marine geophysicist at Lamont – Doherty. “You have these two different forces [magma vs. tectonics] that could play a role, and it’s hard to tell which triggers the eruption. Here, we can make the argument for one dominating, because we see this series of events, and then multiple magma chambers erupting at the same time.”

The authors say that according to their observations, about 85 percent of the lava emerged within two days, with remnants dribbling out over the course of a week. The eruptions produced some 22 million cubic meters of seafloor – about enough to cover 13 football fields 1,000 feet deep.

Cynthia Ebinger, a professor at the University of Rochester who studies eruptions at spreading sites both on land and under the ocean, said in an email that very few seafloor eruptions have been so directly observed. The study “adds a new factor to consider,” she said. “It shows that tectonic stresses can trigger large – volume intrusions and eruptions” to create new seafloor.

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Michael Perfit, a professor at the University of Florida who also studies undersea eruptions, said the study “tells a remarkable story.” But, he said, the authors may have overstated the relative role of tectonic stress versus magma pressure. “I think it’s really got to be both,” he said. He cited a 2014 geochemical study he coauthored suggesting that the magma was replenished with new material from below about 6 weeks before the eruption. This suggests pressure could have played a more substantial role, he said.

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