ST/ Rare ‘black widow’ binary star with the shortest orbit ever discovered

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Paradigm

35 min readMay 11, 2022

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Space biweekly vol.51, 26th April — 11th May

TL;DR

Astronomers discovered a ‘black widow binary’ — a rapidly spinning neutron star circling and slowly consuming a smaller companion star. Named ZTF J1406+1222, the pair has the shortest orbital period yet identified, and is unique in that it appears to host a third star that orbits around the two inner stars every 10,000 years.
Soil on the moon contains active compounds that can convert carbon dioxide into oxygen and fuels, scientists report. They are now exploring whether lunar resources can be used to facilitate human exploration on the moon or beyond.
A planetary scientist worked with engineers to measure the physical limits for a liquid when salty water is at very high pressure. The results suggest where to look for extraterrestrial life in the ice-covered oceans of Jupiter’s moon Europa and Saturn’s moon Titan.
Researchers have found an alternative explanation for a mysterious gamma-ray signal coming from the center of the galaxy, which was long claimed as a signature of dark matter.
Astronomers discovered eight new echoing black hole binaries in our galaxy, enabling them to piece together a general picture of how a black hole evolves during an outburst. The findings will help scientists trace a black hole’s evolution as it feeds on stellar material.
An international group of astronomers has used observations to unlock a puzzling mystery about a stellar explosion discovered several years ago and evolving even now. The results will help astronomers better understand the process of how massive stars live and die.
It’s not unheard of to find a surviving star at the scene of a titanic supernova explosion, which would be expected to obliterate everything around it, but new research has provided a long-awaited clue to a specific type of stellar death. In some supernova cases, astronomers find no trace of the former star’s outermost layer of hydrogen. Suspicions that companion stars are responsible — siphoning away their partners’ outer shell before their death — are supported by the recent identification of a surviving companion star on the scene of supernova 2013ge.
The remnants of a collapsed neutron star, called a pulsar, are magnetically charged and spinning anywhere from one rotation per second to hundreds of rotations per second. These celestial bodies, each 12 to 15 miles in diameter, generate light in the x-ray wavelength range. Researchers have developed a new way spacecraft can use signals from multiple pulsars to navigate in deep space.
As the scientific community searches for worlds orbiting nearby stars that could potentially harbor life, new research suggests that younger rocky exoplanets are more likely to support temperate, Earth-like climates.
Long-duration space flight alters fluid-filled spaces along veins and arteries in the brain, according to new research.
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Space industry in numbers

The global smart space market size is projected to grow from USD 9.4 billion in 2020 to USD 15.3 billion by 2025, at a Compound Annual Growth Rate (CAGR) of 10.2% during the forecast period. The increasing venture capital funding and growing investments in smart space technology to drive market growth.

Analysts at Morgan Stanley and Goldman Sachs have predicted that economic activity in space will become a multi-trillion-dollar market in the coming decades. Morgan Stanley’s Space Team estimates that the roughly USD 350 billion global space industry could surge to over USD 1 trillion by 2040.

Source: Satellite Industry Association, Morgan Stanley Research, Thomson Reuters. *2040 estimates

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A 62-minute orbital period black widow binary in a wide hierarchical triple

by Kevin B. Burdge, Thomas R. Marsh, Jim Fuller, Eric C. Bellm, et al in Nature

The flashing of a nearby star has drawn MIT astronomers to a new and mysterious system 3,000 light years from Earth. The stellar oddity appears to be a new “black widow binary” — a rapidly spinning neutron star, or pulsar, that is circling and slowly consuming a smaller companion star, as its arachnid namesake does to its mate.

Astronomers know of about two dozen black widow binaries in the Milky Way. This newest candidate, named ZTF J1406+1222, has the shortest orbital period yet identified, with the pulsar and companion star circling each other every 62 minutes. The system is unique in that it appears to host a third, far-flung star that orbits around the two inner stars every 10,000 years.

An illustrated view of a black widow pulsar and its stellar companion. The pulsar’s gamma-ray emissions (magenta) strongly heat the facing side of the star (orange). The pulsar is gradually evaporating its partner. Credit: NASA’s Goddard Space Flight Center/Cruz deWilde

This likely triple black widow is raising questions about how such a system could have formed. Based on its observations, the MIT team proposes an origin story: As with most black widow binaries, the triple system likely arose from a dense constellation of old stars known as a globular cluster. This particular cluster may have drifted into the Milky Way’s center, where the gravity of the central black hole was enough to pull the cluster apart while leaving the triple black widow intact.

“It’s a complicated birth scenario,” says Kevin Burdge, a Pappalardo Postdoctoral Fellow in MIT’s Department of Physics. “This system has probably been floating around in the Milky Way for longer than the sun has been around.”

Burdge is the author of a study that details the team’s discovery. The researchers used a new approach to detect the triple system. While most black widow binaries are found through the gamma and X-ray radiation emitted by the central pulsar, the team used visible light, and specifically the flashing from the binary’s companion star, to detect ZTF J1406+1222.

CHIMERA and ZTF light curve of ZTF J1406+1222.

“This system is really unique as far as black widows go, because we found it with visible light, and because of its wide companion, and the fact it came from the galactic center,” Burdge says. “There’s still a lot we don’t understand about it. But we have a new way of looking for these systems in the sky.”

The study’s co-authors are collaborators from multiple institutions, including the University of Warwick, Caltech, the University of Washington, McGill University, and the University of Maryland.

Black widow binaries are powered by pulsars — rapidly spinning neutron stars that are the collapsed cores of massive stars. Pulsars have a dizzying rotational period, spinning around every few milliseconds, and emitting flashes of high-energy gamma and X-rays in the process. Normally, pulsars spin down and die quickly as they burn off a huge amount of energy. But every so often, a passing star can give a pulsar new life. As a star nears, the pulsar’s gravity pulls material off the star, which provides new energy to spin the pulsar back up. The “recycled” pulsar then starts reradiating energy that further strips the star, and eventually destroys it.

“These systems are called black widows because of how the pulsar sort of consumes the thing that recycled it, just as the spider eats its mate,” Burdge says.

Every black widow binary to date has been detected through gamma and X-ray flashes from the pulsar. In a first, Burdge came upon ZTF J1406+1222 through the optical flashing of the companion star. It turns out that the companion star’s day side — the side perpetually facing the pulsar — can be many times hotter than its night side, due to the constant high-energy radiation it receives from the pulsar.

“I thought, instead of looking directly for the pulsar, try looking for the star that it’s cooking,” Burdge explains.

He reasoned that if astronomers observed a star whose brightness was changing periodically by a huge amount, it would be a strong signal that it was in a binary with a pulsar.

Astrometric characterization of ZTF J1406+1222.

To test this theory, Burdge and his colleagues looked through optical data taken by the Zwicky Transient Facility, an observatory based in California that takes wide-field images of the night sky. The team studied the brightness of stars to see whether any were changing dramatically by a factor of 10 or more, on a timescale of about an hour or less — signs that indicate the presence of a companion star orbiting tightly around a pulsar.

The team was able to pick out the dozen known black widow binaries, validating the new method’s accuracy. They then spotted a star whose brightness changed by a factor of 13, every 62 minutes, indicating that it was likely part of a new black widow binary, which they labeled ZTF J1406+1222. They looked up the star in observations taken by Gaia, a space telescope operated by the European Space Agency that keeps precise measurements of the position and motion of stars in the sky. Looking back through decades old measurements of the star? from the Sloan Digital Sky Survey, the team found that the binary was being trailed by another distant star. Judging from their calculations, this third star appeared to be orbiting the inner binary every 10,000 years.

Cross-section of the orbital solution of ZTF J1406+1222 in the Galaxy.

Curiously, the astronomers have not directly detected gamma or X-ray emissions from the pulsar in the binary, which is the typical way in which black widows are confirmed. ZTF J1406+1222, therefore, is considered a candidate black widow binary, which the team hopes to confirm with future observations.

“The one thing we know for sure is that we see a star with a day side that’s much hotter than the night side, orbiting around something every 62 minutes,” Burdge says. “Everything seems to point to it being a black widow binary. But there are a few weird things about it, so it’s possible it’s something entirely new.”

The team plans to continue observing the new system, as well as apply the optical technique to illuminate more neutron stars and black widows in the sky.

The Candidate Progenitor Companion Star of the Type Ib/c SN 2013ge

by Ori D. Fox, Schuyler D. Van Dyk, Benjamin F. Williams, Maria Drout, et al in The Astrophysical Journal Letters

It’s not unheard of to find a surviving star at the scene of a titanic supernova explosion, which would be expected to obliterate everything around it, but the latest research from the Hubble Space Telescope has provided a long-awaited clue to a specific type of stellar death. In some supernova cases, astronomers find no trace of the former star’s outermost layer of hydrogen. What happened to the hydrogen? Suspicions that companion stars are responsible — siphoning away their partners’ outer shell before their death — are supported by Hubble’s identification of a surviving companion star on the scene of supernova 2013ge.

The discovery also lends support to the theory that the majority of massive stars form and evolve as binary systems. It could also be the prequel to another cosmic drama: In time, the surviving, massive companion star will also undergo a supernova, and if both the stars’ remnant cores are not flung from the system, they will eventually merge and produce gravitational waves, shaking the fabric of space itself.

This infographic shows the evolution astronomers propose for supernova (SN) 2013ge. Panels 1–3 show what has already occurred, and panels 4–6 show what may take place in the future. 1) A binary pair of massive stars orbit one another. 2) One star ages into its red giant stage, getting a puffy outer envelope of hydrogen that its companion star siphons off with gravity. Astronomers propose this is why Hubble found no trace of hydrogen in the supernova debris. 3) The stripped-envelope star goes supernova (SN 2013ge), jostling but not destroying its companion star. After the supernova, the dense core of the former massive star remains either as neutron star or black hole. 4) Eventually the companion star also ages into a red giant, maintaining its outer envelope, some of which came from its companion. 5) The companion star also undergoes a supernova. 6) If the stars were close enough to each other not to be flung from their orbits by the supernova blast wave, the remnant cores will continue to orbit one another and eventually merge, creating gravitational waves in the process.

NASA’s Hubble Space Telescope has uncovered a witness at the scene of a star’s explosive death: a companion star previously hidden in the glare of its partner’s supernova. The discovery is a first for a particular type of supernova — one in which the star was stripped of its entire outer gas envelope before exploding. The finding provides crucial insight into the binary nature of massive stars, as well as the potential prequel to the ultimate merger of the companion stars that would rattle across the universe as gravitational waves, ripples in the fabric of spacetime itself.

Astronomers detect the signature of various elements in supernova explosions. These elements are layered like an onion pre-supernova. Hydrogen is found in the outermost layer of a star, and if no hydrogen is detected in the aftermath of the supernova, that means it was stripped away before the explosion occurred.

The cause of the hydrogen loss had been a mystery, and astronomers have been using Hubble to search for clues and test theories to explain these stripped supernovae. The new Hubble observations provide the best evidence yet to support the theory that an unseen companion star siphons off the gas envelope from its partner star before it explodes.

“This was the moment we had been waiting for, finally seeing the evidence for a binary system progenitor of a fully stripped supernova,” said astronomer Ori Fox of the Space Telescope Science Institute in Baltimore, Maryland, lead investigator on the Hubble research program. “The goal is to move this area of study from theory to working with data and seeing what these systems really look like.”

Fox’s team used Hubble’s Wide Field Camera 3 to study the region of supernova (SN) 2013ge in ultraviolet light, as well as previous Hubble observations in the Barbara A. Mikulski Archive for Space Telescopes. Astronomers saw the light of the supernova fading over time from 2016 to 2020 — but another nearby source of ultraviolet light at the same position maintained its brightness. This underlying source of ultraviolet emission is what the team proposes is the surviving binary companion to SN 2013ge.

Hubble images of galaxy NGC 3287 show supernova 2013ge fading over time, revealing the steady source of ultraviolet light astronomers have identified as its binary companion star. Credits: NASA, ESA, and Ori Fox (STScI); Image Processing: Joseph DePasquale (STScI)

Previously, scientists theorized that a massive progenitor star’s strong winds could blow away its hydrogen gas envelope, but observational evidence didn’t support that. To explain the disconnect, astronomers developed theories and models in which a binary companion siphons off the hydrogen.

“In recent years many different lines of evidence have told us that stripped supernovae are likely formed in binaries, but we had yet to actually see the companion. So much of studying cosmic explosions is like forensic science — searching for clues and seeing what theories match. Thanks to Hubble, we are able to see this directly,” said Maria Drout of the University of Toronto, a member of the Hubble research team.

In prior observations of SN 2013ge, Hubble saw two peaks in the ultraviolet light, rather than just the one typically seen in most supernovae. Fox said that one explanation for this double brightening was that the second peak shows when the supernova’s shock wave hit a companion star, a possibility that now seems much more likely. Hubble’s latest observations indicate that while the companion star was significantly jostled, including the hydrogen gas it had siphoned off its partner, it was not destroyed. Fox likens the effect to a jiggling bowl of jelly, which will eventually settle back to its original form.

While additional confirmation and similar supporting discoveries need to be found, Fox said that the implications of the discovery are still substantial, lending support to theories that the majority of massive stars form and evolve as binary systems.

Single-star evolutionary tracks of 10 and 12 M⊙ stars in MIST for different HST/WFC3 filters, plotted in both (left) temperature and (right) color space.

Unlike supernovae that have a puffy shell of gas to light up, the progenitors of fully stripped-envelope supernovae have proven difficult to identify in pre-explosion images. Now that astronomers have been lucky enough to identify the surviving companion star, they can use it to work backward and determine characteristics of the star that exploded, as well as the unprecedented opportunity to watch the aftermath unfold with the survivor.

As a massive star itself, SN 2013ge’s companion is also destined to undergo a supernova. Its former partner is now likely a compact object, such as a neutron star or black hole, and the companion will likely go that route as well. The closeness of the original companion stars will determine if they stay together. If the distance is too great, the companion star will be flung out of the system to wander alone across our galaxy, a fate that could explain many seemingly solitary supernovae.

However, if the stars were close enough to each other pre-supernova, they will continue orbiting each other as black holes or neutron stars. In that case, they would eventually spiral toward each other and merge, creating gravitational waves in the process.

That is an exciting prospect for astronomers, as gravitational waves are a branch of astrophysics that has only begun to be explored. They are waves or ripples in the fabric of spacetime itself, predicted by Albert Einstein in the early 20th century. Gravitational waves were first directly observed by the Laser Interferometer Gravitational-Wave Observatory.

“With the surviving companion of SN 2013ge, we could potentially be seeing the prequel to a gravitational wave event, although such an event would still be about a billion years in the future,” Fox said.

Fox and his collaborators will be working with Hubble to build up a larger sample of surviving companion stars to other supernovae, in effect giving SN 2013ge some company again.

“There is great potential beyond just understanding the supernova itself. Since we now know most massive stars in the universe form in binary pairs, observations of surviving companion stars are necessary to help understand the details behind binary formation, material-swapping, and co-evolutionary development. It’s an exciting time to be studying the stars,” Fox said.
“Understanding the lifecycle of massive stars is particularly important to us because all heavy elements are forged in their cores and through their supernovae. Those elements make up much of the observable universe, including life as we know it,” added co-author Alex Filippenko of the University of California at Berkeley.

Extraterrestrial photosynthesis by Chang’E-5 lunar soil

by Yingfang Yao, Lu Wang, Xi Zhu, Wenguang Tu, Yong Zhou, Rulin Liu, Junchuan Sun, Bo Tao, Cheng Wang, Xiwen Yu, Linfeng Gao, Yuan Cao, Bing Wang, Zhaosheng Li, Wei Yao, Yujie Xiong, Mengfei Yang, Weihua Wang, Zhigang Zou in Joule

Soil on the moon contains active compounds that can convert carbon dioxide into oxygen and fuels, scientists in China report. They are now exploring whether lunar resources can be used to facilitate human exploration on the moon or beyond.

Nanjing University material scientists Yingfang Yao and Zhigang Zou hope to design a system that takes advantage of lunar soil and solar radiation, the two most abundant resources on the moon. After analyzing the lunar soil brought back by China’s Chang’e 5 spacecraft, their team found the sample contains compounds — including iron-rich and titanium-rich substances — that could work as a catalyst to make desired products such as oxygen using sunlight and carbon dioxide.

Based on the observation, the team proposed an “extraterrestrial photosynthesis” strategy. Mainly, the system uses lunar soil to electrolyze water extracted from the moon and in astronauts’ breathing exhaust into oxygen and hydrogen powered by sunlight. The carbon dioxide exhaled by moon inhabitants is also collected and combined with hydrogen from water electrolysis during a hydrogenation process catalyzed by lunar soil.

The process yields hydrocarbons such as methane, which could be used as fuel. The strategy uses no external energy but sunlight to produce a variety of desirable products such as water, oxygen, and fuel that could support life on a moonbase, the researchers say. The team is looking for an opportunity to test the system in space, likely with China’s future crewed lunar missions.

“We use in-situ environmental resources to minimize rocket payload, and our strategy provides a scenario for a sustainable and affordable extraterrestrial living environment,” Yao says.

While the catalytic efficiency of lunar soil is less than catalysts available on Earth, Yao says the team is testing different approaches to improve the design, such as melting the lunar soil into a nanostructured high-entropy material, which is a better catalyst.

Previously, scientists have proposed many strategies for extraterrestrial survival. But most designs require energy sources from Earth. For example, NASA’s Perseverance Mars rover brought an instrument that can use carbon dioxide in the planet’s atmosphere to make oxygen, but it’s powered by a nuclear battery onboard.

“In the near future, we will see the crewed spaceflight industry developing rapidly,” says Yao. “Just like the ‘Age of Sail’ in the 1600s when hundreds of ships head to the sea, we will enter an ‘Age of Space.’ But if we want to carry out large-scale exploration of the extraterrestrial world, we will need to think of ways to reduce payload, meaning relying on as little supplies from Earth as possible and using extraterrestrial resources instead.”

On the pressure dependence of salty aqueous eutectics

by Brooke Chang, Anthony N. Consiglio, Drew Lilley, Ravi Prasher, Boris Rubinsky, Baptiste Journaux, Matthew J. Powell-Palm in Cell Reports Physical Science

Researchers from the University of Washington and the University of California, Berkeley have conducted experiments that measured the physical limits for the existence of liquid water in icy extraterrestrial worlds. This blend of geoscience and engineering was done to aid in the search for extraterrestrial life and the upcoming robotic exploration of oceans on moons of other planets.

“The more a liquid is stable, the more promising it is for habitability,” said co-corresponding author Baptiste Journaux, an acting assistant professor of Earth and space sciences at the UW. “Our results show that the cold, salty, high-pressure liquids found in the deep ocean of other planets’ moons can remain liquid to much cooler temperature than they would at lower pressures. This extends the range of possible habitats on icy moons, and will allow us to pinpoint where we should look for biosignatures, or signs of life.”

Jupiter and Saturn’s icy moons — including Europa, Ganymede and Titan — are leading candidates within our solar system for hosting extraterrestrial life. These ice-encrusted moons are thought to harbor enormous liquid oceans, up to several dozen times the volume of oceans on Earth.

The left panel’s gray and blue layers show the deep, ice-covered ocean on Europa, a moon of Jupiter that could host extraterrestrial life. This ocean is thought to be much deeper than oceans on Earth. New research hints at where liquid water might be found in these environments.Image by NASA/JPL-Caltech, with modifications by Baptiste Journaux.

“Despite its designation as the ‘blue marble,’ Earth is remarkably dry when compared to these worlds,” Journaux said.

The oceans on these moons may contain various types of salts and are expected to range from about 100 miles deep, on Europa, to more than 400 miles deep, on Titan.

“We know that water supports life, but the major part of the oceans on these moons are likely below zero degrees Celsius and at pressures higher than anything experienced on Earth,” Journaux said. “We needed to know how cold an ocean can get before entirely freezing, including in its deepest abyss.”

The study focused on eutectics, or the lowest temperature that a salty solution can remain liquid before entirely freezing. Salt and water are one example — salty water remains liquid below the freezing temperature of pure water, one of the reasons people sprinkle salt on roads in winter to avoid the formation of ice. The experiments used UC Berkeley equipment originally designed for the future cryopreservation of organs for medical applications and for food storage. For this research, however, the authors used it to simulate the conditions thought to exist on other planets’ moons.

Measurement of binary eutectic phase equilibria by isochoric freezing.

Journaux, a planetary scientist and expert on the physics of water and minerals, worked with UC Berkeley engineers to test solutions of five different salts at pressures up to 3,000 times atmospheric pressure, or 300 megapascals — about three times the pressure in Earth’s deepest ocean trench.

“Knowing the lowest temperature possible for salty water to remain a liquid at high pressures is integral to understanding how extraterrestrial life could exist and thrive in the deep oceans of these icy ocean worlds,” said co-corresponding author Matthew Powell-Palm, who did the work as a postdoctoral researcher at UC Berkeley, also co-founder and CEO of the cryopreservation company BioChoric, Inc.

Journaux recently started working with NASA’s Dragonfly mission team, which will send a rotorcraft in 2027 to Saturn’s largest moon, Titan. NASA also is leading the Europa Clipper mission in 2024 to explore Europa, one of the many moons orbiting Jupiter. Meanwhile, the European Space Agency in 2023 will send its JUICE spacecraft, or Jupiter Icy Moons Explorer, to explore three of Jupiter’s largest moons: Ganymede, Callisto and Europa.

“The new data obtained from this study may help further researchers’ understanding of the complex geological processes observed in these icy ocean worlds,” Journaux said.

The NICER “Reverberation Machine”: A Systematic Study of Time Lags in Black Hole X-Ray Binaries

by Jingyi Wang, Erin Kara, Matteo Lucchini, Adam Ingram, Michiel van der Klis, Guglielmo Mastroserio, Javier A. García, Thomas Dauser, Riley Connors, Andrew C. Fabian, James F. Steiner, Ron A. Remillard, Edward M. Cackett, Phil Uttley, Diego Altamirano in The Astrophysical Journal

Scattered across our Milky Way galaxy are tens of millions of black holes — immensely strong gravitational wells of spacetime, from which infalling matter, and even light, can never escape. Black holes are dark by definition, except on the rare occasions when they feed. As a black hole pulls in gas and dust from an orbiting star, it can give off spectacular bursts of X-ray light that bounce and echo off the inspiraling gas, briefly illuminating a black hole’s extreme surroundings.

Now MIT astronomers are looking for flashes and echoes from nearby black hole X-ray binaries — systems with a star orbiting, and occasionally being eaten away by, a black hole. They are analyzing the echoes from such systems to reconstruct a black hole’s immediate, extreme vicinity.

In a study, the researchers report using a new automated search tool, which they’ve coined the “Reverberation Machine,” to comb through satellite data for signs of black hole echoes. In their search, they have discovered eight new echoing black hole binaries in our galaxy. Previously, only two such systems in the Milky Way were known to emit X-ray echoes.

In comparing the echoes across systems, the team has pieced together a general picture of how a black hole evolves during an outburst. Across all systems, they observed that a black hole first undergoes a “hard” state, whipping up a corona of high-energy photons along with a jet of relativistic particles that is launched away at close to the speed of light. The researchers discovered that at a certain point, the black hole gives off one final, high-energy flash, before transitioning to a “soft,” low-energy state.

This final flash may be a sign that a black hole’s corona, the region of high-energy plasma just outside a black hole’s boundary, briefly expands, ejecting a final burst of high-energy particles before disappearing entirely. These findings could help to explain how larger, supermassive black holes at the center of a galaxy can eject particles across vastly cosmic scales to shape a galaxy’s formation.

“The role of black holes in galaxy evolution is an outstanding question in modern astrophysics,” says Erin Kara, assistant professor of physics at MIT. “Interestingly, these black hole binaries appear to be ‘mini’ supermassive black holes, and so by understanding the outbursts in these small, nearby systems, we can understand how similar outbursts in supermassive black holes affect the galaxies in which they reside.”

The study’s first author is MIT graduate student Jingyi Wang; other co-authors include Matteo Lucchini and Ron Remillard at MIT, along with collaborators from Caltech and other institutions.

Hardness-intensity diagram (HID), where the arrows show the typical temporal evolution of BHLMXBs through the four different states (hard state, HIMS, SIMS, and soft state) represented by different colors. The HID is encompassed by representative flux-energy spectrum and power spectrum of each state, using the NICER data of the BHLMXB MAXI J1820+070. The flux-energy spectrum (solid line) is fitted phenomenologically with the sum of a thermal disk emission (diskbb, dashed line), the coronal emission approximated by a power law (dashed–dotted line), and the relativistic iron line approximated by a Gaussian line (dotted line).

Kara and her colleagues are using X-ray echoes to map a black hole’s vicinity, much the way that bats use sound echoes to navigate their surroundings. When a bat emits a call, the sound can bounce off an obstacle and return to the bat as an echo. The time it takes for the echo to return is relative to the distance between the bat and the obstacle, giving the animal a mental map of its surroundings.

In similar fashion, the MIT team is looking to map the immediate vicinity of a black hole using X-ray echoes. The echoes represent time delays between two types of X-ray light: light emitted directly from the corona, and light from the corona that bounces off the accretion disk of inspiraling gas and dust.

The time when a telescope receives light from the corona, compared to when it receives the X-ray echoes, gives an estimate of the distance between the corona and the accretion disk. Watching how these time delays change can reveal how a black hole’s corona and disk evolve as the black hole consumes stellar material.

Spectral states and systematically compare the reverberation lag properties using the power-spectral hue in the PC diagram.

In their new study, the team developed search algorithm to comb through data taken by NASA’s Neutron star Interior Composition Explorer, or NICER, a high-time-resolution X-ray telescope aboard the International Space Station. The algorithm picked out 26 black hole X-ray binary systems that were previously known to emit X-ray outbursts. Of these 26, the team found that 10 systems were close and bright enough that they could discern X-ray echoes amid the outbursts. Eight of the 10 were previously not known to emit echoes.

“We see new signatures of reverberation in eight sources,” Wang says. “The black holes range in mass from five to 15 times the mass of the sun, and they’re all in binary systems with normal, low-mass, sun-like stars.”

As a side project, Kara is working with MIT education and music scholars, Kyle Keane and Ian Condry, to convert the emission from a typical X-ray echo into audible sound waves.

The researchers then ran the algorithm on the 10 black hole binaries and divided the data into groups with similar “spectral timing features,” that is, similar delays between high-energy X-rays and reprocessed echoes. This helped to quickly track the change in X-ray echoes at every stage during a black hole’s outburst.

The team identified a common evolution across all systems. In the initial “hard” state, in which a corona and jet of high-energy particles dominates the black hole’s energy, they detected time lags that were short and fast, on the order of milliseconds. This hard state lasts for several weeks. Then, a transition occurs over several days, in which the corona and jet sputter and die out, and a soft state takes over, dominated by lower-energy X-rays from the black hole’s accretion disk.

During this hard-to-soft transition state, the team discovered that time lags grew momentarily longer in all 10 systems, implying the distance between the corona and disk also grew larger. One explanation is that the corona may briefly expand outward and upward, in a last high-energy burst before the black hole finishes the bulk of its stellar meal and goes quiet.

“We’re at the beginnings of being able to use these light echoes to reconstruct the environments closest to the black hole,” Kara says. “Now we’ve shown these echoes are commonly observed, and we’re able to probe connections between a black hole’s disk, jet, and corona in a new way.”

Millisecond pulsars from accretion-induced collapse as the origin of the Galactic Centre gamma-ray excess signal

by Anuj Gautam, Roland M. Crocker, Lilia Ferrario, Ashley J. Ruiter, Harrison Ploeg, Chris Gordon, Oscar Macias in Nature Astronomy

Researchers from The Australian National University (ANU) have found an alternative explanation for a mysterious gamma-ray signal coming from the centre of the galaxy, which was long claimed as a signature of dark matter.

Gamma-rays are the form of electromagnetic radiation with the shortest wavelength and highest energy.

Co-author of the study Associate Professor Roland Crocker said this particular gamma-ray signal — known as the Galactic Centre Excess — may actually come from a specific type of rapidly-rotating neutron star, the super-dense stellar remnants of some stars much more massive than our sun.

The Galactic Centre Excess is an unexpected concentration of gamma-rays emerging from the centre of our galaxy that has long puzzled astronomers.

“Our work does not throw any doubt on the existence of the signal, but offers another potential source,” Associate Professor Crocker said. “It is based on millisecond pulsars — neutron stars that spin really quickly — around 100 times a second.
“Scientists have previously detected gamma-ray emissions from individual millisecond pulsars in the neighbourhood of the solar system, so we know these objects emit gamma-rays. Our model demonstrates that the integrated emission from a whole population of such stars, around 100,000 in number, would produce a signal entirely compatible with the Galactic Centre Excess.”

The discovery may mean scientists have to re-think where they look for clues about dark matter.

“The nature of dark matter is entirely unknown, so any potential clues garner a lot of excitement,” Associate Professor Crocker said. “But our results point to another important source of gamma-ray production.
“For instance, the gamma-ray signal from Andromeda, the next closest large galaxy to our own may be mostly due to millisecond pulsars.”

Seven Years of SN 2014C: a Multi-Wavelength Synthesis of an Extraordinary Supernova

by Benjamin P. Thomas, J. Craig Wheeler, Vikram V. Dwarkadas, Christopher Stockdale, Jozsef Vinko, David Pooley, Yerong Xu, Greg Zeimann, Phillip MacQueen in The Astrophysical Journal

An international group of astronomers led by Benjamin Thomas of The University of Texas at Austin has used observations from the Hobby-Eberly Telescope (HET) at the university’s McDonald Observatory to unlock a puzzling mystery about a stellar explosion discovered several years ago and evolving even now. The results will help astronomers better understand the process of how massive stars live and die.

When an exploding star is first detected, astronomers around the world begin to follow it with telescopes as the light it gives off changes rapidly over time. They see the light from a supernova get brighter, eventually peak, and then start to dim. By noting the times of these peaks and valleys in the light’s brightness, called a “light curve,” as well as the characteristic wavelengths of light emitted at different times, they can deduce the physical characteristics of the system.

“I think what’s really cool about this kind of science is that we’re looking at the emission that’s coming from matter that’s been cast off from the progenitor system before it exploded as a supernova,” Thomas said. “And so this makes a sort of time machine.”

In the case of supernova 2014C, the progenitor was a binary star, a system in which two stars were orbiting each other. The more massive star evolved more quickly, expanded, and lost its outer blanket of hydrogen to the companion star. The first star’s inner core continued burning lighter chemical elements into heavier ones, until it ran out of fuel. When that happened, the outward pressure from the core that had held up the star’s great weight dropped. The star’s core collapsed, triggering a gigantic explosion. This makes it a type of supernova astronomers call a “Type Ib.” In particular, Type Ib supernovae are characterized by not showing any hydrogen in their ejected material, at least at first.

Thomas and his team have been following SN 2014C from telescopes at McDonald Observatory since its discovery that year. Many other teams around the world also have studied it with telescopes on the ground and in space, and in different types of light, including radio waves from the ground-based Very Large Array, infrared light, and X-rays from the space-based Chandra Observatory. But the studies of SN 2014C from all of the various telescopes did not add up into a cohesive picture of how astronomers thought a Type Ib supernova should behave.

For one thing, the optical signature from the Hobby-Eberly Telescope (HET) showed SN 2014C contained hydrogen — a surprising finding that also was discovered independently by another team using a different telescope.

“For a Type Ib supernova to begin showing hydrogen is completely weird,” Thomas said. “There’s just a handful of events that have been shown to be similar.”

For a second thing, the optical brightness (light curve) of that hydrogen was behaving strangely. Most of the light curves from SN 2014C — radio, infrared, and X-rays — followed the expected pattern: they got brighter, peaked, and started to fall. But the optical light from the hydrogen stayed steady.

“The mystery that we’ve wrestled with has been ‘How do we fit our Texas HET observations of hydrogen and its characteristics into that [Type Ib] picture?’,” said UT Austin professor and team member J. Craig Wheeler.

The problem, the team realized, was that previous models of this system assumed that the supernova had exploded and sent out its shockwave in a spherical manner. The data from HET showed that this hypothesis was impossible — something else must have happened.

“It just would not fit into a spherically symmetric picture,” Wheeler said.

The team proposes a model where the hydrogen envelopes of the two stars in the progenitor binary system merged to form a “common-envelope configuration,” where both were contained within a single envelope of gas. The pair then expelled that envelope in an expanding, disk-like structure surrounding the two stars. When one of the stars exploded, its fast-moving ejecta collided with the slow-moving disk, and also slid along the disk surface at a “boundary layer” of intermediate velocity.

The team suggests that this boundary layer is the origin of the hydrogen they detected and then studied for seven years with HET. Thus the HET data turned out to be the key that unlocked the mystery of supernova SN 2014C.

“In a broad sense, the question of how massive stars lose their mass is the big scientific question we were pursuing,” Wheeler said. “How much mass? Where is it? When was it ejected? By what physical process? Those were the macro questions we were going after.
“And 2014C just turned out to be a really important single event that’s illustrating the process,” Wheeler said.

Longitudinal MRI-visible perivascular space (PVS) changes with long-duration spaceflight

by Kathleen E. Hupfeld, Sutton B. Richmond, Heather R. McGregor, et al in Scientific Reports

Long-duration space flight alters fluid-filled spaces along veins and arteries in the brain, according to new research from Oregon Health & Science University and scientists across the country.

“These findings have important implications as we continue space exploration,” said senior author Juan Piantino, M.D., assistant professor of pediatrics (neurology) in the OHSU School of Medicine. “It also forces you to think about some basic fundamental questions of science and how life evolved here on Earth.”

PVSs identified on a single astronaut. Here we depict a binary mask of PVSs for a single astronaut at the last pre-flight time point (Launch-60 days), for illustrative purposes. PVSs are shown in red, overlaid onto this individual’s native space 3D-rendered white matter segment (left) as well as several sagittal slices of their skull stripped native space structural scan (right). The blue box shows a zoomed-in view of one PVS, inside the blue circle.

The research involved imaging the brains of 15 astronauts before and after extended tours of duty on the International Space Station. Researchers used magnetic resonance imaging to measure perivascular space — or the space around blood vessels — in the brains of astronauts prior to their launch and again immediately after their return. They also took MRI measurements again at one, three and six months after they had returned. Astronauts’ images were compared with those taken of the same perivascular space in the brains of 16 Earth-bound control subjects. Comparing before and after images, they found an increase in the perivascular spaces within the brains of first-time astronauts, but no difference among astronauts who previously served aboard the space station orbiting earth.

“Experienced astronauts may have reached some kind of homeostasis,” Piantino said.

In all cases, scientists found no problems with balance or visual memories that might suggest neurological deficits among astronauts, despite the differences measured in the perivascular spaces of their brains. In comparing a large group of deidentified astronauts, the study is the first to comparatively assess an important aspect of brain health in space.

Changes in PVS metrics and ventricular volume from pre- to post-flight.

Human physiology is based on the fact that life evolved over millions of years while tethered to Earth’s gravitational pull. Unbound by the forces of gravity, the normal flow of cerebrospinal fluid in the brain is altered in space.

“We all adapted to use gravity in our favor,” Piantino said. “Nature didn’t put our brains in our feet — it put them high up. Once you remove gravity from the equation, what does that do to human physiology?”

Researchers decided to find out by measuring perivascular spaces, where cerebrospinal fluid flows in the brain. These spaces are integral to a natural system of brain cleansing that occurs during sleep. Known as the glymphatic system, this brain-wide network clears metabolic proteins that would otherwise build up in the brain. Scientists say this system seems to perform optimally during deep sleep.

The perivascular spaces measured in the brain amount to the underlying “hardware” of the glymphatic system. Enlargement of these spaces occurs in aging, and also has been associated with the development of dementia.

Researchers used a technique developed in the laboratory of co-author Lisa C. Silbert, M.D., M.C.R., professor of neurology in the OHSU School of Medicine, to measure changes in these perivascular spaces through MRI scans. Piantino said the study could be valuable in helping to diagnose and treat Earth-bound disorders involving cerebrospinal fluid, such as hydrocephalus.

“These findings not only help to understand fundamental changes that happen during space flight, but also for people on Earth who suffer from diseases that affect circulation of cerebrospinal fluid,” Piantino said.

Mantle Degassing Lifetimes through Galactic Time and the Maximum Age Stagnant-lid Rocky Exoplanets Can Support Temperate Climates

by Cayman T. Unterborn, Bradford J. Foley, Steven J. Desch, Patrick A. Young, Gregory Vance, Lee Chiffelle, Stephen R. Kane in The Astrophysical Journal Letters

As the scientific community searches for worlds orbiting nearby stars that could potentially harbor life, new Southwest Research Institute-led research suggests that younger rocky exoplanets are more likely to support temperate, Earth-like climates.

In the past, scientists have focused on planets situated within a star’s habitable zone, where it is neither too hot nor too cold for liquid surface water to exist. However, even within this so-called “Goldilocks zone,” planets can still develop climates inhospitable to life. Sustaining temperate climates also requires a planet have sufficient heat to power a planetary-scale carbon cycle. A key source of this energy is the decay of the radioactive isotopes of uranium, thorium and potassium. This critical heat source can power a rocky exoplanet’s mantle convection, a slow creeping motion of the region between a planet’s core and crust that eventually melts at the surface. Surface volcanic degassing is a primary source of CO2 to the atmosphere, which helps keep a planet warm. Without mantle degassing, planets are unlikely to support temperate, habitable climates like the Earth’s.

Histograms of measured abundances of Th (left), Eu (as a proxy for U; middle), and K (right) for samples of stars.

“We know these radioactive elements are necessary to regulate climate, but we don’t know how long these elements can do this, because they decay over time,” said Dr. Cayman Unterborn, lead author of a paper about the research. “Also, radioactive elements aren’t distributed evenly throughout the Galaxy, and as planets age, they can run out of heat and degassing will cease. Because planets can have more or less of these elements than the Earth, we wanted to understand how this variation might affect just how long rocky exoplanets can support temperate, Earth-like climates.”

Studying exoplanets is challenging. Today’s technology cannot measure the composition of an exoplanet’s surface, much less that of its interior. Scientists can, however, measure the abundance of elements in a star spectroscopically by studying how light interacts with the elements in a star’s upper layers. Using these data, scientists can infer what a star’s orbiting planets are made of using stellar composition as a rough proxy for its planets.

“Using host stars to estimate the amount of these elements that would go into planets throughout the history of the Milky Way, we calculated how long we can expect planets to have enough volcanism to support a temperate climate before running out of power,” Unterborn said. “Under the most pessimistic conditions we estimate that this critical age is only around 2 billion years old for an Earth-mass planet and reaching 5–6 billion years for higher-mass planets under more optimistic conditions. For the few planets we do have ages for, we found only a few were young enough for us to confidently say they can have surface degassing of carbon today, when we’d observe it with, say, the James Webb Space Telescope.”

Probability that the host-star/planet age is (left) and ≤Age (right) for the 17 planets in sample as a function of their mass and system age.

This research combined direct and indirect observational data with dynamical models to understand which parameters most affect an exoplanet’s ability to support a temperate climate. More laboratory experiments and computational modeling will quantify the reasonable range of these parameters, particularly in the era of the James Webb Space Telescope, which will provide more in-depth characterization of individual targets. With the Webb telescope, it will be possible to measure the three-dimensional variation of exoplanet atmospheres. These measurements will deepen the knowledge of atmospheric processes and their interactions with the planet’s surface and interior, which will allow scientists to better estimate whether a rocky exoplanet in habitable zones is too old to be Earth-like.

“Exoplanets without active degassing are more likely to be cold, snowball planets,” Unterborn said. “While we can’t say the other planets aren’t degassing today, we can say that they would require special conditions to do so, such as having tidal heating or undergoing plate tectonics. This includes the high-profile rocky exoplanets discovered in the TRAPPIST-1 star system. Regardless, younger planets with temperate climates may be the simplest places to look for other Earths.”

Characterization of Candidate Solutions for X-Ray Pulsar Navigation

by Kevin Lohan, Zachary Putnam in IEEE Transactions on Aerospace and Electronic Systems

The remnants of a collapsed neutron star, called a pulsar, are magnetically charged and spinning anywhere from one rotation per second to hundreds of rotations per second. These celestial bodies, each 12 to 15 miles in diameter, generate light in the x-ray wavelength range. Researchers at The Grainger College of Engineering, University of Illinois Urbana-Champaign developed a new way spacecraft can use signals from multiple pulsars to navigate in deep space.

“We can use star trackers to determine the direction a spacecraft is pointing, but to learn the precise location of the spacecraft, we rely on radio signals sent between the spacecraft and the Earth, which can take a lot of time and requires use of oversubscribed infrastructure, like NASA’s Deep Space Network,” said Zach Putnam, professor in the Department of Aerospace Engineering at Illinois.

“Using x-ray navigation eliminates those two factors, but until now, required an initial position estimate of the spacecraft as a starting point. This research presents a system that finds candidates for possible spacecraft locations without prior information, so the spacecraft can navigate autonomously.”

Image credit: X-ray: NASA/CXC/University of Amsterdam/N.Rea et al; Optical: DSS

“Also, our ground communication systems for deep space missions are overloaded right now,” he said. “This system would give spacecraft autonomy and reduce the dependency on the ground. X-ray pulsar navigation gets us around that and allows us to determine where we are, without calling.”

Putnam said because our atmosphere filters out all the x-rays, you have to be in space to observe them. The pulsars emit electromagnetic radiation that look like pulses because we measure the peak in the x-ray signals every time the pulsar spins around and points toward us — like the ray of light cast from the beacon on a lighthouse.

“Each pulsar has its own characteristic signal, like a fingerprint,” he said. “We have records of the x-rays over time from the 2,000 or so pulsars and how they’ve changed over time.”

Much like the Global Positioning System, location can be determined from intersection of three signals.

“The issue with pulsars is that they spin so fast that the signal repeats itself a lot,” he said. “By comparison, GPS repeats every two weeks. With pulsars, while there are an infinite number of possible spacecraft locations, we know how far apart these candidate locations are from each other.

Intersection of banded regions of three pulsars in 3D.

“We are looking at determining spacecraft position within domains that have diameters on the order of multiple astronomical units, like the size of the orbit of Jupiter — something like a square with one billion miles on a side. The challenge we are trying to address is, how do we intelligently observe pulsars and fully determine all possible spacecraft locations in a domain without using an excessive amount of compute resources,” Putnam said.

The algorithm developed by graduate student Kevin Lohan combines observations from numerous pulsars to determine all the possible positions of the spacecraft. The algorithm processes all the candidate intersections in two dimensions or three dimensions.

“We used the algorithm to study which pulsars we should observe to reduce the number of candidate spacecraft locations within a given domain,” said Putnam. Results showed that observing sets of pulsars with longer periods and small angular separations could significantly reduce the number of candidate solutions within a given domain.