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19 April 2019

Compact Objects Charging Toward Merger

When two compact objects — neutron stars or black holes — merge, will they emit light? A recent study looks at a neglected factor that could affect the answer: electric charge.

Dark or Light?

Artist’s impression of two merging neutron stars producing a gamma-ray burst. [National Science Foundation/LIGO/Sonoma State University/A. Simonnet]

Most theories agree that a compact binary containing a neutron star can emit light when it merges. This is because these systems contain lots of neutron-rich matter that can then radiate in the final stages of merger, in the form of gamma-ray bursts, kilonovae, and afterglows.

But what about compact binaries containing two black holes? Or so-called “plunging” black-hole–neutron-star mergers in which the neutron star plunges directly into the black hole before it can be disrupted? Are these mergers all doomed to darkness?

Possible Charge

Not according to Bing Zhang, a scientist at University of Nevada Las Vegas. Recently, Zhang proposed that black holes might carry electric charge in a surrounding magnetosphere. As charged black holes spiral around and around each other during a merger, they could generate electromagnetic radiation: a characteristic signal that rises sharply just before merger.

Now Zhang is back with a generalized model for the merger of charged compact objects, which also explores possible signatures from electrically charged neutron stars. In a new study, he works out the details and reports on where we might be able to detect these signals.

Searching for a Signal

All compact binaries containing a neutron star should emit radiation from electric charge, since neutron stars are definitely charged — they’re essentially spinning magnets. But for most systems containing a neutron star, Zhang demonstrates, the radiation associated with the object’s charge will be non-detectable, since it’s so much dimmer than other electromagnetic signatures from merger (like a gamma-ray burst).

The Crab pulsar is a highly magnetized, spinning neutron star that powers the Crab nebula seen in this composite image. [X-ray: NASA/CXC/SAO/F.Seward; Optical: NASA/ESA/ASU/J.Hester & A.Loll; Infrared: NASA/JPL-Caltech/Univ. Minn./R.Gehrz]

There’s hope, though, in the scenario of a plunging neutron-star–black-hole merger. If the neutron star is less than 20% the size of the black hole, it can be consumed whole, preventing any of the typical electromagnetic signatures from occurring. In this case, the radiation from the charged, inspiralling neutron star is the only electromagnetic signal present.

If the neutron star in such a system has a magnetic field similar to that of the Crab pulsar — possible in young star clusters — the charge signal can reach detectable levels, according to Zhang’s calculations. In fact, it’s possible that we could observe such a signal as a fast radio burst, the mysterious millisecond radio bursts that we’ve seen originating from beyond our galaxy.

Looking Ahead

Many unknowns are still present in this picture. How is the electric radiation converted into observable emission? How commonly do we expect plunging neutron-star–black-hole mergers to occur as described? Will we be able to link radiation from charged mergers to a gravitational-wave chirp?

One thing is for certain: if we can, indeed, observe the light from charge in a compact-binary merger, this would provide an exciting new opportunity to further probe these distant, exotic systems.

Citation

“Charged Compact Binary Coalescence Signal and Electromagnetic Counterpart of Plunging Black Hole–Neutron Star Mergers,” Bing Zhang 2019 ApJL 873 L9. doi:10.3847/2041-8213/ab0ae8

17 April 2019

Exploring Filaments on the Sun

Images of the Sun’s chromosphere often reveal dark threads cutting across the Sun’s face. New research has now explored how these solar filaments are built from magnetic fields and plasma.

Two-Faced Structures

A solar eruptive prominence as seen in extreme UV light on March 30, 2010, with Earth superimposed for a sense of scale. [NASA/SDO]

Solar filaments may look like deep cracks in the Sun’s façade, but in reality, they are enormous arcs of hot plasma that extend above the Sun’s surface. Because this plasma is slightly cooler than the solar surface below, they appear dark against the hotter background.

Unfamiliar with filaments? You’ve likely seen plenty of them in images — but from a different angle! Filaments are the same structures as solar prominences, the loops of plasma we can see suspended above the Sun’s limbs. When prominences appear on the side of the Sun facing us, they take the form of filaments from our point of view.

Shaped by Fields

Filaments are often associated with various forms of solar activity. They last for days, frequently hanging above active regions of the Sun; filament channels are often the origin of eruptions from the Sun’s surface. To better understand our active and energetic Sun, understanding the structures of filaments is an important step.

Unfortunately, this is challenging! We know that filament structure is largely due to the magnetic fields — whose forces suspend the filaments against the downward pull of gravity — but we don’t have the ability to directly measure the magnetic field in the Sun’s atmosphere. A team of scientists at the University of Science and Technology of China has instead taken an indirect approach: they explored filaments by looking at the motion of plasma along them.

Top: time-distance map characterizing the oscillations at one position on the filament spine. Bottom: a Doppler map, averaged over time, that shows the rotation around the spine of the filament. Blue indicates motion toward the observer, red away. [Adapted from Awasthi et al. 2019]

A Double Decker?

Scientists Arun Awasthi, Rui Liu, and Yuming Wang examined observations of a filament that appeared near active region AR 12685 in October 2017, captured with the 1-m New Vacuum Solar Telescope in China. The team used these high-resolution images to explore bulk motions of plasma in the filament.

Awasthi and collaborators found that the filament displayed two different types of motion: rotation around its central spine, and longitudinal oscillations along its spine. The longitudinal oscillations in the eastern segment of the filament were distinct from those in the west, suggesting that the magnetic field lines underneath these two segments have different lengths and curvatures.

On the whole, the motions observed in the filament indicate that magnetic structure for filaments is complicated. The authors argue that more than one model is likely at work; they propose a “double-decker” picture for the filament in which a flux rope (a twisted bundle of magnetic field lines) sits on top of a sheared arcade (a series of distorted loops).

Proposed “double-decker” configuration of the magnetic field hosting the filament, consisting of a flux rope (red) atop a sheared arcade (blue). Left panel shows the cross section viewed from the west; right panel shows the structure viewed from above. [Adapted from Awasthi et al. 2019]

Awasthi and collaborators conclude with specific predictions of indicators we can look for in future filament observations to test this model. If correct, this view of filament structure brings us a little closer to understanding the complex magnetic fields that control solar activity.

Bonus

Check out the video below showing the motions in the filament in different wavelengths. Top panel: GOES light curve and an HMI magnetogram. Bottom panels: AIA 171 Å image, GONG Hα image, and NVST Hα image.

Citation

“Double-decker Filament Configuration Revealed by Mass Motions,” Arun Kumar Awasthi et al 2019 ApJ 872 109. doi:10.3847/1538-4357/aafdad

15 April 2019

NuSTAR Explores the Aftermath of a Supernova

There’s plenty to learn from the skeletons left behind after supernova explosions tear through their surroundings. An X-ray view from space has revealed new details about a particularly extreme supernova remnant.

Unexpected Plasmas

NuSTAR observations showing the spatial distribution of flux that indicates where overionized plasma resides in supernova remnant W49B. The overionized plasma is more highly concentrated on the western side of the remnant. [Yamaguchi et al. 2018]

When some stars explode as powerful supernovae at the end of their lifetimes, they expel material into their surroundings, enriching the galaxy with heavy elements. As this matter is flung outwards at high speeds, it slams into the interstellar medium, generating shocks that heat the gas and ionize it.

We can study the young remnants of supernovae — the structures of gas and dust left behind shortly after these explosions — to learn more about how supernovae interact with the interstellar medium. One type of source is particularly intriguing: very young, hot supernova remnants that are “overionized”.

Overionized plasmas send us mixed signals: their level of ionization is higher than what we expect from the temperature we measure from their electrons. This is most likely an indication that the plasma has recently started cooling very rapidly.

But what might cause this sudden cooling of the remnant? To learn more, we need detailed observations of a young, hot remnant. Luckily, we’ve got an ideal target — and an ideal instrument.

Artist’s illustration of the NuSTAR spacecraft. [NASA]

Setting Sights

Supernova remnant W49B is one of the first sources in which we discovered signs of an overionized plasma. It’s the youngest (just ~1,000 years old!), hottest, and most highly ionized among all such objects exhibiting this trait. But W49B’s hot plasma is challenging to observe, and we haven’t yet managed to constrain its detailed high-energy properties.

A powerful telescope is up to the task, however: the Nuclear Spectroscopic Telescope Array (NuSTAR). This versatile space observatory is ideally suited for exploring the spectroscopic details of the X-rays emitted from the hot plasma in W49B.

Sudden Expansion

Top panel: Diagram of the 1’ x 1’ box regions NuSTAR resolved for spectral analysis (overlaid on a color image of W49B from the Wide Field Infrared Camera at Palomar Observatory). Bottom panel: Plot of the electron temperature (bottom) vs. the density (right) measured for each of the labeled regions. [Adapted from Yamaguchi et al. 2018]

Led by Hiroya Yamaguchi (Institute of Space and Astronautical Science, JAXA, Japan), a team of scientists used NuSTAR to capture detailed images and spectroscopy of the W49B remnant.

Yamaguchi and collaborators first confirmed that the overionized plasma is most highly concentrated on the western side of the remnant. They then show that lower electron temperatures — i.e., signs of rapid cooling — are found in the same regions that also have lower density. They measure a gradient from lowest electron temperature and density in the west, to highest in the east.

Taken together with previous observations that reveal that W49B’s surroundings also have lower density on the western side, these results provide strong evidence that the remnant is cooling via adiabatic expansion. In this picture, the supernova blast wave punched through dense circumstellar matter in early stages of the explosion, expanding slowly. Now it’s suddenly breaking out into the lower-density interstellar medium — on the west side first, because it’s not exactly symmetric — leading to sudden expansion and cooling.

Does this explanation apply to overionized plasmas in the skeletons of other, similar supernovae? We’ll need more observations to be sure — but NuSTAR has proven itself up to the task!

Citation

“Evidence for Rapid Adiabatic Cooling as an Origin of the Recombining Plasma in the Supernova Remnant W49B Revealed by NuSTAR Observations,” Hiroya Yamaguchi et al 2018 ApJL 868 L35. doi:10.3847/2041-8213/aaf055

12 April 2019

A Faster Way to Form Planets

When interstellar asteroid ‘Oumuamua sped through our solar system in 2017, it revealed that small bodies might make a habit of visiting other planetary systems. Could such objects occasionally be responsible for jump-starting planet formation?

Planet Formation in the Works

An artist’s impression of interstellar asteroid 1I/2017 U1 (‘Oumuamua). [European Southern Observatory / M. Kornmesser]

One of the leading models of planet formation is core accretion, in which dust grains come together to form pebbles, planetesimals, and planets. While the core accretion model explains many features that we observe in protoplanetary disks, it’s not yet clear exactly how dust grains clump together to form larger objects.

It’s also expected to take up to tens of millions of years to form a planet, which clashes with observations of planets forming in as little as a few hundred thousand years in disks that dissipate after just a few million years.

Many models of planet formation show that once planetesimals in a disk reach a certain size, they rapidly accrete gas to form planets. What if core accretion could get a head start from long-lost planetesimals ejected from other stellar systems?

A qualitative illustration of the timeline and mechanisms of interstellar object production. The times here are typical for a solar-mass star. Higher-mass stars would begin the final stage of interstellar object production earlier. Click to enlarge. [Pfalzner & Bannister 2019]

One System’s Trash, Another System’s Treasure?

To estimate how the population of interstellar objects affects the formation of new planets, Susanne Pfalzner (Jülich Supercomputing Center and Max Planck Institute for Radio Astronomy, Germany) and Michele Bannister (Queen’s University Belfast, UK) calculated the number of interstellar objects that survive the journey from their nascent star systems and become incorporated into new systems.

They began with an estimate of ‘Oumuamua-sized objects based on 30 years of observations from the Pan-STARRS1/Catalina Sky Survey: 10¹⁵ objects in a cubic parsec (about 35 cubic light-years).

The authors estimated that only a few percent of these objects can be easily captured by molecular clouds, and even fewer make it into the surroundings of a young star, which consumes all but 1–10% of the interstellar objects capable of acting as seeds for planet formation.

Plentiful Planetesimals

Density of 100-meter interstellar objects over the course of the star and planet formation process. [Pfalzner & Bannister 2019]

Of the initial 10¹⁵ interstellar objects found in a single cubic parsec, the authors estimate that more than 10 million are able to be captured by molecular clouds, survive collisions, withstand heating, and become incorporated into a protoplanetary disk to participate in accretion. Most are about 100 meters in size, but as many as 10⁵ are kilometer sized and a thousand are 100 km or larger.

It’s not yet clear what role the abundant 100-meter objects play in planet formation, but simulations have shown that planetesimals larger than ~200 km rapidly accrete gas from their surroundings, forming both terrestrial and gas-giant planets in a blazingly fast 1,000 years.

Since the number of interstellar objects increases over time, the number of potential seeds for planet formation increases as well, meaning that planet formation is likely faster now than it was billions of years ago. There’s still plenty of work to be done, but one thing is clear: we can’t ignore the importance of interstellar objects in the planet formation process.

Citation

“A Hypothesis for the Rapid Formation of Planets,” S. Pfalzner & M. T. Bannister 2019 ApJL 874 L34. doi:10.3847/2041-8213/ab0fa0

10 April 2019

First Images of a Black Hole from the Event Horizon Telescope

Astronomers have used a telescope that spans the globe to capture the first detailed images of a black hole: the nearby supermassive black hole in the Messier 87 galaxy. The first results from the Event Horizon Telescope (EHT) are detailed in six articles that make up a new Focus Issue in The Astrophysical Journal Letters.

Hubble image of the elliptical galaxy M87, dominated by the visible jet extending to the northwest of the supermassive black hole at the galaxy’s center (click to enlarge). [NASA/ESA/Hubble Heritage Team (STScI/AURA)/P. Cote (Herzberg Institute of Astrophysics)/E. Baltz (Stanford University)]

Why M87?

Obtaining an up-close view of a black hole is a goal that has long remained out of reach. When astronomers first developed a plan to image a supermassive black hole’s event horizon — the close-in boundary from which not even light can escape — two sources were selected as targets: the black hole at the center of our galaxy, Sgr A*; and the gargantuan, jet-producing black hole in the neighboring galaxy M87.

Sgr A*, while the nearest supermassive black hole, suffers from a host of complicating factors for imaging. This monster’s rapid variability and its position in the galactic plane — where it’s blurred by the interstellar medium — provide additional challenges that must be overcome to accurately capture it.

M87 may be further away, but its black hole is a thousand times larger than Sgr A*, giving it longer and more manageable variability timescales. We’re also peering through much less of the interstellar medium when pointed at M87.

These factors contribute to making the neighboring behemoth in M87 comparatively accessible for imaging — and, consequently, it’s the first source for which the EHT is presenting images.

A Planet-Scale Telescope

So how do we zoom in on this distant object? We need a telescope unlike any other.

The eight stations of the EHT 2017 campaign over six geographic locations. [EHT Collaboration et al 2019]

The EHT is an extensive virtual telescope created by combining simultaneous observations from radio arrays and dishes all around the planet. For the images of M87 released today, the observations were made by eight ground-based telescopes in Arizona, Hawai’i, Mexico, Chile, Spain, and the South Pole.

The EHT works by performing very-long-baseline interferometry; by combining different telescopes around the world, the EHT can function like a telescope with an effective size that’s the same as its longest baseline — the distance between component telescopes. In this way, the EHT is able to achieve unprecedented resolution: it can theoretically resolve down to 25 millionths of an arcsecond at its observing wavelength of 1.3 mm.

It’s been more than a decade since the EHT’s bold imaging plans were first begun. Scientists have patiently waited as existing facilities were upgraded and new facilities have been built — and in April 2017, conditions were finally right to obtain the first good look at M87’s event horizon.

Images of a Shadow

M87’s black hole was observed on four days in April 2017. Weather was uniformly good — planet-wide! — during those observations, allowing EHT scientists to combine the data from the eight telescopes and reconstruct images of the black hole.

EHT observations of M87 taken over 4 days reveal a bright, asymmetric ring; north is up and east is left. The images remain consistent over the observations, providing evidence of a stable source. [EHT Collaboration et al 2019]

What they saw was spot-on with predictions: a ring of light spanning ~38–44 µas, with the southern part of the ring appearing brighter than the rest.

While a naked black hole would simply appear dark in an image, an active black hole like M87 is surrounded by dust and gas that forms an accreting disk, as well as funnels at the base of its powerful jets. Images of such an encased black hole were predicted to reveal a dark region — the black hole’s “shadow” — surrounded by a ring of emission produced by the distorted paths of light from the surrounding material. The EHT observations of M87 beautifully confirm this picture!

Why is the southern part of the ring brighter? As fast-moving material rotates around the black hole, it speeds toward us on one side and away from us on the other. On the side of the ring where matter moves toward us, a relativistic effect beams light in our direction, causing this region to appear brighter.

From the asymmetry of the ring, EHT scientists determine that matter on the south side of M87’s black hole is moving toward us. Combined with previous observations of M87’s jet, which show it’s inclined at an angle of 17° relative to our line of sight, this tells us that M87’s black hole likely spins clockwise from our point of view, with its spin axis pointed away from us at an angle.

Three of the authors’ example models of spinning black holes (top row) and their simulated observations (bottom row). All three models produce similar simulated observations that well-match the real observations, indicating a single good fit doesn’t imply that one model is preferred over the others. [EHT Collaboration et al 2019]

Models and Measurements

The EHT’s images of M87’s black hole were compared to an extensive library of synthetic observations obtained from simulations of this source under varying models and conditions. The simulated observations show remarkable consistency with the actual observations, confirming a picture of a spinning supermassive black hole as the source of the emission. The models also suggest the most likely way that M87’s jets are launched: via energy extraction directly from the black hole’s spin.

Previous estimates of the mass for M87’s black hole ranged from ~3–7 billion solar masses, depending on the method used to measure it. From the EHT’s images of M87, the authors were able to estimate a mass for the black hole of 6.5 ± 0.7 billion solar masses, providing an independent check of past measurements.

Future Plans

What’s next for the EHT? These images of M87 are just the start! The team still plans to perform polarimetric analysis of their images, which will probe the magnetic field and help determine the rate of accretion onto M87’s black hole. Future observations will test the stability, shape, and depth of its shadow more accurately. And higher-resolution images will be possible with the addition of new telescopes to the EHT and a possible push to shorter-wavelength observations.

Continued improvements to EHT’s observational technology and analysis techniques should also bring our own supermassive black hole, Sgr A*, into reach. Welcome to a new world of black-hole exploration!

For more information, check out the full ApJL Focus Issue here:
Focus on the First Event Horizon Telescope Results

Citation

“First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole,” EHT Collaboration et al 2019 ApJL 875 L1. doi:10.3847/2041-8213/ab0ec7

5 April 2019

Supernovae, Dark Energy, and the Fate of Our Universe

What’s the eventual fate of our universe? Is spacetime destined to continue to expand forever? Will it fly apart, tearing even atoms into bits? Or will it crunch back in on itself? New results from Dark Energy Survey supernovae address these and other questions.

Uncertain Expansion

The evolution of the scale of our universe; click to enlarge. Measurements suggest that the universe is currently expanding, but does dark energy behaves like a cosmological constant, resulting in continued accelerating expansion like now? Or might we instead be headed for a Big Rip or Big Crunch? [NASA/CXC/M. Weiss]

At present, the fabric of our universe is expanding — and not only that, but the its expansion is accelerating. To explain this phenomenon, we invoke what’s known as dark energy — an unknown form of energy that exists everywhere and exerts a negative pressure, driving the expansion.

Since this idea was first proposed, we’ve conducted decades of research to better understand what dark energy is, how much of it there is, and how it influences our universe.

In particular, dark energy’s still-uncertain equation of state determines the universe’s ultimate fate. If the density of dark energy is constant in time, our universe will continue its current accelerating expansion indefinitely. If the density increases in time, the universe will end in the Big Rip — space will expand at an ever-increasing acceleration rate until even atoms fly apart. And if the density decreases in time, the universe will recollapse in the Big Crunch, ending effectively in a reverse Big Bang.

Which of these scenarios is correct? We’re not sure yet. But there’s a project dedicated to finding out: the Dark Energy Survey (DES).

The Hunt for Supernovae

DES was conducted with the Dark Energy Camera at the Cerro Tololo Inter-American Observatory in Chile. After six years taking data, the survey officially wrapped up observations this past January.

One of DES’s several missions was to make detailed measurements of thousands of supernovae. Type Ia supernovae explode with a prescribed absolute brightness, allowing us to determine their distance from observations. DES’s precise measurements of Type Ia supernovae allow us to calculate the expansion of the space between us and the supernovae, probing the properties of dark energy.

Though DES scientists are still in the process of analyzing the tens of terabytes of data generated by the project, they recently released results from the first three years of data — including the first DES cosmology results based on supernovae.

Refined Measurements

Constraints on the dark energy equation of state w from the DES supernova survey. Combining this data with constraints from the cosmic microwave background radiation suggest an equation of state consistent with a constant density of dark energy (w = –1). [Abbott et al. 2019]

Using a sample of 207 spectroscopically confirmed DES supernovae and 122 low-redshift supernovae from the literature, the authors estimate the matter density of a flat universe to be Ω_m = 0.321 ± 0.018. This means that only ~32% of the universe’s energy density is matter (the majority of which is dark matter); the remaining ~68% is primarily dark energy.

From their observations, the DES team is also able to provide an estimate for the dark-energy equation of state w, finding that w = –0.978 ± 0.059. This result is consistent with a constant density of dark energy (w = –1), which would mean that our universe will continue to expand with its current acceleration indefinitely.

These results are exciting, but they use only ~10% of the supernovae DES discovered over the span of its 5-year survey. This means that we can expect even further refinements to these measurements in the future, as the DES collaboration analyzes the remaining data!

Citation

“First Cosmology Results using Type Ia Supernovae from the Dark Energy Survey: Constraints on Cosmological Parameters,” T. M. C. Abbott et al 2019 ApJL 872 L30. doi:10.3847/2041-8213/ab04fa

3 April 2019

The Variable Jets of Gamma-Ray Bursts

What drives rapid flickering in the jets that are produced in some powerful, high-energy explosions? Recent research explores the role of magnetic fields.

Mapping an Explosion

Artist’s illustration of the gamma-ray-burst jet launched during the merger of two neutron stars in 2017. [NSF/LIGO/Sonoma State University/A. Simonnet]

Gamma-ray bursts — brief flashes of high-energy emission from beyond our galaxy — have been detected since the 1960s. Though we’ve collected many observations of this explosions through the decades, it’s only recently that new evidence has clarified what causes some gamma-ray bursts.

In 2017, the merger of two neutron stars was observed by the Laser Interferometer Gravitational-Wave Observatory (LIGO), just before the detection of a short (less than ~2 seconds) gamma-ray burst from the same location. These observations support the following picture for short gamma-ray bursts:

A neutron-star–neutron-star binary or a neutron-star–black-hole binary merges, generating a potentially observable gravitational-wave signal.
The merger either immediately produces a black hole, or it produces a hypermassive neutron star that collapses into a black hole shortly thereafter.
The remnant material surrounding the newly formed black hole then rapidly accretes, leading to the production of powerful jets along the black-hole rotation axis.

Snapshot from one of the authors’ simulations, in which an axisymmetric jet extends in the +z and -z directions. Only half of the jet is shown, since the simulation is axisymmetric. From left to right, panels represent density, energy distribution, magnetization, and speed of the jet. [Sapountzis & Janiuk 2019]

Jetted Mysteries

This picture, while seemingly straightforward, is loaded with uncertainties. In particular, the jets launched in the third step are not well understood. We’re not sure what drives the production of these jets in the first place, and we also don’t know what collimates the jets, causing them to become tightly beamed as they travel, rather than spraying out in all directions.

What’s more, we observe rapid variability within the gamma-ray-burst jets; for short gamma-ray bursts, the timescales for variability are just ten-thousandths to hundredths of a second! What drives this rapid flickering within the jets?

In a recent study, two researchers at the Center for Theoretical Physics of the Polish Academy of Sciences, Konstantinos Sapountzis and Agnieszka Janiuk, explore the role that magnetic fields might have in the launching and properties of short-gamma-ray-burst jets.

Left column: Variability of jet energy at a point inside the jet as a function of time for five of the authors’ models (each shown in a different row). Right column: same as left, but for a zoomed-in time. Vertical dashed lines show the characteristic timescale of the magnetorotational instability, which matches the jet variability well in all but the bottom model. [Sapountzis & Janiuk 2019]

Magnetic Fields at Work

Sapountzis and Janiuk perform a series of general-relativistic, magnetohydrodynamic simulations of a black hole surrounded by a torus of accreting material.

The authors use these simulations to explore how the magnetic field piles up as hot, ionized gas spirals inward and falls onto the black hole. The building field eventually forms a magnetic barrier that halts the inward flow of gas, leading to the formation of jets along twisting field lines that extend down the black-hole rotation axis.

But the role of the magnetic fields isn’t over with the launch of the jet. In the authors’ simulations, they observe a magnetic instability in the accreting plasma — known as the magnetorotational instability — operating on similar timescales to the variability in gamma-ray-burst jets. This suggests a link between the activity of magnetic fields at the base of the jet and the flickering we observe in the brief gamma-ray-burst jets.

We still have a lot to learn about gamma-ray bursts — and we can hope that future observations, especially now that LIGO is back online, will shed more light on these explosions! It certainly seems clear, however, that magnetic fields have an important role to play.

Citation

“The MRI Imprint on the Short-GRB Jets,” Konstantinos Sapountzis and Agnieszka Janiuk 2019 ApJ 873 12. doi:10.3847/1538-4357/ab0107

Photograph of the rocky surface of an asteroid.

1 April 2019

What’s with Asteroid Ryugu’s Weird Shape?

Recent space missions have served as solar-system paparazzi, stalking a number of near-Earth objects — and the images they’ve sent home give us plenty to ponder. Can we use these observations to determine how one of these photogenic asteroids obtained its shape?

Visits to Asteroids

Asteroids Ryugu (top) and Bennu (bottom) have similar spinning-top shapes. Click to enlarge. [JAXA/U. of Tokyo/Kochi U./Rikkyo U./Nagoya U./Chiba Ins. Tech/Meiji U./U. Aizu/AIST; GSFC/NASA/U. of Arizona]

Near-Earth objects naturally fall under public scrutiny — it’s in our best interests to learn as much as we can about these neighboring (and potentially hazardous!) bodies. Two spacecraft were launched within the past few years to scope out these objects: NASA’s OSIRIS-REx is currently examining the asteroid 101955 Bennu, and JAXA’s Hayabusa2 is exploring the asteroid 162173 Ryugu.

Both missions plan to eventually return samples of their targets to Earth to help us better understand asteroid composition. In the meantime, the spacecraft have provided stunning images of the asteroids’ surfaces and overall structures, allowing us to learn more about these kilometer-scale inhabitants of our solar system.

Details of a Spinning Top

One striking feature of these asteroids is their shape: both Bennu and Ryugu have so-called “spinning-top” shapes, characterized by a raised equatorial ridge that makes the asteroids look more diamond-like than circular when viewed from the side. Such a shape could conceivably be caused by the flow of materials to the equator, but this requires rapid rotation — quicker than Ryugu’s 7.6-hour or Bennu’s 4.3-hour periods.

Ryugu seen from the Hayabusa2 spacecraft from different angles. The western region exhibits a smoother surface and a sharper ridge angle than the rest of the asteroid. [Hirabayashi et al. 2019 via JAXA/U. of Tokyo/Kochi U./Rikkyo U./Nagoya U./Chiba Ins. Tech/Meiji U./U. Aizu/AIST]

Hayabusa2’s detailed images of Ryugu reveal other asymmetries of the asteroid: the western region — termed the western bulge — is smoother than the eastern side, and the ridge angle here is sharper: 95° rather than the 105° seen on the rest of the asteroid. Recent research led by Masatoshi Hirabayashi (Auburn University) uses Hayabusa2’s observations to explore the possibility that Ryugu’s deformation was caused by a faster spin rate in its past.

Failed Structure?

Hirabayashi and collaborators use numerical models derived from Hayabusa2’s observations to analyze Ryugu’s current structure, judging how well different regions of the asteroid would hold up at different spin rates. The authors show that the subsurface region of the western bulge is currently structurally intact, whereas other regions are sensitive to structural failure. This suggests that the western bulge’s subsurface is relaxed — likely because it already experienced structural deformation in the past.

“Failure mode diagram” computed for Ryugu. The shaded region indicates combinations for which Ryugu cannot structurally exist because the cohesive strength of its material would be below that necessary to remain structurally intact at a given spin period. [Hirabayashi et al. 2019]

What would it take to cause this deformation and create Ryugu’s current structure? Hirabayashi and collaborators show that a past spin period of around 3 hours — more than twice the asteroid’s current spin rate — could have caused the asteroid’s structure to fail in places. As material shifted, it could have generated the smooth surface of the western region and eventually settled to form its current configuration at a spin period of ~3.5 hours. Ryugu’s spin likely slowed gradually over time after this, eventually reaching its current leisurely 7.6-hour period.

The authors acknowledge that this scenario is not the only possible explanation for Ryugu’s shape, but it’s a model that produces results consistent with Hayabusa2’s detailed images. As we gain more data from Hayabusa2 — and from OSIRIS-REx at Bennu — we can hope to refine our theories further!

Citation

“The Western Bulge of 162173 Ryugu Formed as a Result of a Rotationally Driven Deformation Process,” Masatoshi Hirabayashi et al 2019 ApJL 874 L10. doi:10.3847/2041-8213/ab0e8b

29 March 2019

A Rare Double-Detonation Supernova Caught in the Act

There’s more than just one way for a star to explode. Supernovae — perhaps the most dramatic form of star death — come in many flavors, and astronomers are still learning about the vast diversity of these stellar explosions.

When Stars Steal Mass

This artist’s rendering depicts one kind of Type Ia supernova mechanism: the singly degenerate model, in which a white dwarf siphons mass from its companion, exceeds the Chandrasekhar mass, and explodes. [NASA/CXC/M. Weiss]

When a white dwarf accretes gas from a binary companion and gains enough mass to exceed the Chandrasekhar limit, it can ignite in a cataclysmic explosion. This is the typical scenario for a Type Ia supernova, a common curtain call for low- to intermediate-mass stars in binary systems.

However, this isn’t the only way a Type Ia supernova can happen. In the double-detonation model, the explosion of the white dwarf is triggered by the ignition of an accreted helium shell. In this case, the white dwarf can be far less massive than the Chandrasekhar limit, leading to unexpectedly dim explosions.

Past studies have explored the minimum helium shell mass necessary (~0.01 solar mass) for this process and found that helium-shell detonations can efficiently cause core detonations, but there’s still plenty we don’t know about these events. The best way to learn about supernovae — double-detonation or otherwise — is to spot them soon after they happen.

A comparison of ZTF 18aaqeasu’s optical light curve (red circles) to normal (orange hexagons) and sub-luminous Type Ia supernovae. [Adapted from De et al. 2019]

A Survey Spies a Supernova

In May 2018, an unusual supernova was detected by the Zwicky Transient Facility, an optical survey that hunts for fleeting events like stellar flares, fast-rotating asteroids, and the visible-light counterparts of gravitational-wave events. Within days of its detection, a team led by Kishalay De (Caltech) began to collect photometric observations and spectra of the object.

The photometry revealed that the object, ZTF 18aaqeasu, was unusually red and less luminous than a typical Type Ia supernova, making it a good candidate for the double-detonation scenario.

Its spectra were unusual even for a sub-luminous supernova, taking much longer to develop the silicon absorption feature typically seen in this type of event. Even stranger, the spectra exhibited a never-before-seen cutoff in the flux at short wavelengths, likely due to the presence of metals like iron and titanium.

Comparison of observed spectra (black) to helium-shell double-detonation models (green and orange). [Adapted from De et al. 2019]

An Unusual Event

In order to derive the properties of ZTF 18aaqeasu, De and collaborators compared their photometric and spectroscopic data to models, finding that the event was likely caused by the ignition of a 0.15 solar mass helium shell, which led to the explosion of a 0.76 solar mass white dwarf.

The combination of a massive helium shell with a low-mass white dwarf makes ZTF 18aaqeasu unique among Type Ia supernovae; SN 2016jhr (one of the only supernovae previously linked to a helium-shell detonation event) featured a much more massive white dwarf with a less massive helium shell.

Can we expect to find more supernovae like ZTF 18aaqeasu? Similarly luminous supernovae should be detectable out to about 1.3 billion light-years, but so far there have been none reported with similar spectral features and unusually red color. This may indicate that double-detonation events featuring massive helium shells might be rare — adding an elusive new member to the Type Ia supernova family.

Citation

“ZTF 18aaqeasu (SN2018byg): A Massive Helium-shell Double Detonation on a Sub-Chandrasekhar-mass White Dwarf,” Kishalay De et al 2019 ApJL 873 L18. doi:10.3847/2041-8213/ab0aec

27 March 2019

Merging Eccentric Pairs of Black Holes

The dense, chaotic centers of star clusters may be a birthplace for binary pairs of black holes like those observed by the Laser Interferometer Gravitational-Wave Observatory (LIGO). A new study now explores how eccentric binaries might arise and merge in these extreme environments.

A Question of Origin

The ten black-hole mergers detected thus far by LIGO/Virgo. Click to enlarge. [Teresita Ramirez/Geoffrey Lovelace/SXS Collaboration/LIGO-Virgo Collaboration]

Since the discovery of the first gravitational-wave signal in September 2015, LIGO and its European counterpart Virgo have detected nine more merging black-hole binaries. After a brief pause for upgrades, the detectors are slated to come back online in April with significantly improved sensitivities — promising many more detections to come.

Though the gravitational-wave signals provide a wealth of information about the pre-merger binaries, we haven’t yet been able to determine how these black-hole binaries formed in the first place. Did these pairs evolve in isolation? Or were they born from interactions in the dense centers of star clusters?

One overlooked piece of data might shed light on these questions in the future: eccentricity. Since black-hole binaries in isolation take a long time to merge, any initial eccentricity in the orbit will be damped by gravitational-wave emission by the time the merger happens. But what if the binary doesn’t evolve in isolation? Could we see an imprint of eccentricity on the gravitational-wave signal then?

A new study led by scientist Michael Zevin (Northwestern University and CIERA) explores one possible channel for eccentric mergers: chaotic interactions between multiple black-hole binaries in the centers of star clusters.

Two examples of the complex evolution of binary–binary encounters, both eventually leading to a gravitational-wave capture. An animation of the second example is shown in the video at the end of the post. [Adapted from Zevin et al. 2019]

Complex Interactions

Zevin and collaborators use models to explore what happens during strong interactions between pairs of black-hole binaries and between black-hole binaries and single black holes.

These interactions are incredibly complex (don’t believe me? Check out the video below!). Systems with more than two bodies evolve chaotically, with small changes in initial conditions leading to vastly different outcomes. To make matters worse, simple Newtonian physics won’t accurately describe these systems; to capture the effects of gravitational-wave dissipation, we must model these interactions taking general relativity into account.

Zevin and collaborators find that these complexities lead to surprising results. Though binary–binary interactions occur 10–100 times less frequently than binary–single interactions in the centers of globular clusters, the long life and complexity of binary–binary interactions means that they are significantly more likely to result in a gravitational-wave capture — the rapid inspiral and merger of a binary pair, which occurs quickly enough that the pair may still have measurable eccentricity at merger time.

An Eccentric Result

Predicted eccentricity distributions and delay times for three populations of binary–binary produced gravitational-wave mergers. The horizontal black lines show minimum measurable eccentricities predicted for LIGO/Virgo and LISA. Solid colored lines show the eccentricities for the three populations at 10 Hz (LIGO/Virgo’s lower limit) and 0.1 Hz (the most sensitive frequency predicted for LISA). [Zevin et al. 2019]

The authors demonstrate that binary–binary interactions contribute a significant fraction (~25–45%) of the eccentric mergers that result when black holes strongly interact in cluster centers. But what are our prospects for being able to detect these eccentric collisions?

The outlook is promising! Gravitational-wave captures generally have eccentricities at merger that should be measurable by LIGO/Virgo, and binary–binary-produced mergers that occur later, either in-cluster or after being ejected from the cluster, could have eccentricities detectable by the future Laser Interferometer Space Antenna (LISA). With enough observations, eccentric binaries may soon help us better understand the origin of black-hole pairs.

Bonus

This video (click here for a larger version) of one of the authors’ binary–binary encounters follows complicated and chaotic interactions over the span of ~25 years, leading to an eventual gravitational-wave capture.

Citation

“Eccentric Black Hole Mergers in Dense Star Clusters: The Role of Binary–Binary Encounters,” Michael Zevin et al 2019 ApJ 871 91. doi:10.3847/1538-4357/aaf6ec

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