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simulation of gravitational waves from merging black holes

How do binary black holes form? Are the two components born together as stars, or do they find each other only after evolving into black holes? Gravitational waves may hold the answer.

Imprints of Formation History

Gravitational-wave detectors like the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo interferometer have allowed us to eavesdrop on the mergers of black holes across the universe. With dozens of black hole mergers detected so far, we can start to question how two black holes end up in a binary system in the first place.

Two main pathways are expected. In the first scenario, two stars, coupled since their formation, engage in a slow gravitational dance over billions of years as they evolve into black holes, grow closer, and merge. In the second scenario, two black holes, born apart, become gravitationally entangled in a dynamic environment like a dense star cluster.

gravitational wave forms

An idealized example of gravitational wave forms for a binary black hole system with perfectly circular orbits (e=0; black) and very elongated orbits (e=0.5; red). Click to enlarge. [Abbott et al. 2019]

These two formation pathways affect how eccentric, or elongated, the orbits of the black hole binary pair will be in the moments before they merge. In the isolated origin scenario, the black holes emit gravitational waves as they slowly slink toward each other, leading their orbits to become circular before they merge. In the dynamic origin scenario, on the other hand, black holes that become entangled are driven to merge quickly, before their orbits can circularize. The eccentricity of the binary system alters the gravitational waves released just before the merger, giving astronomers a way to track down the origins of these systems.

eccentricity probability distributions for black hole binaries

Violin plot showing probability distributions for the eccentricities of the 26 binary black hole mergers analyzed in this study and 10 mergers that were previously analyzed. The wider the violin, the more likely the eccentricity. Most events are weighted toward e=0, though two events (GW190620A and GW190521A) show clear signs of nonzero eccentricity. Click to enlarge. [Romero-Shaw et al. 2021]

Examining Eccentricity

A team led by Isobel Romero-Shaw (Monash University and ARC Center of Excellence for Gravitational Wave Discovery, Australia) performed a statistical analysis of gravitational-wave signals from 26 binary black hole mergers in the LIGO/Virgo catalog to determine the most likely eccentricity — and therefore the most likely origin — for each merging system.

Romero-Shaw and collaborators found that while the majority of the events analyzed likely had circular orbits, two events showed clear signs of eccentricity, with 50% of their probability distributions falling above an eccentricity of 0.05. The team’s results suggest that 27% or more of the binary black holes in the LIGO/Virgo catalog formed dynamically, likely in a dense cluster environment.

Emerging Possibilities

ground-based image of a star cluster

Black hole binaries formed in young star clusters, like NGC 3293 shown here, may resemble those formed in dense clusters or isolated environments. [ESO/G. Beccari; CC BY 4.0]

Aside from the systems that were clearly eccentric, Romero-Shaw and collaborators found a further 10 events that showed hints of eccentricity but were still consistent with circular orbits. It’s not yet clear what these marginal detections mean, since random fluctuations could cause perfectly circular binaries to appear eccentric. If these marginal cases truly are eccentric, dense star clusters may not be sufficient to produce the number of observed dynamically assembled systems. This may mean that eccentric black hole binaries can form in other places, such as young open star clusters, which might serve as a gravitational middle ground between dynamic and isolated environments, or the disks surrounding active galactic nuclei.

There’s still much to learn about merging black holes, and luckily our gravitational-wave detectors are hard at work. Hopefully, future detections of black hole mergers help us discern how these systems form!

Citation

“Signs of Eccentricity in Two Gravitational-wave Signals May Indicate a Subpopulation of Dynamically Assembled Binary Black Holes,” Isobel Romero-Shaw et al 2021 ApJL 921 L31. doi:10.3847/2041-8213/ac3138

Pulsar and a larger star

Particles are accelerated all over the universe, but exactly how they get so energetic often remains a mystery. A team of scientists has come up with a model involving stellar winds to explain some intense bursts from binary systems.  

LHAASOing High-Energy Particles

High-energy blasts of radiation can come from all different types of sources, from streams of charged particles from supermassive black holes to high-energy rays produced by our own Sun. Some sources of especially high-energy photons — energetic gamma rays — also produce accompanying bursts of neutrinos.  

Image showing how the IceCube detector works

A diagram of the IceCube detector. [IceCube Collaboration] 

Gamma-ray observatories such as the Large High Altitude Air Shower Observatory (LHAASO) and the Carpet-2 experiment have captured phenomenally energetic photons from more than a dozen sources in the past ten years. Some of these detections have been accompanied by neutrino outflows, which have been observed by the IceCube detector, an observatory at the South Pole containing thousands of sensors distributed throughout a cubic kilometer of Antarctic ice. These detections point to astronomical sources able to accelerate particles to mind-boggling petaelectronvolt (PeV) energies. One PeV is one quadrillion electronvolts — one thousand times the kinetic energy of a flying mosquito, packed into a single particle. A group led by Andrei Bykov from the Ioffe Physical-Technical Institute in Russia may have now come up with a model to explain what powers these ultra-energetic accelerators. 

A Compact Explanation 

A diagram of the Be star and complex object, showing how the winds collide to accelerate particles.

A diagram of the model. The pulsar wind is in pink, the cross-hatched green is the zone of contact discontinuity, the blue is the stellar wind. [Bykov et al. 2021]

The authors’ model involves a binary system consisting of a Be star — a B spectral giant that emits Balmer lines in its emission — and some type of compact object. The two objects each have a stellar wind, and the collision of those stellar winds accelerates protons up to the PeV range. From there, the protons collide with photons to produce PeV-regime gamma rays and neutrinos. This model allows a large fraction of the kinetic energy of the outflows of compact object binary systems to be converted into energy in the PeV range and provides a high flux of neutrinos. 

This scheme explains how these events get so energetic, and it can even explain lower-energy gamma-ray events that coincide with high-energy neutrinos. These neutrinos, which may greatly contribute to the high-energy neutrino flux in the galaxy, should be detectable by IceCube, so future observations may provide a test of this model. 

Citation 

“PeV Photon and Neutrino Flares from Galactic Gamma-Ray Binaries,” A. M. Bykov et al 2021 ApJL 921 L10. doi:10.3847/2041-8213/ac2f3d 

artist's impression of a pulsar

Does Einstein’s general theory of relativity stand the test of time, or does this famed description of gravity waver under scrutiny? A team of astronomers has used a pair of ultra-dense stellar remnants to put the theory to the test.

reproduction of a 1919 photograph of the solar corona during a total solar eclipse

An image of the Sun based on a reproduction of a photograph negative created during Sir Arthur Eddington’s 1919 expedition to verify the general theory of relativity. [Frank Watson Dyson]

New Test of an Old Theory

In 1919, scientists journeyed to Brazil and São Tomé and Príncipe to witness a solar eclipse and measure the subtle bending of starlight caused by the Sun’s mass. This experiment was one of the first of a highly publicized series of tests of the general theory of relativity, which governs the warping of spacetime by matter and energy. The theory has withstood test after test, but that hasn’t stopped astronomers from trying to confound it.

As our understanding of the universe and the tools at our disposal have grown more sophisticated, so too have our tests; now, a team led by Hao Ding (Swinburne University of Technology and ARC Center of Excellence for Gravitational Wave Discovery, Australia) has joined the storied quest to probe the century-old theory of gravitation using a type of object not yet known to the world when Einstein penned the theory in 1915 — pulsars.

artist's impression of a pair of pulsars

An artist’s impression of a pair of pulsars. [Michael Kramer (Jodrell Bank Observatory, University of Manchester)]

Tracking a Binary System

Pulsars are highly magnetized, extremely dense stellar remnants that emit beams of radiation. In rare cases, the two stars in a binary system can both evolve into pulsars; the 1974 discovery of a pulsar in a binary system and its importance as a test bed for theories of gravitation won Russell Hulse and Joseph Taylor a Nobel Prize.

Astronomers use binary pulsars to test the predictions of general relativity by measuring the rate at which their orbits decay as they lose energy in the form of gravitational waves. Past measurements of the orbital decay rate for one of the 16 confirmed pulsar binaries, PSR J1537+1155, disagreed with the predictions of relativity by 9% — a far larger discrepancy than has been found for other systems.

A major difficulty of performing this test lies in determining the distance to PSR J1537+1155, which moves across the sky at an unusually quick pace; unfortunately, the best way to measure the distance to a pair of pulsars uses the predictions of general relativity — so it can’t be used in a test of the theory. Now, Ding and collaborators have determined a new distance to PSR J1537+1155 the old-fashioned way: by measuring the system’s apparent motion relative to background stars as Earth moves around the Sun.

Questing for Precision

radio telescope in front of a mountain range

One of the 25-meter radio telescopes in Owens Valley, California, which is one of the 10 sites that make up the VLBA. [NRAO/AUI/NSF; CC BY 4.0]

For their updated distance measurement, Ding and collaborators used data from the Very Long Baseline Array (VLBA) — a network of radio telescopes scattered across the globe with the resolving power of a single 5,351-mile-wide radio dish. The team carefully combined measurements taken over a six-year period to get the most precise distance to a binary pulsar system ever obtained without assuming relativity to be correct: 0.94 kiloparsec (3,066 light-years).

With this new measurement, the disagreement between observations and the predictions of relativity shrinks to just 2.3%. Looking to the future, Ding and collaborators anticipate that high-sensitivity very long baseline interferometry measurements will further refine our estimate of the distance to PSR J1537+1155 and potentially resolve the remaining tension between observations and theory.

Citation

“The Orbital-decay Test of General Relativity to the 2% Level with 6 yr VLBA Astrometry of the Double Neutron Star PSR J1537+1155,” Hao Ding et al 2021 ApJL 921 L19. doi:10.3847/2041-8213/ac3091

image of the sun's surface

Sophisticated machine-learning techniques may finally answer a question astronomers have pondered for decades: what makes the Sun launch solar flares?

Predicting and Pattern-Finding

extreme-ultraviolet image of the sun with a magnetic field model overlaid

Why study the causes of solar flares? Flares are just one example of how the Sun’s complex magnetic field interacts with its dynamic plasma environment. [NASA/GSFC/Solar Dynamics Observatory]

In recent years, researchers have explored using machine learning to understand the causes of solar outbursts like solar flares and predict their onset. Computers excel at sifting through mountains of data and finding subtle patterns that would elude a manual search. After these patterns have been dredged up, researchers can interpret and model them, yielding new insights. This process may not only help predict solar flares before they happen, but it also might help us understand the physical mechanism that launches these powerful events and gain a better understanding of the complex plasma environment of the Sun’s atmosphere.

In a new study, a team led by Magnus Woods (Bay Area Environmental Research Institute and Lockheed Martin Solar and Astrophysics Lab) applied a machine-learning algorithm to ultraviolet solar spectra to identify signs that a solar flare is imminent — should any such signs exist.

image of the sun

A sample image of the Sun from IRIS, showing twisting structures dubbed “mini-tornadoes.” [NASA/IRIS/Pereira]

Spectral Signatures of Future Flares

Woods and collaborators used spectra from the Interface Region Imaging Spectrograph (IRIS) — an Earth-orbiting spacecraft dedicated to understanding the region of the Sun’s atmosphere in which the temperature increases sharply from a few thousand kelvin to millions of kelvin. The team selected three sets of observations for analysis: quiet Sun (no areas of enhanced magnetic fields), quiescent active region (enhanced magnetic fields but no solar flares), and flares. By comparing observations known to contain solar flares to those without, the team aimed to narrow in on spectral features that are uniquely associated with flares.

The team focused on the magnesium h and k lines, which are prominent features in the wavelength range monitored by IRIS. Their machine-learning technique pinpointed interesting behavior in these spectral lines; typically, the h and k lines are double peaked, but under the conditions that precede a solar flare — up to 40 minutes in advance — they retain only a single peak.

More To Learn

spectral lines of magnesium

The two types of spectral signatures associated with pre-flare conditions. Individual profiles are in black and a representative profile is in orange. The data are reported in counts per second. [Adapted from Woods et al. 2021]

Woods and collaborators found that the single-peaked magnesium lines also appeared in quiescent active regions, but they were far more prevalent in the pre-flare regions. The quiescent active regions with single-peaked spectral lines also exhibited small-scale brightening events — not full-on solar flares, but further evidence that this spectral feature is associated with solar heating.

The team determined that the observed spectral features were associated with rising temperatures roughly 1,000 kilometers above the Sun’s surface. This suggests that the warning signs of solar flares are generated in this region, though more work is needed to confirm this result. The enormous wealth of solar spectral and magnetic data makes teasing out the fleeting signatures of solar flares a tall order, but this work makes it clear that machine learning can bring us a step closer to understanding the causes of solar flares.

Citation

“Unsupervised Machine Learning for the Identification of Preflare Spectroscopic Signatures,” Magnus M. Woods et al 2021 ApJ 922 137. doi:10.3847/1538-4357/ac2667

multiwavelength image of supernova remnant IC 443

Sometimes, the properties of massive astrophysical objects are determined by some of their smallest components: electrons. Today’s article explores an intriguing idea: what if the differences between some mysterious explosions across our universe can be explained by the behavior of electrons?

image of AT2018cow's location

Image from the Sloan Digital Sky Survey with cross hairs pinpointing the location of AT2018cow. It is spatially coincident with the galaxy CGCG 137-068 in the constellation Hercules. [Sloan Digital Sky Survey; CC BY 4.0]

New Kid in the Universe

In 2018, a telescope searching for near-Earth asteroids witnessed a fleeting explosion nearly 100 times more luminous than a typical supernova in a galaxy 200 million light-years distant. The event — dubbed AT2018cow — was the first of a new class of phenomena called fast blue optical transients. Astronomers have yet to agree on the cause of these rare events, but many explanations center on an exploding object that expands into surrounding gas.

These mysterious new transients aren’t the only astronomical example of explosions colliding with nearby material; supernovae that emit strongly at radio wavelengths — aptly named radio supernovae — arise when an expanding shock wave plows into circumstellar gas. However, their spectra are very different from those of fast blue optical transients. Are radio supernovae and AT2018cow-like events completely unrelated, or could a common thread tie them together?

Shocking Similarities

spectral energy distributions

Example spectral energy distributions showing peak emission dominated by thermal electrons (top) and non-thermal electrons bottom). [Adapted from Margalit & Quataert 2021]

Picture this: millions of light-years away, there’s an explosion. Maybe it’s a garden-variety supernova, or maybe it’s something far stranger. Either way, the explosion launches a shock wave that sweeps up material and accelerates electrons to relativistic speeds. Typically, astronomers assume that many of these shock-accelerated electrons are zipping around faster than they would be if they were all in thermal equilibrium. This non-thermal electron model can explain the emission from radio supernovae, but it can’t explain rare transient events like AT2018cow. Recently, researchers modeled a population of thermal electrons instead and were able to reproduce the emission of an AT2018cow-like event.

This suggests that seemingly disparate phenomena like radio supernovae and AT2018cow-like events could both be explained by the shock-accelerated electron model — just by tweaking a few properties of the electrons. In a new study, Ben Margalit (University of California, Berkeley) and Eliot Quataert (Princeton University) modeled the emission from a mixture of thermal and non-thermal electrons to explore how different electron behaviors affect the emission we observe.

Varied Outcomes

Margalit and Quataert found that thermal electrons are critical for modeling some — but not all! — shock waves, and whether or not thermal electrons are important is mainly determined by the speed of the shock. For example, shocks that move relatively slowly, like those seen in radio supernovae, show virtually no emission from thermal electrons. As a result, a solely non-thermal electron model can reproduce those events. For AT2018cow-like events, where the shock moves at a substantial fraction of the speed of light, including thermal electrons is critical for reproducing their spectra. The authors found that including a thermal electron component becomes necessary when modeling shocks moving faster than 20% the speed of light.

illustration of the aftermath of two neutron stars merging

The collision of two neutron stars, illustrated here, may be a target for the authors’ model. [NASA Goddard Space Flight Center/CI Lab]

Although this work is motivated by observations of radio supernovae and AT2018cow-like events, the authors note that their model should apply to other astrophysical blasts, such as shock waves produced by colliding neutron stars or fading gamma ray-bursts. Future modeling of these and other events may further highlight role that humble electrons play in influencing the appearance of explosions in our universe.

 

Citation

“Thermal Electrons in Mildly Relativistic Synchrotron Blast Waves,” Ben Margalit and Eliot Quataert 2021 ApJL 923 L14. doi:10.3847/2041-8213/ac3d97

image of triton

Decades of observations have taught us a great deal about the planets, moons, and small bodies in our solar system. As our explorations have progressed, a new class of solar system objects has emerged: ocean worlds. In a focus issue of the Planetary Science Journal, a set of articles details proposed missions and methods to explore the icy ocean worlds in our solar system. These articles provide a potential framework for embarking on an exploration of these distant worlds, where life might develop beneath icy crusts in oceans heated by tidal forces.

Move Over, Mars!

What makes ocean worlds such intriguing targets in the search for life, as opposed to more familiar terrestrial worlds like Mars and Venus? Ocean worlds have all or most of the ingredients necessary for life as we know it: liquid water, organic material, and internal heating provided by the tidal tugs of the planets they orbit. In order of increasing distance from Earth, here are just a few of the many known and suspected ocean worlds in our solar system:

  • photograph of europa's surface

    This image from the Galileo spacecraft showcases the linae, pits, ridges, domes, and jumbled terrains called “chaos” that cover Europa’s surface. [NASA/JPL-Caltech/University of Arizona]

    Europa: The smallest of Jupiter’s Galilean moons has a young, smooth surface marred by a pattern of cracks called linae. These cracks may be evidence for a type of plate tectonics on Europa caused by the flexing of its ice shell.
  • Enceladus: This tiny moon of Saturn ejects plumes of water from fissures in its ice shell, broadcasting the contents of its global ocean hundreds of kilometers above its surface. The majority of the plume particles fall to the moon’s surface, where a lander mission could collect and study these particles with relative ease.
  • Titan: Famous for its Earth-like surface features, substantial atmosphere, and methane rain, Saturn’s largest moon Titan is the only ocean world on which we’ve made landfall. The Huygens probe dropped to Titan’s surface in 2005, revealing in great detail what appear to be drainage channels, coastlines, and rocks made of water ice.
  • photograph of Uranus's moon Titania

    Titania, the largest moon of Uranus — not to be confused with Titan or Triton! — is shown here in the highest-resolution image available from Voyager 2. [NASA/JPL]

    Uranian satellites: During its 1986 flyby of the Uranus system, Voyager 2 glimpsed the southern hemispheres of six of Uranus’s 27 known moons. Though our explorations are now limited to ground-based telescopic observations, researchers believe that the ice giant’s five largest moons — and likely some of its smaller satellites — have or once had layers of liquid water below their surfaces. A Uranus orbiter might finally reveal the nature of the dark material mixed into these moons’ surface ice.
  • Triton: Neptune’s largest moon is thought to be a captured Kuiper belt object, which may explain its backwards orbit. Like many of the moons in the outer solar system, the only spacecraft to visit Triton was Voyager 2 in 1989. During its visit, Voyager 2 observed Triton’s young surface, which is coated with nitrogen frost and a reddish organic material.
poster of the juice spacecraft

Though its primary target is Ganymede — the largest moon in the solar system and the only one with a global magnetic field — JUICE will also make some much-needed observations of Europa from its berth in the Jupiter system. [ESA]

Missions in the Making

The missions described in this special issue would increase our knowledge of the many icy satellites in our solar system through images, spectra, radar maps, and particle measurements of their surfaces, subsurfaces, and near-space environments. Though some of these missions are still in the planning phase, others are already ramping up: in the wake of major discoveries by the Galileo, Cassini, Dawn, and New Horizons missions, the European Space Agency (ESA) and NASA have planned several new missions that will explore ocean moons in the outer solar system in the near future.

In 2023, ESA’s JUpiter ICy moons Explorer (JUICE) will begin its long journey to observe Jupiter’s moons Ganymede, Callisto, and Europa — first from Jupiter orbit and later from orbit around Ganymede, becoming the first spacecraft to orbit a moon other than our own. NASA’s Europa Clipper mission, planned for launch the following year, will undertake further observations of Europa to determine if its ocean is suitable for life. And in 2026, NASA’s Dragonfly mission will embark on a seven-year cruise to Titan. Although the spacecraft will touch down far from Titan’s northern hemisphere oceans, it will greatly enhance our understanding of this frosty world. While we’re waiting for these and other proposed missions to bring us new discoveries from our solar system’s ocean worlds, browse the articles in the focus issue below to whet your appetite for ocean-world exploration!

Citation

Articles belonging to the Ocean Worlds focus issue will be collected here: Ocean Worlds: Motivations for a Multi-Decadal Exploration Program

Artist's rendition of the Parker Solar Probe getting very close to the Sun

What did the Parker Solar Probe observe when it got closer to the Sun than any spacecraft has ever ventured? Astronomers peer into the solar magnetic field to find out. 

Probing the Sun’s Surface 

Obtaining high-quality observations of the Sun has been important to astronomers for centuries, in large part because the Sun is capable of unleashing space weather events that not only produce the spectacular northern lights, but can also totally knock out our power grids and GPS systems. Therefore, understanding the Sun’s atmosphere and magnetic fields, where these weather events originate, is key. 

Image of the solar corona as seen during a solar eclipse

The solar corona as seen during a solar eclipse. [NASA/Aubrey Gemignani]

The Parker Solar Probe, which launched in 2018, completed its closest encounter with the Sun last month, entering its corona (the hot, tenuous upper atmosphere). This marked the closest any spacecraft has ever gotten to the solar surface. The mission’s primary goal is to fly into the corona and sample particles and magnetic fields to get a better understanding of what drives the solar wind. A group of plasma physicists has now used these data to analyze the magnetic fields and plasma waves in the Sun’s atmosphere in unprecedented detail.  

 

Waving Hello to Results at Small and Large Scales 

Graphs showing the normal, tangential, and radial components of the magnetic field and velocity of particles within the solar corona on two different days. In general, they decrease very slightly over time

The normal, tangential, and radial components of the magnetic field and velocity of particles observed by the Parker Solar Probe on two different days. Click to enlarge. [Zhao et al. 2021]

A group led by Lingling Zhao from The University of Alabama in Huntsville, focused on properties of different types of plasma waves in the corona. Plasma waves are present everywhere in our solar system, from Jupiter’s magnetosphere to the scorching solar atmosphere. These waves transfer energy and momentum and can heat and cool plasma.

The team looked at two scales: magnetohydrodynamic (which treats the plasma as a conducting fluid) and ion-kinetic (which treats the plasma as a collection of individual particles). Using Parker Solar Probe’s observations, the authors analyzed the flow of particles at these scales and found that different types of waves occur at each scale. The presence of waves at both large and small scales supports specific models of magnetic fluctuations and instabilities within the solar system, especially close to the Sun, which could lead to a better understanding of the solar wind. The authors’ work helps to home in on how the solar wind accelerates from a gentle breeze near the solar surface to the 400-kilometer-per-second gale seen near Earth.

The Parker Solar Probe has allowed scientists to get a closer glimpse of the solar surface than ever before. Future measurements with the Parker Solar Probe and the Solar Orbiter, a satellite launched in February 2020 to study the inner heliosphere and solar wind, will give more insight into the solar wind under different conditions (such as different temperatures and speeds) and will allow us to get a better handle on what heats the solar wind. 

Bonus

Check out this video from NASA/the Johns Hopkins Applied Physics Laboratory showing the Parker Solar Probe during its closest encounter with the Sun. Streaks in the gif are coronal streamers, which are part of the magnetic field of the Sun and are usually only seen from Earth during solar eclipses. The Milky Way can be seen in the background.

Citation  

“MHD and Ion Kinetic Waves in Field-aligned Flows Observed by Parker Solar Probe,” L.-L. Zhao et al 2021 ApJ 922 188. doi:10.3847/1538-4357/ac28fb 

magnetar

Astronomers are starting to close in on the origins of fast radio bursts — powerful, fleeting flashes of radio waves seen at extragalactic distances. Highly magnetized neutron stars called magnetars might be responsible for many of these far-flung events, but how exactly do these extreme objects generate fast radio bursts?

A Cosmic Conundrum

radio and X-ray image of a magnetar in a supernova remnant

X-ray and radio observations of the magnetar associated with the first fast radio burst seen in our galaxy. The magnetar is the bright blue X-ray source in the center of the supernova remnant. [Zhou et al. 2020]

The mystery of where fast radio bursts come from — some of them, anyway! — seemed to be solved when the first radio burst within the Milky Way was found to come from a magnetar — an ultra-dense stellar remnant with a magnetic field roughly 100 billion to 10 trillion times stronger than a typical refrigerator magnet. But as is so often the case in astrophysics, solving one puzzle prompted many more: how do magnetars generate these strong, brief bursts of radio waves, and do they arise from near the magnetar’s surface or from material surrounding it?

In today’s article, Andrei Beloborodov (Columbia University and Max Planck Institute for Astrophysics, Germany) explored whether a fast radio burst generated close to the surface of a magnetar could escape the confines of its magnetosphere — the region of space where charged particles bend to the will of the magnetar’s intense magnetic field.

illustration of the size of jupiter's magnetosphere

Illustration of the size of Jupiter’s magnetosphere on the night sky. Although a magnetar’s magnetosphere is physically smaller, its magnetic fields are far stronger. [NASA/ESA]

Magnetar Models

Magnetospheres have great importance within our solar system — Earth’s magnetosphere shields us from energetic particles generated by the Sun, and Jupiter’s magnetosphere would dominate the sky if our eyes were adapted to see radio wavelengths. The magnetosphere of a magnetar, though, is far stranger than these nearby examples; the magnetized neutron star at the center generates a plasma of electrons and their positively charged counterparts, positrons, that fills the magnetosphere and might prevent a radio burst generated near a magnetar’s surface from escaping into space.

Beloborodov used plasma physics equations to understand how a radio burst might interact with the charged particles and magnetic fields within a magnetar’s magnetosphere. As the radio burst travels outward, it compresses the magnetosphere, transferring its momentum to the magnetic fields and the plasma. The oscillating electrons and positrons emit gamma rays that can collide to produce even more electrons and positrons — creating a cascade of particles and gamma rays that scatter the radio wave and sap its energy. For the radio wave, there’s no escape; Beloborodov found that it’s extremely unlikely that a fast radio burst could escape the magnetosphere’s grasp if it were generated within 100,000 km of the magnetar’s surface.

More to Learn

While Beloborodov’s findings rule out the possibility of fast radio bursts arising from within the inner magnetosphere, there are other ways that magnetars can power these events. Another possibility is that fast radio bursts form much farther from the magnetar where magnetospheric flares collide with the wind flowing out from the magnetar.

Luckily, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) is poised to add new observations to its existing catalog of hundreds of fast radio bursts, helping us to understand the sources of these mysterious events and the complex physics behind them.

Citation

“Can a Strong Radio Burst Escape the Magnetosphere of a Magnetar?,” Andrei M. Beloborodov 2021 ApJL 922 L7. doi:10.3847/2041-8213/ac2fa0

Artist's impression of an exoplanet orbiting its host star

How did the 14 Herculis planetary system get into an unusual configuration? Did the system have a run-in with another star, or was there a troublemaker within the system itself? Astronomers peek deep into the characteristics of the system to find out! 

Planets Going the Distance 

Light curve from 14 Her, showing time (JD) vs. V (magnitude)

Light curve for 14 Herculis, as measured in the ASAS-SN (All-Sky Automated Survey for Supernovae) program. [Adapted from Bardalez Gagliuffi et al. 2021]

When a star forms, leftover material from the protostellar nebula creates a disk around the star and may eventually become planets. Once these planets form, their characteristics, such as mass and inclination, can determine how the system will evolve. Some planetary systems, like our own, have relatively circular orbits, but others have strange configurations. One example of an oddball system is 14 Herculis, a middle-aged K0 dwarf star that’s orbited by two giant planets with very high eccentricities. Exactly how did the planets end up on these peculiar orbits? A team led by Daniella C. Bardalez Gagliuffi at the American Museum of Natural History examined the full orbital configuration of the system to find out. 

When the Planets Don’t Align 

Corner plot showing distributions for mass, inclination, eccentricity, and semi-major axis

Distributions of parameters for planet b. Click to enlarge. [Bardalez Gagliuffi et al. 2021]

14 Her was one of the first stars targeted in radial-velocity searches for exoplanets because of its proximity and brightness. Astronomers detected the closer in of its two known planets, 14 Her b, in 2003, and its outer planet, 14 Her c, four years later. Using archival radial-velocity data from the high-resolution echelle spectrometer on the Keck Telescope and proper motions inferred from the Hipparcos–Gaia Catalog of Accelerations, the team was able to calculate the orbital parameters of the two planets.

They found that planet b orbits at a semimajor axis of ~3 au and has a moderate eccentricity, and that planet c is 27 au from the star and has a highly eccentric orbit. Because the authors only had data for ~15% of 14 Her c’s orbit, they were only able to get a broad distribution of inclinations relative to 14 Her b, but the data pointed to the two orbits being misaligned by nearly 70 degrees. There are only three other known systems with giant planets that have misaligned orbits. 

Corner plot showing distributions for mass, inclination, eccentricity, and semi-major axis

Distribution of parameters for planet c. Click to enlarge. [Bardalez Gagliuffi et al. 2021]

Feeling Inclined to Search for Answers 

What caused this strange misalignment? The answer surely lies in the system’s dynamic past, but what kind of dynamic past is still up in the air. The most likely explanation is planet–planet scattering: multiple planets of similar mass formed together in circular orbits and gravitationally influenced one another, eventually leading to the ejection of one planet and affecting the orbits of the others. It’s also possible that a passing star came close enough to the system that it perturbed the orbits and launched one planet out of the system; to reach a stable configuration, the surviving planets got farther apart, and their eccentricities grew. 

Imaging the system in the mid-infrared could reveal a hidden planetary sibling that might shed further light on 14 Her’s dynamic past, so we’ll just have to wait for JWST and the Nancy Grace Roman Space Telescope to help solve this mystery! 

Citation 

“14 Her: A Likely Case of Planet–Planet Scattering,” Daniella C. Bardalez Gagliuffi et al 2021 ApJL 922 L43. doi: 10.3847/2041-8213/ac382c 

Hubble image of the Ring Nebula

When a star ends its main-sequence lifetime, loses its outer layers, and becomes a white dwarf, how massive will it be? Astronomers turn to white dwarfs’ binary companions to help answer this question.

A Frequent Finale

artist's impression of Sirius B next to Earth

Stars with masses less than roughly eight solar masses leave behind their dense cores, which are approximately the same size as Earth. This artist’s impression depicts Sirius B, the nearest white dwarf. [ESA and NASA; CC BY 4.0]

Except for the few percent of stars that will end their lives in massive cosmic explosions, nearly all the stars in our galaxy are destined to become white dwarfs. Our own star is headed that route: in five billion years or so, the Sun will exhaust its supply of core hydrogen, balloon into a red giant, and loft its outer layers into space to expose its blazing hot core. The core will shine for a further ten billion years — or more! — as a white dwarf before fading from view.

Even though the vast majority of stars become white dwarfs, many questions remain about these compact stellar remnants. One lingering uncertainty is the connection between a star’s main-sequence mass and the mass of the white dwarf it leaves behind. Is the mass of the white dwarf correlated with the star’s main-sequence mass, or do other factors, like the star’s chemical makeup, play a defining role? Pinning down the initial-to-final mass relation can help us gain a better grasp on the intricacies of how stars make the tumultuous journey from the main sequence to their final phase.

diagram of the mass-determination process

Diagram of the white dwarf mass-determination process, shown for two white dwarfs in the sample. Stellar models (solid lines) allow progenitor star lifetimes to be mapped to initial stellar masses. [Barrientos & Chanamé 2021]

Companion Chronology

Manuel Barrientos and Julio Chanamé (Pontifical Catholic University of Chile) approached this question by studying white dwarfs that are separated by thousands of astronomical units from their binary companions. These widely separated binary systems are a useful tool for studying white dwarfs; the two stars in the binary system are likely to be the same age, and the wide separation makes it unlikely that the stars have exchanged mass, which could complicate their evolutionary tracks.

Barrientos and Chanamé used models of stellar evolution and parameters extracted from spectroscopic and photometric observations to determine the masses of the white dwarfs, how long ago they formed, and the ages of their stellar companions. They then used evolutionary models to link the main-sequence lifetime of the white dwarf’s progenitor star — determined by subtracting the lifetime of the white dwarf from the age of the companion star — to its mass.

Far-Reaching Implications

plot of final mass versus initial mass

Initial and final masses of the stars in this work (blue symbols) and others. The two panels use different sets of stellar parameters. Click to enlarge. [Barrientos & Chanamé 2021]

The team was particularly interested in the least massive stars, for which the initial-to-final mass relation has been only loosely constrained by previous work. They found that the lowest-mass progenitor stars yielded a wide range of white-dwarf masses — some were more massive than white dwarfs that originated from progenitor stars twice as massive. However, the authors find evidence that these white dwarfs may actually be two closely situated white dwarfs, which would complicate the analysis.

The authors hope that future observations will refine the initial-to-final mass relationship further — the dispersion seen in their work isn’t predicted by models, cannot be fully explained by differences in stellar composition, and would have major implications for research that relies on precise determination of this relationship.

Citation

“Improved Constraints on the Initial-to-final Mass Relation of White Dwarfs Using Wide Binaries,” Manuel Barrientos and Julio Chanamé 2021 ApJ 923 181. doi:10.3847/1538-4357/ac2f49

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