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merging neutron stars

Now that the hubbub of GW170817 — the first coincident detection of gravitational waves and an electromagnetic signature — has died down, scientists are left with the task of taking the spectrum-spanning observations and piecing them together into a coherent picture. Researcher Iair Arcavi examines one particular question: what caused the blue color in the early hours of the neutron-star merger?

kilonova

Observations of the GW170817 kilonova by Hubble over a ~week-long span. [ESA/Hubble]

Early Color

When the two neutron stars of GW170817 merged in August of last year, they produced not only gravitational waves, but a host of electromagnetic signatures. Chief among these was a flare of emission thought to be powered by the radioactive decay of heavy elements formed in the merger — a kilonova.

The emission during a kilonova can come from a number of different sources — from the heavy-element-rich tidal tails of the disrupting neutron stars, or from fast, light polar jets, or from a wind or a disk outflow — and each of these components could reveal different information about the original neutron stars and the merger.

It’s therefore important that we understand the sources of the emission that we observed in the GW170817 kilonova. In particular, we’d like to know where the early blue emission came from that was spotted in the first hours of the kilonova.

light curve of the GW170817

The combined ultraviolet–optical–infrared light curve of the GW170817 kilonova. The rise in the emission occurs on roughly a day-long timescale. [Arcavi 2018]

Comparing Models

To explore this question, Iair Arcavi (Einstein Fellow at University of California, Santa Barbara and Las Cumbres Observatory) compiled infrared through ultraviolet observations of the GW170817 kilonova from nearly 20 different telescopes. To try to distinguish between possible sources, Arcavi then compared the resulting combined light curves to a variety of models.

Arcavi found that the light curves for the GW170817 kilonova indicate an initial ~24-hour rise of emission. This rise is best matched by models in which the emission is produced by radioactive decay of ejecta with lots of heavier elements (likely from tidal tails). The subsequent decline of the emission, however, is fit as well or better by models that include lighter, faster outflows, or additional emission due to shock-heating from a wind or a cocoon surrounding a jet.

optical and ultraviolet lightcurves

Optical and ultraviolet light curves for the first 3 days after merger, as compared to four different emission models. Observations at earlier times, where the models differ more substantially, could provide stronger constraints for future mergers. [Arcavi 2018]

Missing Ultraviolet

The takeaway from Arcavi’s work is that we can’t yet eliminate any models for the GW170817 kilonova’s early blue emission — we simply don’t have enough data.

Why not? It turns out we had some bad luck with GW170817: a glitch in one of the detectors slowed down localization of the source, preventing earlier discovery of the kilonova. The net result was that the electromagnetic signal of this merger was only found 11 hours after the gravitational waves were detected — and the ultraviolet signal was detected 4 hours after that, when the kilonova light curves are already decaying.

If we had ultraviolet observations that tracked the earlier, rising emission, Arcavi argues, we would be able to differentiate between the different emission models for the kilonova. So while this may be the best we can do with GW170817, we can hope that with the next merger we’ll have a full set of early observations — allowing us to better understand where its emission comes from.

Citation

Iair Arcavi 2018 ApJL 855 L23. doi:10.3847/2041-8213/aab267

NGC 1052-DF2

You may have seen recent news about NGC 1052–DF2, a galaxy that was discovered to have little or no dark matter. Now, a new study explores what NGC 1052–DF2 does have: an enigmatic population of unusually large and luminous globular clusters.

NGC 1052–DF2 cluster spectra

Keck/LRIS spectra (left and right) and HST images (center) of the 11 clusters associated with NGC 1052–DF2. The color images each span 1” × 1”. [van Dokkum et al. 2018]

An Unusual Dwarf

The ultra-diffuse galaxy NGC 1052–DF2, originally identified with the Dragonfly Telescope Array, has puzzled astronomers since the discovery that its dynamical mass — determined by the motions of globular-cluster-like objects spotted within it — is essentially the same as its stellar mass. This equivalence implies that the galaxy is strangely lacking dark matter; the upper limit set on its dark matter halo is 400 times smaller than what we would expect for such a dwarf galaxy.

Led by Pieter van Dokkum (Yale University), the team that made this discovery has now followed up with detailed Hubble Space Telescope imaging and Keck spectroscopy. Their goal? To explore the objects that allowed them to make the dynamical-mass measurement: the oddly bright globular clusters of NGC 1052–DF2.

cluster sizes

Sizes (circularized half-light radii) vs. absolute magnitudes for globular clusters in NGC1052–DF2 (black) and the Milky Way (red). [Adapted from van Dokkum et al. 2018]

What’s Up with the Globular Clusters?

Van Dokkum and collaborators spectroscopically confirmed 11 compact objects associated with the faint galaxy. These objects are globular-cluster-like in their appearance, but the peak of their luminosity distribution is offset by a factor of four from globular clusters of other galaxies; these globular clusters are significantly brighter than is typical.

Using the Hubble imaging, the authors determined that NGC 1052–DF2’s globular clusters are more than twice the size of the Milky Way’s globular clusters in the same luminosity range. As is typical for globular clusters, they are an old (~9.3 billion years) population and metal-poor.

Rethinking Formation Theories

The long-standing picture of galaxies has closely connected old, metal-poor globular clusters to the galaxies’ dark-matter halos. Past studies have found that the ratio between the total globular-cluster mass and the overall mass of a galaxy (i.e., all dark + baryonic matter) holds remarkably constant across galaxies — it’s typically ~3 x 10-5. This has led researchers to believe that properties of the dark-matter halo may determine globular-cluster formation.

cluster luminosity function

The luminosity function of the compact objects in NGC 1052–DF2. The red and blue curves show the luminosity functions of globular clusters in the Milky Way and in the typical ultra-diffuse galaxies of the Coma cluster, respectively. NGC 1052–DF2’s globular clusters peak at a significantly higher luminosity. [Adapted from van Dokkum et al. 2018]

NGC 1052–DF2, with a globular-cluster mass that’s >3% of the mass of the galaxy (~1000 times the expected ratio!), defies this picture. This unusual galaxy therefore demonstrates that the usual relation between globular-cluster mass and total galaxy mass probably isn’t due to a fundamental connection between the dark-matter halo and globular-cluster formation. Instead, van Dokkum and collaborators suggest, globular-cluster formation may ultimately be a baryon-driven process.

As with all unexpected discoveries in astronomy, we must now determine whether NGC 1052–DF2 is simply a fluke, or whether it represents a new class of object we can expect to find more of. Either way, this unusual galaxy is forcing us to rethink what we know about galaxies and the star clusters they host.

Citation

Pieter van Dokkum et al 2018 ApJL 856 L30. doi:10.3847/2041-8213/aab60b

Mars asteroids

Like evidence left at a crime scene, the mineral olivine may be the clue that helps scientists piece together Mars’s possibly violent history. Could a long-ago giant impact have flung pieces of Mars throughout our inner solar system? Two researchers from the Tokyo Institute of Technology in Japan are on the case.

A Telltale Mineral

olivine

Olivine, a mineral that is common in Earth’s subsurface but weathers quickly on the surface. Olivine is a major component of Mars’s upper mantle. [Wilson44691]

Olivine is a major component of the Martian upper mantle, making up 60% of this region by weight. Intriguingly, olivine turns up in other places in our solar system too — for instance, in seven out of the nine known Mars Trojans (a group of asteroids of unknown origin that share Mars’s orbit), and in the rare A-type asteroids orbiting in the main asteroid belt.

How did these asteroids form, and why are they so olivine-rich? An interesting explanation has been postulated: perhaps this olivine all came from the same place — Mars — as the result of a mega impact billions of years ago.

Evidence for Impact

Mars bears plenty of signs pointing to a giant impact in its past. The northern and sourthern hemispheres of Mars look very different, a phenomenon referred to as the Mars hemisphere dichotomy. The impact of a Pluto-sized body could explain the smooth Borealis Basin that covers the northern 40% of Mars’s surface. 

Mars topography

This high-resolution topographic map of Mars reveals the dichotomy between its northern and sourthern hemispheres. The smooth region in the northern hemisphere, the Borealis basin, may have been formed when a giant object impacted Mars billions of years ago. [NASA/JPL/USGS]

Other evidence piles up: Mars’s orbit location, its rotation speed, the presence of its two moons — all could be neatly explained by a large impact around 4 billion years ago. Could such an impact have also strewn debris from Mars’s mantle across the solar system?

To test this theory, we need to determine if a mega impact is capable of producing enough ejecta — and with the appropriate compositions and orbits — to explain the Mars trojans and the A-type asteroids we observe. Tackling this problem, researchers Ryuki Hyodo and Hidenori Genda have performed numerical simulations to explore the ejecta from such a collision.

Distributing Debris

Hyodo and Genda examine the outcomes of a Mars mega impact using smoothed particle hydrodynamics simulations. They test different impactor masses, impactor speeds, angles of impact, and more to determine how these properties affect the properties of the Martian ejecta that result.

Mars mega-impact ejecta

Debris ejected in a Mars mega impact, at 20 hours post-impact. Blue particles are from the impactor, red particles are from Mars, yellow particles are clumps of >10 particles. [Hyodo & Genda 2018]

The authors find that a large amount of debris can be ejected from Mars during such an impact and distributed between ~0.5–3 AU in the solar system. Roughly 2% of this debris could originate from Mars’s olivine-rich, unmelted upper mantle — which could indeed be the source of the olivine-rich Mars Trojan asteroids and rare A-type asteroids.

How can we further explore this picture? Debris from a Mars mega impact would not just have been the source of new asteroids; the debris likely also collided with pre-existing asteroids — or even transferred to early Earth. Signatures of a Mars mega impact may therefore be recorded in main-belt asteroids or in meteorites found on Earth, providing tantalizing targets for future studies in the effort to map out Mars’s past.

Citation

Ryuki Hyodo and Hidenori Genda 2018 ApJL 856 L36. doi:10.3847/2041-8213/aab7f0

S106 star-forming region

Deep within giant molecular clouds, hidden by dense gas and dust, stars form. Unprecedented data from the Atacama Large Millimeter/submillimeter Array (ALMA) reveal the intricate magnetic structures woven throughout one of the most massive star-forming regions in the Milky Way.

How Stars Are Born

Horsehead Nebula

The Horsehead Nebula’s dense column of gas and dust is opaque to visible light, but this infrared image reveals the young stars hidden in the dust. [NASA/ESA/Hubble Heritage Team]

Simple theory dictates that when a dense clump of molecular gas becomes massive enough that its self-gravity overwhelms the thermal pressure of the cloud, the gas collapses and forms a star. In reality, however, star formation is more complicated than a simple give and take between gravity and pressure. The dusty molecular gas in stellar nurseries is permeated with magnetic fields, which are thought to impede the inward pull of gravity and slow the rate of star formation.

How can we learn about the magnetic fields of distant objects? One way is by measuring dust polarization. An elongated dust grain will tend to align itself with its short axis parallel to the direction of the magnetic field. This systematic alignment of the dust grains along the magnetic field lines polarizes the dust grains’ emission perpendicular to the local magnetic field. This allows us to infer the direction of the magnetic field from the direction of polarization.

Magnetic field vectors

Magnetic field orientations for protostars e2 and e8 derived from Submillimeter Array observations (panels a through c) and ALMA observations (panels d and e). Click to enlarge. [Adapted from Koch et al. 2018]

Tracing Magnetic Fields

Patrick Koch (Academia Sinica, Taiwan) and collaborators used high-sensitivity ALMA observations of dust polarization to learn more about the magnetic field morphology of Milky Way star-forming region W51. W51 is one of the largest star-forming regions in our galaxy, home to high-mass protostars e2, e8, and North.

The ALMA observations reveal polarized emission toward all three sources. By extracting the magnetic field orientations from the polarization vectors, Koch and collaborators found that the molecular cloud contains an ordered magnetic field with never-before-seen structures. Several small clumps on the perimeter of the massive star-forming cores exhibit comet-shaped magnetic field structures, which could indicate that these smaller cores are being pulled toward the more massive cores.

These findings hint that the magnetic field structure can tell us about the flow of material within star-forming regions — key to understanding the nature of star formation itself.

sin omega maps

Maps of sin ω for two of the protostars (e2 and e8) and their surroundings. [Adapted from Koch et al. 2018]

Guiding Star Formation

Do the magnetic fields in W51 help or hinder star formation? To explore this question, Koch and collaborators introduced the quantity sin ω, where ω is the angle between the local gravity and the local magnetic field.

When the angle between gravity and the magnetic field is small (sin ω ~ 0), the magnetic field has little effect on the collapse of the cloud. If gravity and the magnetic field are perpendicular (sin ω ~ 1), the magnetic field can slow the infall of gas and inhibit star formation.

Based on this parameter, Koch and collaborators identified narrow channels where gravity acts unimpeded by the magnetic field. These magnetic channels may funnel gas toward the dense cores and aid the star-formation process.

The authors’ observations demonstrate just one example of the broad realm ALMA’s polarimetry capabilities have opened to discovery. These and future observations of dust polarization will continue to reveal more about the delicate magnetic structure within molecular clouds, further illuminating the role that magnetic fields play in star formation.

Citation

Patrick M. Koch et al 2018 ApJ 855 39. doi:10.3847/1538-4357/aaa4c1

bent jets

Powerful jets emitted from the centers of distant galaxies make for spectacular signposts in the radio sky. Can observations of these jets reveal information about the environments that surround them?

Signposts in the Sky

seven bent DLRGs

VLA FIRST images of seven bent double-lobed radio galaxies from the authors’ sample. [Adapted from Silverstein et al. 2018]

An active supermassive black hole lurking in a galactic center can put on quite a show! These beasts fling out accreting material, often forming intense jets that punch their way out of their host galaxies. As the jets propagate, they expand into large lobes of radio emission that we can spot from Earth — observable signs of the connection between distant supermassive black holes and the galaxies in which they live.

These distinctive double-lobed radio galaxies (DLRGs) don’t all look the same. In particular, though the jets are emitted from the black hole’s two poles, the lobes of DLRGs don’t always extend perfectly in opposite directions; often, the jets become bent on larger scales, appearing to us to subtend angles of less than 180 degrees.

Can we use our observations of DLRG shapes and distributions to learn about their surroundings? A new study led by Ezekiel Silverstein (University of Michigan) has addressed this question by exploring DLRGs living in dense galaxy-cluster environments.

Projected density of DLRG–central galaxy matches (black) compared to a control sample of random positions–central galaxy matches (red) for different distances from a cluster center. DLRGs have a higher likelihood of being located close to a cluster center. [Silverstein et al. 2018]

Living Near the Hub

To build a sample of DLRGs in dense environments, Silverstein and collaborators started from a large catalog of DLRGs in Sloan Digital Sky Survey quasars with radio lobes visible in Very Large Array data. They then cross-matched these against three galaxy catalogs to produce a sample of 44 DLRGs that are each paired to a nearby massive galaxy, galaxy group, or galaxy cluster.

To determine if these DLRGs’ locations are unusual, the authors next constructed a control sample of random galaxies using the same selection biases as their DLRG sample.

Silverstein and collaborators found that the density of DLRGs as a function of distance from a cluster center drops off more rapidly than the density of galaxies in a typical cluster. Observed DLRGs are therefore more likely than random galaxies to be found near galaxy groups and clusters. The authors speculate that this may be a selection effect: DLRGs further from cluster centers may be less bright, preventing their detection.

Bent Under Pressure

bent vs. unbent DLRGs

The angle subtended by the DLRG radio lobes, plotted against the distance of the DLRG to the cluster center. Central galaxies (red circle) experience different physics and are therefore excluded from the sample. In the remaining sample, bent DLRGs appear to favor cluster centers, compared to unbent DLRGs. [Silverstein et al. 2018]

In addition, Silverstein and collaborators found that location appears to affect the shape of a DLRG. “Bent” DLRGs (those with a measured angle between their lobes of 170° or smaller) are more likely to be found near a cluster center than “unbent” DLRGs (those with angles of 170°–180°). The fraction of bent DLRGs is 78% within 3 million light-years of the cluster center, and 56% within double that distance — compared to a typical fraction of just 29% in the field.

These results support the idea that ram pressure — the pressure experienced by a galaxy as it moves through the higher density environment closer to the center of a cluster — is what bends the DLRGs.

What’s next to learn? This study relies on a fairly small sample, so Silverstein and collaborators hope that future deep optical surveys will increase the completeness of cluster catalogs, enabling further testing of these outcomes and the exploration of other physics of galaxy-cluster environments.

Citation

Ezekiel M Silverstein et al 2018 AJ 155 14. doi:10.3847/1538-3881/aa9d2e

Galactic center

Far from the galactic suburbs where the Sun resides, a cluster of stars in the nucleus of the Milky Way orbits a supermassive black hole. Can chemical abundance measurements help us understand the formation history of the galactic center nuclear star cluster?

Studying Stellar Populations

Stellar populations comparison

Metallicity distributions for stars in the inner two degrees of the Milky Way (blue) and the central parsec (orange). [Do et al. 2018]

While many galaxies host nuclear star clusters, most are too distant for us to study in detail; only in the Milky Way can we resolve individual stars within one parsec of a supermassive black hole. The nucleus of our galaxy is an exotic and dangerous place, and it’s not yet clear how these stars came to be where they are — were they siphoned off from other parts of the galaxy, or did they form in place, in an environment rocked by tidal forces?

Studying the chemical abundances of stars provides a way to separate distinct stellar populations and discern when and where these stars formed. Previous studies using medium-resolution spectroscopy have revealed that many stars within the central parsec of our galaxy have very high metallicities — possibly higher than any other region of the Milky Way. Can high-resolution spectroscopy tell us more about this unusual population of stars?

Spectral Lines on Display

Tuan Do (University of California, Los Angeles, Galactic Center Group) and collaborators performed high-resolution spectroscopic observations of two late-type giant stars located half a parsec from the Milky Way’s supermassive black hole.

Spectral comparison

Comparison of the observed spectra of the two galactic center stars (black) with synthetic spectra with low (blue) and high (orange) [Sc/Fe] values. Click to enlarge. [Do et al. 2018]

In order to constrain the metallicities of these stars, Do and collaborators compared the observed spectra to a grid of synthetic spectra and used a spectral synthesis technique to determine the abundances of individual elements. They found that while one star is only slightly above solar metallicity, the other is likely more than four times as metal-rich as the Sun.

The features in the observed and synthetic spectra generally matched well, but the absorption lines of scandium, vanadium, and yttrium were consistently stronger in the observed spectra than in the synthetic spectra. This led the authors to conclude that these galactic center stars are unusually rich in these metals — trace elements that could reveal the formation history of the galactic nucleus.

Old Stars, New Trends?

[Sc/Fe] trend

Scandium to iron ratio versus iron abundance for stars in the disk of the Milky Way (blue) and the stars in this sample (orange). The value reported for this sample is a 95% lower limit. [Do et al. 2018]

For stars in the disk of the Milky Way, the abundance of scandium relative to iron tends to decrease as the overall metallicity increases, but the stars investigated in this study are both iron-rich and anomalously high in scandium. This hints that the nuclear star cluster might represent a distinct stellar population with different metallicity trends.

However, it’s not yet clear what could cause the elevated abundances of scandium, vanadium, and yttrium relative to other metals. Each of these elements is linked to a different source; scandium and vanadium are mainly produced in Type II and Type Ia supernovae, respectively, while yttrium is likely synthesized in asymptotic giant branch stars. Future observations of stars near the center of the Milky Way may help answer this question and further constrain the origin of our galaxy’s nuclear star cluster.

Citation

Tuan Do et al 2018 ApJL 855 L5. doi:10.3847/2041-8213/aaaec3

NGC 3201

How many black holes lurk within the dense environments of globular clusters, and how do these powerful objects shape the properties of the cluster around them? One such cluster, NGC 3201, is now helping us to answer these questions.

Hunting Stellar-Mass Black Holes

Since the detection of merging black-hole binaries by the Laser Interferometer Gravitational-Wave Observatory (LIGO), the dense environments of globular clusters have received increasing attention as potential birthplaces of these compact binary systems.

NGC 3201 center

The central region of the globular star cluster NGC 3201, as viewed by Hubble. The black hole is in orbit with the star marked by the blue circle. [NASA/ESA]

In addition, more and more stellar-mass black-hole candidates have been observed within globular clusters, lurking in binary pairs with luminous, non-compact companions. The most recent of these detections, found in the globular cluster NGC 3201, stands alone as the first stellar-mass black hole candidate discovered via radial velocity observations: the black hole’s main-sequence companion gave away its presence via a telltale wobble.

Now a team of scientists led by Kyle Kremer (CIERA and Northwestern University) is using models of this system to better understand the impact that black holes might have on their host clusters.

A Model Cluster

The relationship between black holes and their host clusters is complicated. Though the cluster environment can determine the dynamical evolution of the black holes, the retention rate of black holes in a globular cluster (i.e., how many remain in the cluster when they are born as supernovae, rather than being kicked out during the explosion) influences how the host cluster evolves.

Kremer and collaborators track this complex relationship by modeling the evolution of a cluster similar to NGC 3201 with a Monte Carlo code. The code incorporates physics relevant to the evolution of black holes and black-hole binaries in globular clusters, such as two-body relaxation, single and binary star evolution, galactic tides, and multi-body encounters. From their grid of models with varying input parameters, the authors then determine which fit best to NGC 3201’s final observational properties.

globular cluster models

Surface brightness profiles for all globular-cluster models at late times compared to observations of NGC 3201 (yellow circles). Blue lines represent models with few retained black holes; black lines represent models with many retained black holes. [Kremer et al. 2018]

Retention Matters

Kremer and collaborators find that the models that best represent NGC 3201 all retain more than 200 black holes at the end of the simulation; models that lost too many black holes due to natal kicks did not match observations of NGC 3201 as well. The models with large numbers of retained black holes also harbored binaries just like the one recently detected in NGC 3201.

Models that retain few black holes, on the other hand, may instead be good descriptions of so-called “core-collapsed” globular clusters observed in the Milky Way. The authors demonstrate that these clusters could contain black holes in binaries with stars known as blue stragglers, which may also be detectable with radial velocity techniques.

Kremer and collaborators’ results suggest that globular clusters similar to NGC 3201 contain hundreds of invisible black holes waiting to be discovered, and they indicate some of the differences in cluster properties caused by hosting such a large population of black holes. We can hope that future observations and modeling will continue to illuminate the complicated relationship between globular clusters and the black holes that live in them.

Citation

Kyle Kremer et al 2018 ApJL 855 L15. doi:10.3847/2041-8213/aab26c

Voyager 1's journey

In 2012, Voyager 1 zipped across the heliopause. Five and a half years later, Voyager 2 still hasn’t followed its twin into interstellar space. Can models of the heliopause location help determine why?

How Far to the Heliopause?

heliosphere

Artist’s conception of the heliosphere with the important structures and boundaries labeled. [NASA/Goddard/Walt Feimer]

As our solar system travels through the galaxy, the solar outflow pushes against the surrounding interstellar medium, forming a bubble called the heliosphere. The edge of this bubble, the heliopause, is the outermost boundary of our solar system, where the solar wind and the interstellar medium meet. Since the solar outflow is highly variable, the heliopause is constantly moving — with the motion driven by changes in the Sun.

NASA’s twin Voyager spacecraft were poised to cross the heliopause after completing their tour of the outer planets in the 1980s. In 2012, Voyager 1 registered a sharp increase in the density of interstellar particles, indicating that the spacecraft had passed out of the heliosphere and into the interstellar medium. The slower-moving Voyager 2 was set to pierce the heliopause along a different trajectory, but so far no measurements have shown that the spacecraft has bid farewell to our solar system.

In a recent study, a team of scientists led by Haruichi Washimi (Kyushu University, Japan and CSPAR, University of Alabama-Huntsville) argues that models of the heliosphere can help explain this behavior. Because the heliopause location is controlled by factors that vary on many spatial and temporal scales, Washimi and collaborators turn to three-dimensional, time-dependent magnetohydrodynamics simulations of the heliosphere. In particular, they investigate how the position of the heliopause along the trajectories of Voyager 1 and Voyager 2 changes over time.

Modeled location of the heliopause

Modeled location of the heliopause along the paths of Voyagers 1 (blue) and 2 (orange). Click for a closer look. The red star indicates the location at which Voyager 1 crossed the heliopause. The current location of Voyager 2 is marked with a red circle. [Washimi et al. 2017]

A Time-Varying Barrier

The authors consider the impact that solar flares, coronal mass ejections, and other disturbances in the solar outflow have on the heliopause distance. These solar disturbances intermingle as they travel outward to form what the authors call global merged interaction regions.

Using their hydrodynamical simulations, Washimi and collaborators capture the complex behavior of the global merged interaction regions as they propagate through the termination shock and collide with the heliopause. Part of the shock is transmitted into the local interstellar medium, while part of it is reflected back toward and collides with the termination shock, which is pushed toward the Sun. This complex interplay of transmitted and reflected shocks — combined with the nonuniformity of the local interstellar medium — causes the heliopause location to vary dramatically in time as well as space.

What Does this Mean for Voyager 2?

Washimi and collaborators find that the location of the heliopause along the trajectories of Voyagers 1 and 2 has changed considerably over the past decade. In particular, they find that the heliopause has been pushed outward over the past few years due to an increase in the solar wind ram pressure. According to their simulations, Voyager 2 is currently traveling outward faster than the heliopause is advancing, which means that the spacecraft should soon cross the boundary — perhaps even this year — to become Earth’s second interstellar messenger.

Citation

Haruichi Washimi et al 2017 ApJL 846 L9. doi:10.3847/2041-8213/aa8556

NICER

What happens to a neutron star’s accretion disk when its surface briefly explodes? A new instrument recently deployed at the International Space Station (ISS) is now watching bursts from neutron stars and reporting back.

Deploying a New X-Ray Mission

NICER launch

Launch of NICER aboard a Falcon 9 rocket in June 2017. [NASA/Tony Gray]

In early June of 2017, a SpaceX Dragon capsule on a Falcon 9 rocket launched on a resupply mission to the ISS. The pressurized interior of the Dragon contained the usual manifest of crew supplies, spacewalk equipment, and vehicle hardware. But the unpressurized trunk of the capsule held something a little different: the Neutron star Interior Composition Explorer (NICER).

In the two weeks following launch, NICER was extracted from the SpaceX Dragon capsule and installed on the ISS. And by the end of the month, the instrument was already collecting its first data set: observations of a bright X-ray burst from Aql X-1, a neutron star accreting matter from a low-mass binary companion.

Impact of Bursts

NICER’s goal is to provide a new view of neutron-star physics at X-ray energies of 0.2–12 keV — a window that allows us to explore bursts of energy that neutron stars sometimes emit from their surfaces.

X-ray binary

Artist’s impression of an X-ray binary, in which a compact object accretes material from a companion star. [ESA/NASA/Felix Mirabel]

In X-ray burster systems, hydrogen- and helium-rich material from a low-mass companion star piles up in an accretion disk around the neutron star. This material slowly funnels onto the neutron star’s surface, forming a layer that gravitationally compresses and eventually becomes so dense and hot that runaway nuclear fusion ignites.

Within seconds, the layer of material is burned up, producing a burst of emission from the neutron star that outshines even the inner regions of the hot accretion disk. Then more material funnels onto the neutron star and the process begins again.

Though we have a good picture of the physics that causes these bursts, we don’t yet understand the impact that these X-ray flashes have on the accretion disk and the environment surrounding the neutron star. In a new study led by Laurens Keek (University of Maryland), a team of scientists now details what NICER has learned on this subject.

Extra X-Rays

Aql X-1 burst

Light curve (top) and hardness ratio (bottom) for the X-ray burst from Aql X-1 captured by NICER on 3 July 2017. [Keek et al. 2018]

In addition to thermal emission from the neutron star, NICER revealed an excess of soft X-ray photons below 1 keV during Aql X-1’s burst. The authors propose two possible models for this emission:

  1. The burst radiation from the neutron star’s surface was reprocessed — i.e., either scattered or absorbed and re-emitted — by the accretion disk.
  2. The persistent, usual accretion flow was enhanced as a result of the burst’s radiation drag on the disk, briefly bumping up the disk’s X-ray flux.

While we can’t yet conclusively state which mechanism dominates, NICER’s observations do show that bursts have a substantial impact on their accretion environment. And, as there are over 100 such X-ray burster systems in our galaxy, we can expect that NICER will allow us to better explore the effect of X-ray bursts on neutron-star disks and their surroundings in many different systems in the future.

Bonus

Check out the awesome gif below, provided by NASA, which shows NICER being extracted from the Dragon capsule’s trunk by a robotic arm.

Citation

L. Keek et al 2018 ApJL 855 L4. doi:10.3847/2041-8213/aab104

Cassiopeia A supernova remnant

Supernova explosions enrich the interstellar medium and can even briefly outshine their host galaxies. However, the mechanism behind these massive explosions still isn’t fully understood. Could probing the asymmetry of supernova remnants help us better understand what drives these explosions?

SN1987a

Hubble image of the remnant of supernova 1987A, one of the first remnants discovered to be asymmetrical. [ESA/Hubble, NASA]

Stellar Send-Offs

High-mass stars end their lives spectacularly. Each supernova explosion churns the interstellar medium and unleashes high-energy radiation and swarms of neutrinos. Supernovae also suffuse the surrounding interstellar medium with heavy elements that are incorporated into later generations of stars and the planets that form around them.

The bubbles of expanding gas these explosions leave behind often appear roughly spherical, but mounting evidence suggests that many supernova remnants are asymmetrical. While asymmetry in supernova remnants can arise when the expanding material plows into the non-uniform interstellar medium, it can also be an intrinsic feature of the explosion itself.

Single-lobe explosion

Simulation results clockwise from top left: Mass density, calcium mass fraction, oxygen mass fraction, nickel-56 mass fraction. Click to enlarge. [Adapted from Wollaeger et al. 2017]

Coding Explosions

The presence — or absence — of asymmetry in a supernova remnant can hold clues as to what drove the explosion. But how can we best observe asymmetry in a supernova remnant? Modeling lets us explore different observational approaches.

A team of scientists led by Ryan T. Wollaeger (Los Alamos National Laboratory) used radiative transfer and radiative hydrodynamics simulations to model the explosion of a core-collapse supernova. Wollaeger and collaborators introduced asymmetry into the explosion by creating a single-lobed, fast-moving outflow along one axis.

Their simulations showed that while some chemical elements lingered near the origin of the explosion or were distributed evenly throughout the remnant, calcium was isolated to the asymmetrical region, hinting that spectral lines of calcium may be good tracers of asymmetry.

Synthetic light curves

Bolometric (top) and gamma-ray (bottom) synthetic light curves for the authors’ model for a range of simulated viewing angles. [Adapted from Wollaeger et al. 2017]

Synthesizing Spectra

Wollaeger and collaborators then generated synthetic light curves and spectra from their models to determine which spectral features or characteristics indicated the presence of the asymmetric outflow lobe. They found that when an asymmetric outflow lobe is present, the peak luminosity of the explosion depends on the angle at which you view it; the highest luminosity occurs when the lobe is viewed from the side, while the lowest luminosity — nearly 40% dimmer — is seen when the explosion is viewed “down the barrel” of the lobe. The dense outflow shades the central radioactive source from view, lowering the luminosity.

This effect also plays out in the gamma-ray light curves; when viewed down the barrel, the shading of the central source by a high-density lobe slows the rise of the gamma-ray luminosity and changes the shape of the light curve compared to views from other vantage points.

Another promising avenue for exploring asymmetry is a near-infrared band encompassing an emission line of singly-ionized calcium near 815 nm. Since calcium is confined within the outflow lobe in the simulation, its emission lines are blueshifted when the lobe points toward the observer.

The authors point out that there is much more to be done in their models, such as including the effects of shock heating of circumstellar material, which can contribute strongly to the light curve, but these simulations bring us a step closer to understanding the nature of asymmetrical supernova remnants — and the explosions that create them.

Citation

Ryan T. Wollaeger et al 2017 ApJ 845 168. doi:10.3847/1538-4357/aa82bd

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