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Voyager spacecraft

In 1977, two space probes were launched from Earth, flung out toward the farthest reaches of our solar system. Now, 43 years later, Voyager 1 and Voyager 2 are journeying through interstellar space — and they’re still providing new insights.

Voyaging to the Outer Edge

The original mission of the Voyager spacecraft was to study the giant planets in our outer solar system. But across 43 years and three mission extensions, these little probes have gone on to do so much more — most recently crossing out of the heliosphere and providing our first up-close look at interstellar space.

heliosphere

Artist’s conception of the heliosphere (shown in the opposite orientation as in the cover image above). The heliospheric nose is on the left here, and the tail is on the right. Click to enlarge. [NASA/Goddard/Walt Feimer]

What’s the heliosphere? As the solar wind streams from the Sun, it carries magnetic fields outward, inflating a bubble around the solar system that separates us from the surrounding interstellar medium (ISM). As the Sun orbits through the galaxy, the heliosphere is compressed on one side and elongated on the other, forming a blunt “nose” and a streaming “tail”.

Into the Unknown

When Voyagers 1 and 2 were launched, they were sent in slightly different directions — so they’re now exploring two different regions of the interface between the heliosphere and the interstellar medium. In 2012, Voyager 1 crossed the boundary of the heliosphere on one side of the nose, at a distance of ~122 au from the Sun. Voyager 2 followed suit in 2018, crossing the other side of the nose at a distance of ~119 au.

Voyager Antennae

This artist’s impression of one of the Voyager spacecraft shows, on the left, the V-shaped pair of antennae used to detect plasma oscillations. [NASA/JPL]

Now, both spacecraft are traveling through the very local ISM beyond the heliosphere. But despite their distance (the one-way light travel time to Voyager 1 is ~21 hours!), the probes are still reporting back data — including from the Plasma Wave Science (PWS) instrument on each craft, which uses the long, V-shaped pair of antennae to measure oscillations in the surrounding plasma. From these oscillations, we can infer the electron density of the ISM that the Voyager spacecraft are traveling through.

Denser and Denser

In a new publication, University of Iowa scientists William Kurth and Donald Gurnett report the latest PWS measurement from Voyager 2, which indicates that the electron density of the ISM is currently increasing as the probe travels away from the Sun. This discovery is neatly consistent with the data from Voyager 1, which has also been reporting an increasing radial density gradient since crossing the boundary of the heliosphere and entering interstellar space.

electron density

Electron density vs. radial distance from the Sun, as measured by the Voyager 1 (black) and Voyager 2 (red) spacecraft. The radial density gradient in the ISM can be seen in the data from both probes at distances above ~120 au. Click to enlarge. [Kurth & Gurnett 2020]

Voyagers 1 and 2 have trajectories that differ by 67° in latitude and 43° in longitude — so with the new Voyager 2 data published by Kurth and Gurnett, we now have confirmation that the radial density gradient first measured by Voyager 1 is a large-scale feature around the heliospheric nose.

Still More to Learn

What’s causing the gradient? Two theories have been put forward:

  1. the interaction of the solar wind with the very local ISM creates a pile-up region outside of the heliosphere, or
  2. draping of magnetic field lines over the outer boundary of the heliosphere depletes the plasma just inside the heliosphere.

We’ll potentially be able to differentiate between these two models once we have density measurements from even farther out in the ISM — so we’ll have to see if the Voyager probes last long enough to provide them!

Citation

“Observations of a Radial Density Gradient in the Very Local Interstellar Medium by Voyager 2,” W. S. Kurth and D. A. Gurnett 2020 ApJL 900 L1. doi:10.3847/2041-8213/abae58

BH-NS merger

On August 16, 2019, both the Fermi Gamma-ray Burst Monitor (GBM) and the Laser Interferometer Gravitational-wave Observatory (LIGO) detected faint blips that didn’t quite register as events. But could these ghost signals actually correspond to the first collision we’ve detected of a black hole with a neutron star?

Neutron Stars and Black Holes: Mix and Match

LIGO’s first detection of gravitational waves was GW150914, the collision of a pair of black holes. In the years since, LIGO has partnered with its European counterpart, Virgo, to make another dozen confirmed detections of binary black holes merging. The collaboration also spotted two instances of binary neutron stars colliding — one of which, GW170817, was accompanied by a short gamma-ray burst and emission spanning the electromagnetic spectrum.

Stellar Graveyard GW190521

A recent version of the “stellar graveyard”, a plot that shows the masses of the different components of confirmed compact binary mergers. No definite NS–BH mergers have been detected yet. Click to enlarge. [LIGO-Virgo/Northwestern U./Frank Elavsky & Aaron Geller]

But LIGO/Virgo’s mix-and-match collection is incomplete: we’re still waiting to detect a definite neutron-star–black-hole collision. In particular, we’d like to spot a merger in which the neutron star is tidally destroyed by the black hole, lighting up the sky with accompanying electromagnetic emission.

Could it be that such an event is actually hidden in the reject data from LIGO/Virgo and Fermi?

A Pair of Intriguing (Non-?)Events

The results from LIGO-Virgo’s third observing run, cut short by the pandemic in March 2020, are still being carefully analyzed by the collaboration. The O3 alert data, however, is publicly available — and a team of scientists have taken advantage of this to do some independent analysis, recently detailed in a publication led by Yi-Si Yang (Nanjing University, China).

Yang and collaborators take note of two faint signals that occurred on August 16, 2020:

Fermi light curve

The accumulated light curve for GBM-190816 shows the duration of the gamma-ray burst, roughly 0.1 seconds. [Adapted from Yang et al. 2020]

  1. A subthreshold gravitational-wave event in the LIGO/Virgo data — i.e., an event with a signal-to-noise ratio below 12, the threshold to qualify as a significant candidate.
  2. A subthreshold gamma-ray burst, GBM-190816, that was picked up by Fermi/GBM just 1.57 seconds after the gravitational-wave event.

If these two signals are both real and related, then GBM-190816 represents a short gamma-ray burst emitted from the merger of two compact objects — and Yang and collaborators’ analysis shows that, with a mass ratio of q ~ 2.26, the system is most likely a neutron-star–black-hole binary. In the simplest explanation, the neutron star was torn apart before the bodies ultimately merged, producing the pair of signals.

Identifying What’s Real

So are these subthreshold events real? We can’t say, yet! The public alerts from LIGO/Virgo only contain a portion of the signal information, so Yang and collaborators had to make a number of assumptions to analyze the event.

short vs. long GRBs

The parameters of GBM-190816 (marked by a red star), like the peak-to-background flux ratio vs. duration shown here, are consistent with typical short gamma-ray bursts (blue triangles). [Adapted from Yang et al. 2020]

That said, the faintness of both signals is reasonable given the parameters of this potential merger: if real, it took place at a distance of 1.4 billion light-years, roughly nine times farther away than GW170817. The gamma-ray spike was also extremely short — just ~0.1 seconds, compared to the ~2 second duration of GW170817 — which is what caused it to register below the Fermi/GBM trigger threshold.

If confirmed, this event could provide interesting insight into how the light emitted by such a merger escapes and travels to us. Now we just have to wait for the official joint analysis from the LIGO/Virgo/Fermi team!

Citation

“Physical Implications of the Subthreshold GRB GBM-190816 and Its Associated Subthreshold Gravitational-wave Event,” Yi-Si Yang et al 2020 ApJ 899 60. doi:10.3847/1538-4357/ab9ff5

Artist's impression of an M dwarf planetary system

Indirect detections of exoplanets rely heavily on the properties of their host stars. However, stellar features can sometimes masquerade as planetary signals. This issue is especially prominent for M dwarfs. So how do we know for sure if we’ve found a planet around an M dwarf?

Starring M Dwarfs

Despite the observational challenges they pose, M dwarfs present exciting prospects for exoplanet science. With their low masses, they are especially susceptible to the gravitational influence of any orbiting planets. When an M dwarf is tugged on by an orbiting planet, this affects the star’s radial velocity, creating a strong Doppler signal that we can detect in its spectrum.

Habitable Zones of A, G, and M stars

The extent of the habitable zone (highlighted in green) for A stars, G stars, and M stars. As indicated in the figure, the Sun is a G star. [NASA]

In addition, the habitable zone of an M dwarf is located very close to the star. This means that planets in the habitable zone of an M dwarf would produce a stronger Doppler signal than planets in the habitable zone of a higher mass, hotter A star.

But there’s a catch! While planets can produce detectable Doppler signals, so can M dwarfs themselves. Starspots, or cool regions on the surface of stars, can be prominent in the stellar spectra, and as an M dwarf rotates, starspots can mimic the effects of a planet.

So what’s to be done? M dwarfs are complicated beasts, so astronomers have been attempting to characterize them in more detail. In a new study, a group of researchers led by Paul Robertson (The University of California, Irvine) used optical and near-infrared observations of M dwarfs to understand the effects of starspots and other surface features.

Artist's impression of an M dwarf flaring

An artist’s impression of an M dwarf flaring like AD Leo was observed to do so earlier this year. However, AP Leo is not confirmed to host any planets. [National Astronomical Observatory of Japan]

Spinning Around and Around

For their study, Robertson and collaborators selected four rapidly-rotating M dwarfs. Here, “rapid” means the rotational speed at the surface of the star is about 3 to 11 kilometers per second. Three of the stars in the sample were chosen because they were similar to the fourth, AD Leo. AD Leo is only 16 light-years from the Earth and has been studied extensively in the context of its magnetic activity, flares, and starspots.

Robertson and collaborators pulled observations from a number of instruments, including the Transiting Exoplanet Survey Satellite and the HARPS Spectrograph. They also used data taken by the Habitable-zone Planet Finder on the Hobby-Eberly Telescope, which was built especially for finding low-mass planets around M dwarfs.

Separating Out Planetary Signals

RV Observations of GJ 3959

The phased radial-velocity/Doppler signals from the M dwarf GJ 3959 as observed by the Habitable-zone Planet Finder (HPF) and the High Resolution Echelle Spectrometer (HIRES). The phased signal represents a stacking of the observations based on the rotational period of GJ 3959, which is about half a day. [Robertson et al. 2020]

All four M dwarfs showed prominent Doppler signals that persisted for longer than expected — hundreds of stellar rotations! — and also synced up with the stars’ rotational periods. Since the stars in this sample aren’t known exoplanet hosts, these stable Doppler signals are likely caused by surprisingly persistent surface features like starspots.

These findings contain a blessing and a curse for exoplanet searchers. Any planets whose orbital period is similar to the rotational period of the star will be very hard to distinguish. The longevity of the signals means it’ll likely be a long wait for the surface features to die out so we can verify if a planet was contributing to the signal. However, since the Doppler signals are stable and fairly predictable, they could be modeled and removed.

This study highlights the challenges of searching for planets around M dwarfs. But the finds would definitely be worth the trouble!

Citation:

“Persistent Starspot Signals on M Dwarfs: Multiwavelength Doppler Observations with the Habitable-zone Planet Finder and Keck/HIRES,” Paul Robertson et al 2020 ApJ 897 125. doi:10.3847/1538-4357/ab989f

AGN outflow star formation

The size of a supermassive black hole seems to track with the size of its host galaxy. But is this a statistical fluke, or is there a physical reason for the connection? Recent modeling provides new clues.

Growing Together

Centaurus A

Composite image of Centaurus A, a galaxy whose appearance is dominated by the large-scale jets emitted by the supermassive black hole at its center. [ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray); CC BY 4.0]

The supermassive black holes — black holes of millions to tens of billions of solar masses — that lurk at the active centers of galaxies have a peculiar quirk: their masses correlate with the stellar masses of their hosts. This means that the larger a galaxy is, the larger we can expect its central black hole to be.

But why the connection? How can the black hole know about the galaxy size around it, and vice versa? There are a few proposed explanations for the correlation between central-black-hole mass and its host galaxy’s stellar mass:

  1. It’s caused by active galactic nucleus (AGN) feedback.
    In this scenario (recently described here), jets and winds from the accreting black hole can both trigger star formation and quench it by expelling extra gas — star-formation fuel — from the galaxy. This feedback causes the rate of star formation to roughly track the rate of black hole accretion.
  2. It’s caused by black-hole growth and galaxy star formation both relying on the same fuel source.
    If black-hole growth and galactic star formation both arise from the same source of fuel, then these two growths should correlate even if they don’t influence each other. One example of a fuel source that could cause sudden growth is a wet merger — the collision of two galaxies rich in gas.
  3. It’s simply a consequence of statistics, and not caused by a physical mechanism.
    A theorem known as the central limit theorem suggests that the correlation we observe could arise naturally as a statistical consequence of building up galaxies hierarchically from smaller structures over time. In this scenario, there’s no physical connection between the black hole and its host galaxy — it’s all statistics.

It’s a Matter of the Model

To test which of these explanations is most likely, we need to combine models with observations. A new study led by Xuheng Ding (University of California, Los Angeles) recently set out to do so.

M-sigma correlation

Plots showing the correlation between black hole mass (MBH) and stellar mass of the host galaxy (M*. The actual observations are plotted in orange. The blue dots in the top plot show the simulated correlation from the hydrodynamic simulation; the green dots in the bottom plot show the simulated correlation from the semianalytic model. The hydro simulation provides a better match to the data. [Adapted from Ding et al. 2020]

Ding and collaborators first compiled observations of a sample of 32 accreting supermassive black holes and their host galaxies. The sample is from the redshift range 1.2 < z < 1.7, a time in our universe’s history when most supermassive black holes acquired their mass.

The authors then compared these data to the outputs of two state-of-the-art models: a hydrodynamic simulation that focuses on AGN feedback, and a semianalytic model that is especially sensitive to wet galaxy-merger events.

Connecting via Feedback

The results of the authors’ models — in particular, the tightness of the modeled correlation between black-hole and galaxy size across redshifts — strongly support a physical mechanism driving the connection, rather than it being a consequence of statistics.

Of the two models, the hydrodynamic simulation better reproduced the scatter of the correlation, suggesting that AGN feedback may indeed be the driver ensuring that supermassive black holes grow at the same rate as their host galaxies. Observations out to higher redshifts — which may be possible with the upcoming James Webb Space Telescope — will help us further tease out the explanation for this intriguing link.

Citation

“Testing the Fidelity of Simulations of Black Hole–Galaxy Coevolution at z ~ 1.5 with Observations,” Xuheng Ding et al 2020 ApJ 896 159. doi:10.3847/1538-4357/ab91be

Taurus Molecular Cloud complex

What as-yet unidentified molecules lurk in the dark clouds of our nearby universe? Answering this requires observation, experiment, and theory — and GOTHAM is on the case.

The Search for New Chemistry

Taurus molecular cloud

Another view of the Taurus Molecular Cloud, captured here in millimeter wavelengths by the APEX telescope. [ESO]

In the census of the molecular makeup of our universe, the interstellar medium (ISM) is the best target for diversity: we’ve spotted just over 200 different molecules in our galaxy’s ISM. We expect that there are many more out there, however — and identifying where these different molecules are found will help us to understand when and how they form.

To this end, a team of scientists is undertaking the GOTHAM project: a radio search of a cold, dark cloud located 450 light-years away called the Taurus Molecular Cloud 1 (TMC-1). This chilly cloud has not yet collapsed to form a star, providing us with an opportunity to identify new molecules that are able to form in a cold, pre-stellar environment.

Hiding in Slow Motion

But the hunt for molecules in a cold, dark cloud is challenging! We generally identify molecules by searching for their signature transition lines in ISM spectra. But in cold clouds, molecules aren’t moving much, which makes their spectral lines very narrow. This means that we need extremely high-spectral-resolution telescopes to be able to identify them. Fortunately, GOTHAM leverages the 100-meter Green Bank Telescope (GBT), which is up to the task!

Green Bank Telescope

The 100-meter Green Bank Telescope in West Virginia. [NRAO/AUI/NSF]

In a new article led by Brett McGuire (MIT, NRAO, and Center for Astrophysics | Harvard & Smithsonian), a team of scientists details the GOTHAM project and its early science results. This is just one of six new articles that describe the first molecular detections by GOTHAM.

An Attack from All Angles

How does a new molecular detection work? As an example, we can look at GOTHAM’s discovery of propargyl cyanide (HCCCH2CN) in TMC-1.

First, due to the GBT’s high spectral resolution, the team needed to produce new, fine-detail guides for the forest of spectral lines expected from propargyl cyanide. This required new laboratory measurements of the molecule.

propargyl cyanide detections

Individual line detections of propargyl cyanide in the GOTHAM data. [McGuire et al. 2020]

Next, the team had to search for these lines in the GBT data. Propargyl cyanide has 3,700 transitions that fall within GOTHAM’s observing range, all contributing to the total flux seen for this molecule. Teasing out these signatures requires complex data analysis.

Finally, after achieving a significant detection of the molecule, the team had to do a sanity check. They conducted simulations of TMC-1 using astrochemical codes and included different channels that could form and destroy propargyl cyanide. They then checked that the abundances they measured for this molecule matched expectations from the simulations.

More Discoveries Ahead

This multi-faceted process has already led to a number of new detections in addition to propargyl cyanide. The detections are announced across a set of six articles — including an additional ApJ Letters publication, in which Ci Xue (University of Virginia, Charlottesville) and collaborators detail the first astronomical detection of isocyanodiacetylene (HC4NC) and the implications for how this molecule and others like it form in the ISM. 

What’s more, all of these new results still only make up 30% of the eventual data that will be collected for the GOTHAM project. There’s plenty more to look forward to in the future as we continue to expand our understanding of the chemistry of the universe around us.

Citation

“Early Science from GOTHAM: Project Overview, Methods, and the Detection of Interstellar Propargyl Cyanide (HCCCH2CN) in TMC-1,” Brett A. McGuire et al 2020 ApJL 900 L10. doi:10.3847/2041-8213/aba632

“Detection of Interstellar HC4NC and an Investigation of Isocyanopolyyne Chemistry under TMC-1 Conditions,” Ci Xue et al 2020 ApJL 900 L9. doi:10.3847/2041-8213/aba631

Y-dwarf star

We’ve tallied up a lot of the stars and substars that lie within our solar neighborhood, but we’re missing a key population: the coolest, dimmest substar dwarfs that lurk nearby. A citizen science study is now filling in this gap with the discovery of 95 new “backyard worlds.”

A Gap in the Census

types of brown dwarfs

Three types of progressively cooler brown dwarfs and their characteristics (click to enlarge). Y dwarfs are the coldest of substars. [NASA/JPL-Caltech/Backyard Worlds]

Stellar classification roughly tracks with star temperature, ranging from the wildly hot and bright O-type stars (which burn at more than 30,000 K) all the way down to dim brown-dwarf substars of T and Y types (which can be nearly as cool as Earth, at just 300 K).

To better understand how stars and substars are distributed across this range, we’ve attempted to take a census of the bodies in our local solar neighborhood and observe their characteristics. The challenge in this lies on the brown-dwarf end: we’ve observed very few of the smallest, coldest substars in our solar backyard, because they’re so dim and hard to spot.

How to Spot a Hidden Substar

The key to finding these lurkers is all-sky surveying at long infrared wavelengths. The Wide-field Infrared Survey Explorer (WISE) is an ideal telescope for the task: this spacecraft has surveyed the entire sky in infrared 14 times over the span of a decade, providing us with a wealth of archived data. By searching for cool, dim objects that move quickly between successive images, we can identify the nearby brown dwarfs that we’ve been missing.

The catch? WISE’s imaging archive contains over 30 trillion pixels! Identifying small, dim, moving objects requires human eyes — a lot of them. It’s definitely time for crowd-sourcing.

A Job for 200,000 Eyes

The citizen science project Backyard Worlds: Planet 9 (which we’ve previously talked about) relies on more than a hundred thousand volunteers to examine WISE images and spot cool, nearby star and substar candidates — and after roughly three years of work, the project has now completed around 1.5 million classifications!

Backyard Worlds flipbook

This gif illustrates how Backyard Worlds volunteers identify nearby, cold substars: by looking for objects that move (like the “dipole” and “mover” marked here) between successive WISE images. [Backyard Worlds]

In a recent publication, a team of scientists led by Aaron Meisner (NSF’s NOIRLab) describes the follow-up of some of the most likely nearby, cold brown dwarf candidates from Backyard Worlds with the Spitzer space telescope.

Discoveries in the Data

Backyard Worlds spatial distribution

Full-sky distribution of the 96 Backyard Worlds targets followed up with Spitzer. [Meisner et al. 2020]

Meisner and the Backyard Worlds team used Spitzer to confirm 75 objects as newly discovered members of the solar neighborhood. Their discoveries include:

  • A number of Y-dwarf candidates. These are the coldest of substars, of which there are only a handful known.
  • Two new worlds that lie within 30 light-years of the Sun.
  • Two sources moving faster than 2” per year — a potential indicator that they’re relatively old objects with low metallicity.
  • A T-dwarf substar that appears to be in a binary with a white dwarf.
brown dwarf white dwarf

Artist’s impression of a brown dwarf orbiting a white dwarf. [NOIRLab/NSF/AURA/P. Marenfeld, Acknowledgement: William Pendrill]

There’s still ~1,500 more Backyard Worlds candidates to be followed up, and many of the team’s discoveries will make excellent targets for future observations and characterization. The continued exploration of these coldest substellar neighbors will help us to bridge the gap between low-mass stars and massive planets, expanding our understanding of the worlds that lurk in our solar neighborhood.

Citation

“Spitzer Follow-up of Extremely Cold Brown Dwarfs Discovered by the Backyard Worlds: Planet 9 Citizen Science Project,” Aaron M. Meisner et al 2020 ApJ 899 123. doi:10.3847/1538-4357/aba633

FRB

Fast radio bursts are perplexing astrophysical phenomena. As their name suggests, they’re essentially short radio signals, but they pack a surprising amount of energy. More unusual is that some fast radio bursts repeat, while others are one-off events.

Repeating fast radio bursts present an opportunity to study bursts in more detail. So what do we see when we observe a burst at multiple frequencies simultaneously?

FRB 121102

FRB 121102, the first fast radio burst found to repeat, was also the first to be localized in the sky. [Gemini Observatory/AURA/NSF/NRC]

A Tendency to Repeat Itself

Fast radio bursts (FRBs) typically last only a few milliseconds, but the strength with which they’re detected suggests that FRBs are produced by extremely energetic processes. What these processes are is an open question. Practically all known FRBs originated outside the Milky Way, though that might no longer be the case.

Some FRBs are known to repeat, allowing for their origin to be pinpointed far more accurately than one-off FRBs. The first known repeating FRB, called FRB 121102, lives in a dwarf galaxy over 2 billion light-years away. FRB 121102 has produced hundreds of bursts since its discovery, and studies have determined that it can be detected at multiple radio frequencies.

A new study led by Walid Majid (Jet Propulsion Laboratory/California Institute of Technology) revisited FRB 121102 using DSS-43, a 70-meter radio telescope in NASA’s Deep Space Network. The goal of this study was to probe FRB 121102’s bursts at higher frequencies than previously studied and to examine the bursts’ appearance in broadband observations.

The radio telescope DSS-43, which is located in Canberra, Australia. [NASA]

Only One Right Frequency?

Broadband observations of FRBs provide spectra of the bursts, and spectra are extremely useful. In the case of FRBs, spectral features could either be caused by the mechanism of the burst itself, or they could instead have been added as the signal propagated through the host environment, across intergalactic space, and then through the Milky Way to reach us.

Majid and collaborators observed FRB 121102 with DSS-43 for nearly six hours on September 19, 2019. The observations were centered at 2.25 (S band) and 8.36 gigahertz (X band) with usable bandwidths of ~100 and ~430 megahertz respectively. Six bursts were observed in this time — but they were only seen in the lower-frequency S band!

Example Burst from FRB 121102

The brightest burst observed from FRB 121102 as seen in the S band (bottom panel) and not seen in the X band (middle panel). The top panel shows the strength of the burst signal in the S band (black line) and the X band (grey line) as the signal-to-noise ratio versus time. In the middle and bottom panels, the signal is shown as frequency versus time, with dark areas corresponding to the burst. The time unit is milliseconds. [Adapted from Majid et al. 2020]

It All Depends on How You Look at It

The lack of a high-frequency detection for FRB 121102 is interesting, especially since the X band had a larger bandwidth than the S band. Does this frequency dependence provide insight into the FRB emission mechanism? Or does it only arise as the signal propagates to us?

Majid and collaborators explored the possibility that scintillation in our galaxy could be responsible for the lack of visible activity in the X band. In the context of FRBs, galactic scintillation is the observation of multiple bursts at various frequencies, caused by burst photons interacting with material in the Milky Way. The authors show that galactic scintillation can’t account for FRB 121102’s observations, suggesting the frequency dependence may have more to do with intrinsic properties of the emission mechanism or properties of the FRB’s host galaxy.

As with most things in astronomy, more observations are required. Wajid and collaborators concluded that dense, multi-frequency observations of FRB 121102 would go a long way to understanding its behavior. And so the mystery of FRBs continues!

Citation

“A Dual-band Radio Observation of FRB 121102 with the Deep Space Network and the Detection of Multiple Bursts,” Walid A. Majid et al 2020 ApJL 897 L4. doi:10.3847/2041-8213/ab9a4a

coronal mass ejection

Can we tell when a solar flare will lead to a potentially hazardous eruption of plasma from the Sun? A new look at hundreds of past solar flares may provide some clues.

Belches from the Sun

Solar flare

A large solar flare may or may not be accompanied by a violent ejection of matter in a CME. [NASA/SDO]

Two of the most energetic phenomena in our solar system are solar flares and coronal mass ejections (CMEs). While both are explosions produced in active regions on the Sun, they are distinctly different: solar flares are intense bursts of radiation spanning the electromagnetic spectrum, whereas CMEs are violent, directed eruptions of hot, magnetized plasma — sometimes containing more than a billion tons of matter — into space. While the two phenomena sometimes arise hand in hand, this is not always the case.

The Earth’s magnetic field does a good job of protecting us from the greatest impact of these eruptions, but a CME directed at Earth still has the potential to be hazardous to our technology and communications systems, as well as to any unshielded life (such as astronauts on a lunar mission). Scientists are therefore interested in better understanding which solar flares are likely to be eruptive — i.e., accompanied by a CME — and which ones will instead be confined.

confined and eruptive flares

Top: For active regions with low magnetic flux, more flares tend to be eruptive (blue), whereas for active regions with high magnetic flux, more flares tend to be confined (red). Bottom: The proportion of eruptive flares (PE) decreases with increasing active region magnetic flux. [Adapted from Li et al. 2020]

A Decade of Activity

To this end, a team of scientists led by Ting Li (Chinese Academy of Sciences) recently constructed an extensive catalog of large (M- and X-class) solar flares and associated CMEs observed in 2010–2019 — a time period that spans nearly the entirety of the Sun’s most recent solar activity cycle.

The team then analyzed the 322 resulting flares and cataloged them as either eruptive or confined, depending on whether or not there was an associated CME. Finally, Li and collaborators analyzed the properties of the active regions from which the flares and CMEs arose.

Exercising Magnetic Restraint

Li and collaborators found an anticorrelation between the total magnetic flux of active regions and the proportion of eruptive flares. That means that the active regions with especially small magnetic fluxes were very likely to have flares with accompanying CMEs, whereas the active regions with especially large magnetic fluxes were more likely to have confined flares with no eruptions.

decay index v. total magnetic flux

The critical decay index height vs. active region magnetic flux for eruptive (blue) and confined (red) flares shows that regions with more magnetic flux are also more confined. [Adapted from Li et al. 2020]

Why? Perhaps counterintuitively, the stronger magnetic flux actually helps to hold back the flares, preventing them from breaking out and expelling matter in a CME. The authors find additional evidence supporting this picture: the critical decay index height — a measure of how quickly the magnetic field drops off with height — indicates that the active regions with greater flux also have stronger confinement.

Space Weather Predictions

Li and collaborators’ publicly available catalog and results provide us with valuable clues to help forecast CMEs in association with large flares. In addition, these outcomes could have implications beyond our own solar system: they may help us to better understand the flares we’ve witnessed from other stars and even assess the potential habitability of their planets. As we learn more, future solar and stellar belches may become just a little less unpredictable.

Citation

“Magnetic Flux of Active Regions Determining the Eruptive Character of Large Solar Flares,” Ting Li et al 2020 ApJ 900 128. doi:10.3847/1538-4357/aba6ef

Illustration of a space probe with two sets of circular solar panels in the foreground of a small, rocky body.

Next year, the Lucy space probe will launch on a journey to visit several asteroids in our solar system. One of its destinations, 3548 Eurybates, has recently been discovered to harbor a satellite — providing Lucy with a new target to explore.

A Trojan Population

Trojan orbits

Animated figure illustrating the orbits of the inner planets, Jupiter (orange disk and orbit), and Jupiter’s trojan asteroids (green disks). The trojans fall into two camps: the Greek camp (the cluster leading Jupiter) and the Trojan camp (the cluster trailing Jupiter). [SwRI]

In astronomy, trojans refers to small bodies that share the orbit of a larger one, residing in stable orbits that either lead or trail the large body. Jupiter provides a prime example of this phenomenon: roughly a million trojan asteroids trace the same path as the giant planet, clumping together into one cluster ahead and one behind Jupiter.

The Jupiter trojans are ancient vestiges of our solar system’s formation — leftover building materials from the construction of the outer planets. Scientists hope that by examining the composition, structure, and dynamics of these time capsules, we’ll learn more about the history of our solar system and the origin of organic materials on Earth.

Getting Up Close and Personal

So far, we’ve detected around 7,000 Jupiter trojans, and we now hope to get a better look at these small, distant bodies. This is where Lucy comes in: this upcoming NASA space probe, slated to launch in October 2021, will fly by six different Jupiter trojan asteroids over the span of six years (2027–2033), providing us with up-close views of these fossils of planet formation.

To prepare for Lucy’s launch, the next step is to characterize the mission’s targets, thereby ensuring that we can plan the spacecraft’s route and observing schedule effectively and efficiently. To this end, a team of scientists led by Keith Noll (NASA Goddard SFC) recently examined one of Lucy’s targets, the trojan asteroid 3548 Eurybates, using the Hubble Space Telescope — and they found more than they bargained for.

Hubble Eurybates

This 2” x 2” view of Eurybates from Hubble in January 2020 confirmed the presence of a satellite, identified here by a green circle. [Adapted from Noll et al. 2020]

Pixel Hunting

Noll and collaborators first spotted hints of a satellite body near the 64-kilometer-diameter Eurybates in two sets of Hubble images from September 2018. A 1.7-pixel blip was visible next to Eurybates in both observations, and it shifted location between the two images — an indication this was a blip worth follow-up.

But it’s tough to get follow-up time on one of the world’s most popular telescopes! The team was granted just three opportunities with Hubble to try to confirm this possible satellite. The first two attempts, made in December 2019, failed — the satellite was likely too close to Eurybates to be resolved. But in the third set of observations, the blip reappeared — a faint observation that allowed Noll and collaborators to confirm the presence of a ~1.2-km satellite for Eurybates.

Time to Study Some Collisions

asteroids and their satellites

The relative size of known satellite/asteroid pairs in the solar system, plotted against their orbital distance. Symbol sizes are proportional to satellite diameter. Eurybates (blue symbol) is one of the smallest and most distant primaries for which a satellite has been detected from an Earth-based telescope. Click to enlarge. [Noll et al. 2020]

Noll and collaborators use their three detections to place early constraints on the orbit of the satellite and compare it to other known asteroid satellites in our solar system. The relative size of this satellite is remarkably tiny — it’s just 1.9% the size of Eurybates!

The properties of this system suggest the satellite was formed from Eurybates by a collision — providing us with a golden opportunity to study a collisional satellite at close range. With seven years to go before Lucy’s close encounter with Eurybates, we’ve got time to learn more and prepare!

Bonus

Curious how Lucy’s going to manage to visit six different targets, all in one journey? Click below to check out a video from SwRI (or visit the SwRI website directly, if it doesn’t play in your browser) that shows Lucy’s complex planned orbit.

Citation

“Detection of a Satellite of the Trojan Asteroid (3548) Eurybates—A Lucy Mission Target,” K. S. Noll et al 2020 Planet. Sci. J. 1 44. doi:10.3847/PSJ/abac54

magnetar outburst

Have we recently spotted the first equivalent of a fast radio burst (FRB) — a mysterious and brief extragalactic flash of radio emission — in our own galaxy? Some astronomers think so, and argue that the new discovery solidifies the connection between these exotic radio bursts and powerfully magnetized neutron stars.

Origin of a Burst

fast radio burst

Artist’s impression of telescopes observing an extragalactic fast radio burst. [CSIRO/Andrew Howells]

Observations of bright, millisecond-duration, extragalactic radio flashes continue to pile up, yet the cause of these odd transients remains uncertain. One popular theory: FRBs may be somehow connected to the birth or evolution of magnetars, neutron stars threaded with especially strong magnetic fields.

There’s plenty of evidence pointing to magnetars as the source of FRBs, from polarization measurements that suggest FRB sources are strongly magnetized, to localizations of several FRBs to star-forming regions typical of magnetar environments. And some magnetars, known as soft gamma-ray repeaters (SGRs), emit repeated high-energy flares and bursts across their lifetime — another sign of volatility that could tie into the FRB picture.

But there’s a major challenge to the magnetar model for FRBs: we’ve never observed radio emission remotely similar to an FRB coming from a magnetar in our own galaxy.

…that is, until now.

A Missing Link Found?

In a new study led by Sandro Mereghetti (INAF, Italy), scientists have reported the detection of a series of bright X-ray bursts from the magnetar SGR 1935+2154 using the International Gamma-Ray Astrophysics Laboratory (INTEGRAL) spacecraft.

X-ray light curve

The X-ray light curve from INTEGRAL (black line) is shown here with the positions of the associated radio pulses (orange line) marked for comparison. The inset box shows the brightest part of the burst. Click to enlarge. [Mereghetti et al. 2020]

In itself, this announcement might not have been newsworthy — but the observed X-ray bursts from this known magnetar were also accompanied by a very bright millisecond radio burst detected by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and the Survey for Transient Astronomical Radio Emission 2 (STARE2) radio telescopes.

The radio burst exhibits similar structure to the associated X-ray burst from the magnetar, occurred at roughly the same time, and is within a factor of 10 of the energy of some extragalactic FRBs. These clues strongly suggest that SGR 1935+2154’s outburst may be the missing link that connects magnetars with FRBs.

Pinpointing Production

Can we use these new observations of SGR 1935+2154 to narrow down models of FRB production?

two magnetar populations

The existence of a second, rare population of magnetars with stronger magnetic fields and higher activity levels could explain a number of properties of the FRBs we’ve observed. As an example, the observed rates of repeating and non-repeating FRBs can be reproduced with the authors’ two-population model. [Adapted from Margalit et al. 2020]

In another recent study, Ben Margalit (NASA Einstein Fellow at UC Berkeley) and collaborators use the new data from this source first to argue that there must be two different populations of magnetars: “ordinary” magnetars like SGR 1935+2154 and other galactic magnetars, and more active but shorter lived magnetars that are responsible for cosmological FRBs. The latter population may form through different channels than galactic magnetars.

Margalit and collaborators also use the radio and X-ray observations from SGR 1935+2154 to evaluate specific magnetar emission mechanisms, providing constraints on models of how these energetic flashes are produced. 

SGR 1935+2154 may have more X-ray and radio activity in store for us in the future, so you can bet we’ll be keeping an eye on it. With any luck, upcoming observations will help us to further address the mystery of FRBs — right here in our own galaxy.

Citation

“INTEGRAL Discovery of a Burst with Associated Radio Emission from the Magnetar SGR 1935+2154,” S. Mereghetti et al 2020 ApJL 898 L29. doi:10.3847/2041-8213/aba2cf

“Implications of a Fast Radio Burst from a Galactic Magnetar,” Ben Margalit et al 2020 ApJL 899 L27. doi:10.3847/2041-8213/abac57

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