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How do we find extraterrestrial intelligence (ETI) — not just life beyond Earth, but advanced extraterrestrial civilizations? One approach is to seek signals from ETI that may be attempting to communicate with us — but the problem of where, when, and how to look for such communications is a complex one. A new study explores one way we could optimize this hunt: by searching for communication signals that are synchronized with the merger of two neutron stars.

“Hey, Look Over Here!”

transmission geometry

Schematic of how ETI in a distant galaxy could send a communication that would arrive at Earth (receiver) at the same time as the signal from the merger of two neutron stars in the ETI’s galaxy. The distance l between the ETI and the merger is much shorter than the distance between their galaxy and Earth. [Nishino & Seto 2018]

Transient, flaring astronomical phenomena — like supernovae and gamma-ray bursts — draw our eye and cause us to point our telescopes in the direction of the transient. A clever advanced civilization might take advantage of this, knowing that the moment when we on Earth observe such a transient astronomical source is the perfect time for us to receive communication from ETI near the transient.

But this strategy requires the ETI to predict the instant such an explosion near them — i.e., within the same galaxy — would occur, far enough in advance that they can send a communication signal that will arrive here on Earth at the same time as the transient signal.

Conveniently, there’s a bright transient that can be predicted in advance: the electromagnetic and gravitational-wave radiation from the merger of two neutron stars. In a new study from Kyoto University in Japan, researchers Yuki Nishino and Naoki Seto explore a scenario in which extragalactic intelligent life synchronizes a signal to us with a binary-neutron-star merger in their own galaxy.

Hulse-Taylor binary

The orbital decay of the binary neutron star PSR B1913+16 has been precisely measured over decades using the timing of its radio pulses. With this approach, ETI could estimate when a binary in their galaxy will eventually merge. [Inductiveload]

A Merger Predicted

Binary-neutron-star systems — which are prolific across the universe — are often discoverable and measurable due to pulses of light from one or both components of the system. From the exact timing of pulsars in a compact binary system in their own galaxy, ETI could measure the orbit and decay of the binary, allowing them to calculate when the system will merge.

Nishino and Seto argue that the ETI could then target a distant galaxy like ours for a communication signal that would arrive at roughly the same time as the gravitational-wave burst from the merger.

Powering Up

What kind of technology is needed for such a signal? The authors estimate that, for a civilization in a galaxy 130 million light-years away, ten megabytes of data could be sent to a receiver like the Square Kilometer Array on Earth using a ~1 terawatt radio transmitter. A terawatt is a lot of power — it’s 10% of the current energy consumption on Earth — but Nishino and Seto argue that such outputs are not out of reach for advanced civilizations.

SKA

Artist’s impression of the antennas in the 5-km central core of the future Square Kilometer Array. [SKA/Swinburne Astronomy Productions]

This feasibility indicates that, in the future, we might consider narrowing the search for ETI by focusing on the host galaxies of the many neutron-star mergers we hope to detect in the future with gravitational-wave observatories like LIGO, Virgo, and LISA. By doing so, there’s a chance we could spot a signal from a civilization that was hoping we’d look their way!

Citation

“The Search for Extra-Galactic Intelligence Signals Synchronized with Binary Neutron Star Mergers,” Yuki Nishino and Naoki Seto 2018 ApJL 862 L21. doi:10.3847/2041-8213/aad33d

Blanco Telescope Dome

Over billions of years, globular clusters and dwarf galaxies orbiting the Milky Way have been torn apart and stretched out by tidal forces. The disruption of these ancient stellar populations results in narrow trails of stars called stellar streams. These stellar streams can help us understand how the Milky Way halo was constructed and what our galaxy’s dark matter distribution is like — but how do we find them? 

Millennium Simulation

Along with cosmological simulations, like the Millennium Simulation pictured here, stellar streams can help us understand how dark matter is distributed in galaxies like the Milky Way. [Max Planck Institute for Astrophysics]

On the Trail of Tidal Streams

Understanding how our galaxy came to look the way it does is no easy task. Trying to discern the structure and formation history of the outer reaches of the Milky Way from our vantage point on Earth is a bit like trying to see the forest for the trees — while also trying to learn how old the forest is and where the trees came from!

One way to do so is to search for the stellar streams that form when globular clusters and dwarf galaxies are disrupted and torn apart by our galaxy. Stellar streams tend to be faint, diffuse, and obscured by foreground stars, which makes them tricky to observe. Luckily, recent data releases from the Dark Energy Survey are perfectly suited to the task.

Shipp et al. 2018 Fig. 4

Top: Density map of stars with a distance modulus of 16.7 for a given matched-filter isochrone. Bottom: Stellar streams identified in this work, including those previously known. Click to enlarge. [Shipp et al. 2018]

Dark Energy Survey Brings Faint Stars to Light

Nora Shipp (University of Chicago) and collaborators analyzed three years of data from the Dark Energy Survey in search of these stellar streams. The Dark Energy Survey is well-suited for stellar-stream hunts since it covers a wide area (5,000 square degrees of the southern sky) and can observe objects as faint as 26th magnitude.

Shipp and collaborators use a matched-filter technique to pinpoint the old, low-metallicity stars that belong to stellar streams. This method uses the modeled properties of stars of a certain age — synthetic isochrones — to identify stars within a background stellar stream with minimal contamination from foreground stars.

Using their matched filters, the authors found 15 stellar streams, 11 of which had never been seen before. They then estimated the age, metallicity, and distance modulus for each stream — all critical to understanding how the individual streams fit into the larger picture of galactic structure. 

Shipp et al. 2018 Fig. 5

A closer look at the stellar streams in the first quadrant of the surveyed area. Top: Density map of stars with a distance modulus of 15.4. Bottom: Stars with a distance modulus of 17.5. [Adapted from Shipp et al. 2018]

Reconstructing the Galactic Halo

These 11 newly discovered stellar streams will greatly enhance our understanding of the history of the galactic halo. Spectroscopy can help clarify the ages of these structures, while kinematic studies can help us understand if and how these structures are associated.

Future work may also help us discern the origin of the streams; the stark dichotomy in the mass-to-light ratios of the stellar streams discovered in this work hints that it may be possible to link some streams to globular clusters and others to dwarf galaxies. Look for this and more exciting results from galactic archaeologists in the future!

Citation

N. Shipp et al 2018 ApJ 862 114. doi:10.3847/1538-4357/aacdab

Segue 1

The mysterious object Segue 1 has intrigued astronomers since it was first discovered in 2007. A new study has now measured the first proper motions for this tiny neighboring galaxy, providing clues as to how it came to be in orbit around the Milky Way.

An Unusual Neighbor

NGC 147

NGC 147 is another example of a dwarf spheroidal galaxy in the Local Group. [Ole Nielsen]

Until recently, the distinction between globular clusters and dwarf galaxies was fairly clear-cut. In the last decade, however, new objects have been discovered that have muddied the waters: compact, faint dwarf galaxies that are so small and dim as to be nearly indistinguishable from globular clusters.

Segue 1, a Milky Way satellite just ~75,000 light-years away (that’s half the distance to the Large Magellanic Cloud!), was the first of these ultra-faint dwarf spheroidal galaxies to be discovered — and scientists have been debating its nature ever since.

It’s not just Segue 1’s nature that’s debated, however; we also want to understand where this tiny galaxy came from, and how it ended up in orbit around the Milky Way. To address these questions, a team of scientists led by Tobias Fritz (University of Virginia; IAC and University of La Laguna, Spain) have made the first proper-motion observations of this unusual object.

Segue 1 stars

Color-magnitude diagram of stars in Segue 1; spectroscopically identified members are shown in blue and non-members in red. [Fritz et al. 2018]

Pinning Down Proper Motion

Fritz and collaborators measured Segue 1’s proper motion using data from the Sloan Digital Sky Survey and the Large Binocular Camera over a baseline of 10 years. Their measurements put Segue 1 on an orbit that circles the Milky Way once every ~600 million years — which suggests it’s quite tightly bound to our galaxy.

The authors point out that the orbit inferred for Segue 1 support its classification as a galaxy rather than a disrupted star cluster: Segue 1 doesn’t pass close enough to the Milky Way to have been tidally disrupted. But how did this tiny galaxy arrive in orbit around the Milky Way in the first place?

Simulated Origins

There are two main options: Segue 1 was either accreted on its own, or it was the satellite of a larger, classical satellite that was accreted by the Milky Way. The parameters of its orbit rule out the possibility that it once orbited one of the known massive classical satellites of the Milky Way, but it’s still possible that it could have initially orbited a long-since destroyed massive satellite.

Infall times

Infall times for Segue 1 analogs in the cosmological simulations. The median first infall time is about 8 billion years ago. [Adapted from Fritz et al. 2018]

The authors explore cosmological zoom-in simulations of Milky-Way-mass galaxies to statistically determine which of these scenarios is statistically the most likely for Segue-1-like satellites, based on the tiny galaxy’s measured properties.

Fritz and collaborators find a 25% likelihood that Segue 1 first arrived in orbit around a long-since-destroyed satellite, with both it and its host accreted by the Milky Way ~12 billion years ago. Even more likely, at 75% probability, Segue 1 accreted on its own perhaps 8 billion years ago.

More detailed future measurements will likely give us an even clearer picture of Segue 1’s orbit and potential origins. In the meantime, there’s a whole wealth of other ultra-faint dwarfs like Segue 1 to explore!

Citation

T. K. Fritz et al 2018 ApJ 860 164. doi:10.3847/1538-4357/aac516

Sun magnetic fields

Solar flares, coronal mass ejections, prominences, sunspots — the most exciting features of the Sun are all driven by complex magnetic activity within the Sun’s interior and at its surface. Indeed, observations of the Sun have long revealed regions of magnetic flux of various magnitudes and polarities across the Sun’s disk.

full-disk solar images

Sample full-disk image of the Sun, with various features contoured in different colors, including plages (red), enhanced networks (blue); sunspots (green), and active networks (yellow). Panel F shows all contours overplotted on a magnetogram, with the grey regions corresponding to the background field. [Bose & Nagaraju 2018]

But it wasn’t until 1970 that scientists started measuring a different aspect of the Sun’s magnetization: its mean magnetic field. By treating the Sun like a distant star and measuring its net magnetic field by integrating across its whole disk, scientists have been able to effectively measure the imbalance in the magnetic flux of opposite polarities across its visible disk. A new study explores what might create this overall imbalance.

Learning from a Mean Field

The Sun’s mean magnetic field can reveal information about global behavior of our home star, helping us to better understand how magnetic fields form and evolve in stars and providing a deeper understanding of the interplanetary magnetic field that threads the space throughout our solar system.

Scientists have now regularly monitored the Sun’s mean magnetic field variations for more than two full solar cycles, on timescales ranging from a few days to several years, and we’ve learned that the mean field doesn’t stay constant — it can vary from ± 0.2 G to ± 2 G (for reference, a typical refrigerator magnet has a strength of ~50 G).

What we haven’t yet determined is the origin of the Sun’s mean magnetic field: does it come from the Sun’s large-scale, background magnetic field structure? Or is it driven by the magnetic fields of smaller active regions, like sunspots? A new study by scientists Souvik Bose (University of Oslo, Norway) and K. Nagaraju (Indian Institute of Astrophysics) explores these relative contributions.

Active Regions and Backgrounds

By decomposing Solar Dynamics Observatory observations of the Sun’s full disk into different regions, Bose and Nagaraju track the magnetic flux resulting from three categories:

  1. sunspots, dark spots that appear in the Sun’s photosphere as magnetic flux emerges;
  2. plages, enhanced networks, and active networks, surface features created by emergence and dispersion of weaker magnetic fields; and
  3. the Sun’s large-scale magnetic field — i.e., background regions that don’t fall into categories (1) and (2).

They then explore the variability of these fluxes over the span of several years.

magnetic field variation

Magnetic field variability vs. time, for A) the solar mean magnetic field; B) the background field; C) plages, enhanced networks, and active networks; and D) sunspots. The largest contributor to solar mean magnetic field variability is clearly the large-scale background field. [Bose & Nagaraju 2018]

Bose and Nagaraju’s calculations show that the variation in the solar mean magnetic field most closely tracks that of the background field, and it shows very little correlation with active regions. In particular, about 89% of the variability in the mean solar field is contributed by the background field, with only ~10% contributed by plages and the network field.

The authors point out that their work only indicates the origin of the solar mean magnetic field’s variability; its amplitude may yet be governed by the presence of sunspot activity on the surface of the Sun. Nonetheless, this study brings us a little closer to understanding the complex magnetic activity of our nearest star. 

Citation

Souvik Bose and K. Nagaraju 2018 ApJ 862 35. doi:10.3847/1538-4357/aaccf1

accretion disk

How can stellar-mass black holes attain the large sizes we’ve recently observed in merging binaries? A recent study explores an intriguing possibility: perhaps these smaller black holes grow and collide while trapped within the accretion disks that can surround enormous supermassive black holes.

A Windy Mass Limit

BBH masses

The masses of the black holes observed by LIGO in the five confirmed black-hole mergers (and one trigger). Of the ten progenitor black holes, six exceed 20 solar masses. [LIGO/VIRGO]

To date, the Laser Interferometer Gravitational-wave Observatory (LIGO) has spotted five confirmed binary black-hole mergers. These mergers have provided us with a wealth of information about stellar-mass black holes, but they’ve also raised a number of new questions. One surprising property of these merging black holes is their mass: of the ten progenitor black holes that we saw colliding, six weighed more than 20 solar masses. 

The common picture of how we get stellar-mass black holes is from the evolution of massive stars. But there’s a theoretical limit to the size of a black hole that can be formed this way: massive stars lose a lot of their mass over their lifetimes due to strong stellar winds, which generally constrains them to produce black holes no larger than ~10–15 solar masses. So how did the black holes spotted with LIGO form?

Possible Explanations

One proposed source of larger stellar-mass black holes is low-metallicity stars. High-mass stars with only minimal metals (i.e., elements that aren’t hydrogen or helium) are thought to have been common in the early universe, and these stars don’t lose so much mass via stellar winds. Therefore, low-metallicity, high-mass stars could develop into larger black holes — perhaps like the black holes seen by LIGO.

A recently published study led by Shu-Xu Yi (The University of Hong Kong), however, proposes an alternative explanation for the large black-hole sizes we’ve seen — an explanation that doesn’t rely on low-metallicity environments. Yi and collaborators suggest that the key to attaining large stellar-mass black holes might be the disks of gas and dust that surround supermassive black holes in active galactic nuclei (AGN).

Trapped in a Disk

BH masses at coalescence

Distributions of final black-hole masses at coalescence for black-hole binaries that start out at around ~7 solar masses each in the authors’ models. A significant fraction of black holes embedded in AGN disks are able to increase their mass to more than 20 solar masses. [Yi et al. 2018]

The dense centers of galaxies tend to form and accumulate large numbers of stars — and it stands to reason that many of these stars will evolve into stellar-mass black holes. But pairs of black holes that lie in the very heart of the galaxy could easily become trapped in the enormous accretion disk that surrounds the supermassive black hole at the galactic center, feeding and growing larger as they spiral into each other.

Yi and collaborators demonstrate that a pair of black holes, each starting at only ~7 solar masses, can accrete and grow to >20 solar masses before coalescing within a low-mass AGN disk. The gravitational-wave signal from this merger could therefore indicate two black holes of perhaps 20–30 solar masses, despite the fact that the black holes were both initially much smaller.

Could this be the explanation for LIGO’s large detections? Or are low-metallicity stars the more likely progenitors? Or could both models be at work? Future detections from gravitational-wave detectors, paired with electromagnetic observations, will help us to answer these questions. In the meantime, it’s exciting to watch the field unfold.

Citation

Shu-Xu Yi et al 2018 ApJL 859 L25. doi:10.3847/2041-8213/aac649

Sombrero Galaxy (NGC 4594)

What’s going on in the galactic bulge? The discovery of the double red clump — two groupings of stars seen in the color-magnitude diagram of the galactic bulge — has raised questions about the structure and formation history of the stars surrounding the center of our galaxy.

Lee et al. 2018 Figure 1

A color-magnitude diagram of the galactic bulge from the Two Micron All Sky Survey. In panel b, the red, blue, and black symbols indicate the bright red clump, faint red clump, and background red giant branch stars, respectively. The double red clump is more clearly visible in the luminosity function in panel c. [Lee et al. 2018]

Two Theories for the Double Red Clump

The double red clump, which is separated into bright and faint components, has inspired various theories about its origin:

  1. X-shaped structure scenario: The presence of the double red clump was initially thought to indicate that the Milky Way bulge contains an X-shaped structure similar to that seen in other galaxies. The difference in apparent magnitude between the two components of the double red clump is simply due to the different distances to two of the arms of the structure as seen by an observer on Earth, with the arm reaching toward Earth forming the bright red clump and the arm stretching away from Earth forming the faint red clump.
  1. Multiple-population scenario: Alternatively, the double red clump could be composed of multiple generations of stars that formed in globular clusters, where later generations of stars would be enhanced in helium, nitrogen, and sodium. Stellar evolution models predict that the first-generation stars would be fainter than subsequent generations, so the magnitude difference between the bright and faint components would be due to intrinsic luminosity differences between the two populations.
Lee et al. 2018 Figure 3

An illustration of the expected magnitude versus CN strength plots for the competing scenarios. Click to enlarge. [Lee et al. 2018]

A Closer Look at Red Clump Stars

How can we distinguish between these two scenarios? Young-Wook Lee (Yonsei University, South Korea) and collaborators posited that these theories can be separated by taking a closer look at the spectra of the double red clump stars.

Using low-resolution spectroscopy from a 2.5-meter telescope at Las Campanas Observatory, Lee and collaborators set out to measure the CN band strength of nearly 500 stars in the double red clump. Why CN? The CN band strength correlates with the nitrogen abundance, which is expected to be enhanced in later generations of stars.

If the two components of the double red clump represent separate generations of stars, the brighter stars will show the expected nitrogen enhancement. If, on the other hand, the double red clump arises from an X-shaped structure in the galactic bulge, there will be no difference in CN band strength between the bright and faint components.

Lee et al. 2018 Figure 4

CN indices for bright red clump (red), faint red clump (blue), and red giant branch (black) stars. Click to enlarge. [Lee et al. 2018]

From Globular Clusters to Galactic Bulges?

By comparing the spectra of the bright and faint red clump stars, Lee and collaborators found that the bright population had stronger CN absorption than the faint population at the 5.3σ level.

While precise parallax distances from Gaia and high-resolution spectroscopy are still needed to completely disentangle the stellar populations near the heart of our galaxy, this result supports the theory that the double red clump contains multiple generations of stars. The abundance patterns inferred from the stars’ spectra imply that the double red clump stars may have formed in young globular clusters, which could have profound implications for how galactic bulges are assembled — in both our home galaxy and afar.

Citation

Young-Wook Lee et al 2018 ApJL 862 L8. doi:10.3847/2041-8213/aad192

solar atmosphere

Despite the Sun’s proximity, there’s still a lot we don’t know about our nearest star! A new study, however, has brought us a step closer to understanding one of its mysteries: how the solar atmosphere is heated.

solar temperatures

The temperatures of different layers of the Sun. The Sun’s atmosphere gets progressively warmer with increasing distance from the core, from the photosphere at 6,000 K, to the chromosphere at 10,000 K, to the corona at 1,000,000 K. [ISAS/JAXA]

Blame Shocks for Heating?

The outer layers of the Sun present an interesting conundrum: instead of continuing to drop in temperature with increasing distance from the Sun’s hydrogen-fusing core — the way the Sun’s inner layers do — the atmosphere above the Sun’s surface becomes progressively hotter with distance. How is heat getting delivered from the Sun’s center to these outer layers? 

One possible culprit is shocks. Oscillations continuously travel through the Sun’s interior to its surface, launching compressible waves into its atmosphere. These waves can then evolve into shocks as they propagate outward — and the dissipation of these shocks can deposit energy into the Sun’s atmosphere, contributing to its heating.

Besides their potential role in atmospheric heating, these shocks serve a number of other purposes that should encourage us to study them. They are thought to serve as the trigger for observed phenomena, such as the tiny chromospheric jets driven in active regions. Furthermore, they can be used to probe the solar atmosphere — measuring shock properties can tell us more about the structure of the environment through which they propagate.

Theory and Observation Combined

Observational data used by the authors to derive the properties of two successive shock waves observed by IRIS. a) IRIS image, spanning 47” x 47”. The vertical line indicates the location of the spectrograph slit. b) the Doppler velocity of Si IV 1393.76 Å in the region marked by the horizontal white lines in panel (a). c) A wavelength-time plot of Si IV 1393.76 Å. [Adapted from Ruan et al. 2018]

Until now, however, we haven’t observationally measured the key parameters of shocks in the solar atmosphere — like the temperature, density, and speeds of the plasma upstream and downstream of the shock waves.

To address this challenge, a team of scientists at Peking University, the Chinese Academy of Sciences, and KU Leuven in Belgium have now developed a means of quantitatively analyzing the shocks we observe in the Sun’s atmosphere. In a recently published study led by Wenzhi Ruan, the team presents analysis that determine shock properties by using the theoretical physics of shocks in combination with quantities derived from simultaneous imaging and spectroscopic observations.

Testing the Approach

To test their analysis, the team takes two approaches. First, they apply the analysis to real shock observations — imaging and spectroscopy of two successive shock waves captured by the Interface Region Imaging Spectrograph (IRIS) observatory — and demonstrate that the shock properties they find are physically reasonable and consistent with each other.

Next, the authors model shocks using numerical simulations, and they generate synthetic observations of how these shocks would appear to an observatory like IRIS from different viewing angles. They then apply their analysis to the synthetic observations and demonstrate that they can recover the original properties of the shocks in all cases, indicating that their method is reliable and independent of viewing direction.

The authors encourage other researchers to use their analysis code (you can find it here) as a standard approach for quantitatively exploring shocks in the solar atmosphere. This work is a crucial step toward better understanding how these shocks deposit heat, and how they interact with and influence the atmosphere of the Sun.

Citation

Wenzhi Ruan et al 2018 ApJ 860 99. doi:10.3847/1538-4357/aac0f8

PSR B1257+12 planet

Finding planets around ordinary stars is great, but what if we could also hunt for planets around the black holes, neutron stars, and white dwarfs that live in binaries with companion stars? A new study shows it’s possible!

A New Place for Planets?

X-ray binary

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

X-ray binaries are gravitationally interacting binary star systems in which a compact object — a white dwarf, neutron star, or black hole — accretes material from its companion star. In these systems, we can often detect the stars in optical or radio light, but the system additionally shines in X-rays as a result of radiation from the very hot, accreting gas.

Planets orbiting within binaries may be common — we’ve spotted around 70 examples so far of planets orbiting one member of a binary, and another dozen or so in which a planet orbits both members on a circumbinary path. It stands to reason, then, that some planets should survive the evolution of one of the binary stars into a compact remnant, eventually becoming a planet orbiting an X-ray binary.

X-ray light curves

Several example transit X-ray light curves for circumbinary planets, including an Earth-mass planet (solid lines) and a Jupiter-mass planet (dashed lines), for different values of μ, which relates the mass of the compact remnant to the companion star. Here the companion mass remains fixed and the different panels show different values for the remnant mass. [Imara & Dr Stefano 2018]

Looking at X-Rays

So how would we detect such a planet? Optical and radio searches for planets can be challenging, since a planet transit often means a very small dip in a light curve that might not be detectable. Two scientists from the Harvard-Smithsonian Center for Astrophysics, Nia Imara and Rosanne Di Stefano, propose an alternative: why not specifically look in the X-rays?

Because the area emitting X-rays is very compact — all smaller than the size of a white dwarf’s surface, i.e., the size of the Earth — the expected dip in the X-ray light curve due to a planet transit is quite large. This increases our chances of being able to detect it.

Exploring Challenges

A few challenges exist with this approach. To increase detection odds, the planet would ideally need to be orbiting within a similar plane to that of the binary, and preferably close to the inner cutoff for stable orbits around the binary. This is not an unreasonable assumption, however, given expected orbital dynamics as star systems evolve.

transit probabilities

Transit probability versus binary mass ratio, μ, for planet circumbinary orbits around coplanar x-ray binaries with white dwarfs (solid lines), neutron stars (dashed lines), or black holes (dotted lines) as the primaries. The black and magenta lines represent the probability calculations for an Earth-like and Jupiter-like planet, respectively. [Imara & Di Stefano 2018]

Another potential difficulty is that X-ray photons are scarce! We’d have to observe systems for an extended time in order to gather enough light in X-rays to definitively detect light-curve dips. Imara and Di Stefano show, however, that we’ve observed a number of X-ray binaries over several hundred thousand seconds — long enough time periods that the dips would be detectable.

A Positive Outlook

With those challenges in mind, Imara and Di Stefano demonstrate through a series of calculations that circumbinary planets are reasonably likely to transit — transit probabilities range from roughly 0.1%–40%, depending on the mass ratio of the binary and the size of the X-ray-emitting region — and that our detection capabilities are such that we could actually spot these transits with present-day technology.

Future X-ray missions, like the proposed Lynx X-ray space telescope — which may have 50 times the sensitivity of the Chandra X-ray telescope! — will dramatically extend the opportunities for transit detection. Indeed, it seems like the hunt for these exotic exoplanetary systems have very good prospects.

Citation

Nia Imara and Rosanne Di Stefano 2018 ApJ 859 40. doi:10.3847/1538-4357/aab903s

NGC 1313

At some point in a galaxy’s life, it transitions from a star-forming factory into an old, red, inactive relic. Can new observations of a recently transitioned galaxy help us understand what drives that change?

Causes of Quenching

galaxy types

Spiral starburst galaxies (top) may precede red, inactive ellipticals (bottom) evolutionarily. [Adapted from Hubble/Galaxy Zoo]

Across the nearby universe, we see two main types of galaxies: blue, active spirals, and red, quiescent ellipticals. It’s generally believed that these represent two evolutionary states: galaxies first undergo starburst periods in which many young, hot, blue stars are born. Later in their lifetimes, these galaxies then settle into red, inactive states.

But what shuts down the star formation, triggering the transition between these two states? There are a number of possible quenching explanations related to galaxy mergers:

  1. Gas heating
    The collision of two galaxies could heat up the gas supply via shocks, preventing it from gravitationally collapsing to form stars.
  2. Compaction
    Mergers may lead to compaction, in which the star-forming gas migrates inward and triggers a central starburst. The remainder of the galaxy is depleted of gas and stops forming stars.
  3. Outflows
    Mergers can trigger high-velocity outflows driven by radiation from new stars or from a central, feeding black hole. The outflows remove gas from the galaxy, shutting down star formation.
  4. Morphological quenching
    A galaxy can stabilize against star formation if the structure of the galaxy changes — say, after a merger — in specific ways, such as if the galactic disk transforms into a spheroid.

A Transitional Galaxy

These different proposed quenching mechanisms should leave distinctive signatures in the stellar populations of recently quenched galaxies. With this in mind, a team of scientists explored SDSS J0912+1523, an intermediate-redshift galaxy that shows signs of having only recently transitioned from a starburst galaxy to a quiescent one.

In a study led by Qiana Hunt, a postbaccalaureate researcher at Princeton University, the scientists combined preexisting ALMA observations of the gas within SDSS J0912+1523 with new Gemini observations of its stellar population. This combination provided a rare opportunity to gain insight into what might shut down a galaxy’s star formation.

Evidence for a Merger

The authors find that SDSS J0912+1523 shows signs of containing two separate cores of stars that now rotate together — which may be evidence of a past merger.

SDSS J0912+1523

The flux density of SDSS J0912+1523 seems to show two separate cores, which may be the sign of a past merger. [Hunt et al. 2018]

In spite of this indication, none of the quenching scenarios above predict all of the characteristics Hunt and collaborators observed in SDSS J0912+1523. There’s lots of cold molecular gas still present in the galaxy, suggesting that neither depletion nor heating of the gas led to its quenching. Minor, gas-rich mergers or morphological quenching may be candidates still, but the kinematics of the gas and stars suggest that the gas didn’t come from an external origin. And there’s no evidence for strong outflows from SDSS J0912+1523.

For now, it looks like the mechanism that shut down star formation in this galaxy remains a mystery. But Hunt and collaborators are optimistic: a number of follow-up observations could shed more light on the problem — like radio hunts for central black-hole activity or high-resolution Hubble images of the galaxy’s morphology. Once these are conducted, we may soon understand what mechanism turns off star formation in an aging galaxy.

Citation

Qiana Hunt et al 2018 ApJL 860 L18. doi:10.3847/2041-8213/aaca9a

X-ray binary

An X-ray telescope recently installed on the International Space Station has been improving our view of distant high-energy sources, one object at a time. Now, this telescope has provided a detailed look at a black hole feeding off its companion star.

How to Spot Black Holes

Stellar-mass black holes lurk throughout our galaxy — but their darkness makes them understandably difficult to spot. Because these small beasts don’t emit light, we have few observations of the stellar-mass black holes around us, limiting what we can learn about these mysterious objects and their behavior.

NICER

An artist’s conception of the NICER telescope installed on the International Space Station. [NASA]

One way that we can observe such black holes is if they exist in an X-ray binary, a binary consisting of a black hole and a donor star. In X-ray binaries, material that is siphoned off the donor star goes to feed the black hole, forming an accretion disk around the black hole as it falls in. As the material in the accretion disk spirals inwards, it radiates strongly in X-rays — resulting in emission that we can observe, even though the black hole itself emits no radiation.

But what is the structure of this accretion disk? How far in toward the black hole does it extend? How fast does the black hole spin at its center? These are among the many questions to which scientists are still seeking answers. In a new study, astronomer Jon Miller (University of Michigan) and collaborators present new views of an X-ray binary that may provide some clues.

MAXI J1535−571

Spectrum of MAXI J1535−571, fit with a simple disk blackbody plus power-law model. The additional features visible in the data/model ratio are likely due to reflection from the disk. [Miller et al. 2018]

Eyes on a New X-Ray Source

Miller and collaborators present new observations of the X-ray binary MAXI J1535-571 made with the Neutron Star Interior Composition Explorer (NICER), an X-ray telescope recently installed on board the International Space Station.

NICER’s observations indicate that the black hole in this binary is likely spinning very rapidly — at more than 99% of the maximum possible speed! The light we observe from MAXI J1535-571 appears to include contributions both from the black hole’s accretion disk and from the corona of very hot gas that lies above the disk. Light from the corona reflects off of the accretion disk, providing us with more information about the structure of the disk.

By modeling the observations, Miller and collaborators show that the disk extends nearly all the way inwards to what’s known as the innermost stable circular orbit, the closest stable orbit you can have to a black hole.

Reflections of a Warp

warped disk

Artist’s impression of a example warped disk of gas and dust surrounding a black hole. [James Gitlin (Space Telescope Science Institute)]

Lastly, the authors point out an additional feature in MAXI J1535-571’s spectrum: a narrow emission line that they suggest might be caused by a warp in the disk, such that the disk no longer lies flat. The warp would locally change the profile of the accretion disk, causing more light from the corona to be reflected to us from that point.

The presence of this potential warp, the extent of the disk, and the spin of the black hole are all pieces of the puzzle that will help us better understand the behavior of stellar-mass black holes feeding off of companions. And we can look forward to the high sensitivity of NICER — which made these observations possible — continuing to produce exciting results in the future!

Citation

J. M. Miller et al 2018 ApJL 860 L28. doi:10.3847/2041-8213/aacc61

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