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29 April 2020

Spins Point to How Black Hole Binaries Formed

Before stellar-mass black holes merge in a spectacular burst of gravitational waves, they’re locked in a fatal dance around each other as a binary black hole. A new study uses clues from black hole spins to explore how these binaries came to be paired together in the first place.

To Build a Binary

The Hanford (top) and Livingston (bottom) LIGO facilities, which work together to detect gravitational-wave signals. [Caltech/MIT/LIGO Lab]

With ten detections of merging stellar-mass black holes made by the LIGO/Virgo gravitational-wave observatories in just their first two observing runs, these detectors have opened a new window through which we can study the evolution of massive stars.

Among the open questions we hope to answer with these and future detections is the following: How were these binary pairs of stellar-mass black holes created? There are two main formation channels proposed:

Field binary evolution
In isolation in the galactic field, the two members of a binary star system independently evolve into black holes, and they remain bound to each other through this process.
Dynamical assembly
Black holes are formed independently and then sink to the centers of high stellar density environments like globular clusters. There, dynamical interactions cause them to pair up and later get ejected from clusters as bound binaries.

Recent research suggests a combination of these two channels is likely at work to produce the black hole binaries we’ve observed. But what fraction of the LIGO/Virgo binaries are created by each channel?

A new study by scientist Mohammadtaher Safarzadeh (Center for Astrophysics | Harvard & Smithsonian; UC Santa Cruz) explores this so-called branching ratio.

Illustration of a pair of black holes with misaligned spins. [LIGO/Caltech/MIT/Sonoma State (A. Simonnet)]

Spinning an Origin Story

Safarzadeh relies on one primary clue: black hole spin. Due to conservation of angular momentum, black holes binaries that form via isolated evolution are likely to have positive black hole spins — the spins of the black holes will be in the same direction as the orbital rotation of the binary. In contrast, the chaos of dynamical assembly should result in binaries with randomly distributed spins.

Safarzadeh statistically models two populations of black hole binaries produced by these two formation channels and compares this model to LIGO/Virgo’s 10 observations of mergers from their first two observing runs. He’s careful to also take into account LIGO/Virgo’s observational biases — the detectors have an easier time observing binaries with positive effective spin.

Dynamics Weigh In

The author’s calculated posterior distribution on the parameter Q, the ratio of field binaries to the total number of observed black hole binaries, shows that contribution from the dynamical channel is more than 55% with 90% confidence. [Safarzadeh]

The result? Safarzadeh estimates that the contribution of the dynamical assembly channel to the total population of binary black holes is more than 55%, with 90% confidence — which means that random pairings of black holes in the chaotic centers of dense star clusters likely dominate the population of black hole binaries we’ve observed.

This outcome can be further refined as we increase our observed sample size — LIGO/Virgo’s third observing run, now being analyzed, is expected to contain perhaps dozens of additional systems, and future detections will hopefully add many more binaries to the list!

Citation

“The Branching Ratio of LIGO Binary Black Holes,” Mohammadtaher Safarzadeh 2020 ApJL 892 L8. doi:10.3847/2041-8213/ab7cdc

27 April 2020

Searching Pulsars for Planets

Are there more hidden exoplanets lurking around extreme pulsar hosts? A recent study explores a well-observed set of pulsars in the hunt for planetary companions.

Ushering in the Age of Exoplanets

An artist’s illustration showing a network of pulsars whose precisely timed flashes of light are observed from Earth. Could some of these pulsars host planets? [David Champion/NASA/JPL]

The first planets ever confirmed beyond our solar system were discovered in 1992 around the pulsar PSR B1257+12. By studying the pulses from this spinning, magnetized neutron star, scientists confirmed the presence of two small orbiting companions. Two years later, a third planet was found in the same system — and it seemed that pulsars showed great promise as hosts for exoplanets.

But then the discoveries slowed. Other detection methods, such as radial velocity and transits, dominated the emerging exoplanet scene. Of the more than 4,000 confirmed exoplanets we’ve discovered overall, a grand total of only six have been found orbiting pulsars.

Is this dearth because pulsar planets are extremely rare? Or have we just not performed enough systematic searches for pulsar planets? A new study led by Erica Behrens (The Ohio State University) addresses this question by using a unique dataset to explore rapidly spinning millisecond pulsars, looking for signs of hidden planets.

The Advantage of Precise Clocks

How are pulsar planets found? Pulsars have beams of hot radiation that flash across our line of sight each time they spin. The regularity of these flashes is remarkably stable, and when we observe them over long periods of time, we can predict the arrival time of the pulses with a precision of microseconds!

Sample periodograms for two pulsars. The top panel includes a simulated planet signal injected into the data, producing a strong peak at the planet’s orbital period. The bottom panel is an actual periodogram for one of the pulsars in this study, showing no evidence of a planetary companion. [Adapted from Behrens et al. 2020]

Because these pulses are so predictable, any perturbation that might change their timing can be measured and modeled. In particular, the presence of a companion body around the pulsar will cause both objects to orbit the system’s center of mass, introducing a periodic signature in the pulsar’s pulse arrival times. This fluctuation in the pulse timing allows us to measure the period and mass of potential companions.

A Multi-Use Dataset

To search for these signatures in pulse data, Behrens and collaborators turn to observations of 45 separate millisecond pulsars, which were made as part of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) project.

NANOGrav’s primary goal is to use the precise timing of these pulsars to measure the warping of spacetime caused by gravitational waves. But in the process of this work, the project has been carefully monitoring pulse arrival times for these pulsars for 11 years, producing a remarkably detailed dataset in which we can search for evidence of planets orbiting any of the 45 pulsars.

Pushing Down to Moon Masses

Lower limits of detectable masses in the 11-year NANOGrav data set, as shown with black lines. The colored data shows the masses of the least massive 10% of confirmed exoplanets we’ve detected with other methods. Pulsar timing provides the ability to detect remarkably low-mass companion bodies. Click to enlarge. [Behrens et al. 2020]

Looking for periodic signals in the data, Behrens and collaborators rule out the presence of planets that have periods between 7 and 2,000 days. By injecting simulated signals into the data, the authors show that their analysis is sensitive to companions with masses of less than the Earth — in fact, for some pulsars, they’ve eliminated the possibility of all companions with more than a fraction of the mass of our Moon!

This study shows the incredible power and sensitivity of extended pulsar monitoring in the hunt for small exoplanets. While it may well be true that pulsar planets are very rare objects, those out there can’t stay hidden for long.

Citation

“The NANOGrav 11 yr Data Set: Constraints on Planetary Masses Around 45 Millisecond Pulsars,” E. A. Behrens et al 2020 ApJL 893 L8. doi:10.3847/2041-8213/ab8121

24 April 2020

A Stellar Method of Catalog Creation

Star catalogs are critical to astronomy research. However, they’re only as reliable as the methods used to create them. As telescopes probe further and fainter regions of the sky, how can we ensure that our methods of catalog creation extract as much information as possible from the returned images?

The “first light” image from the Transiting Exoplanet Survey Satellite. This is a good example of an astronomical research image, featuring the distortion of star shapes and crowded regions. The crosses typically accompany bright stars and are caused by the instrument doing the imaging. The object on the right is the Large Magellanic Cloud and the bright star on the left is R Doradus. [NASA/MIT/TESS]

To Get to the Point

The start of any star catalog is an image taken by a telescope. These images typically look like black and white photographs of the night sky, and the wavelengths of light used to make them are set by a filter, also known as a band. To make a catalog, you just need to identify stars in an image.

This isn’t as straightforward as it sounds, though! Stars, which would ideally appear as points, look like distorted circles due to atmospheric effects and electronics. Brighter ones can drown out fainter ones. They can also overlap, making it hard to tell them apart in crowded regions like the hearts of galaxies. And as telescopes get better, the images they produce will feature fainter stars and consequently be more crowded.

To efficiently create catalogs in the future, we need improvements on our methods of star finding. In a recent study, Richard Feder (California Institute of Technology) and collaborators used a technique called probabilistic cataloging (PCAT) to identify stars in the globular cluster Messier 2 (M2) and compared their results to existing catalogs on M2.

Assuming We Can Have More Than One Catalog

A catalog is generated by applying assumptions of what a star should look like and how bright it should be. This brightness threshold is a key assumption to catalog creation — too low and false stars may be let in, too high and real stars may be left out.

Traditional methods of catalog creation are one and done; assumptions are made and a single catalog is generated per image. In PCAT, multiple catalogs are generated per image, each with a different set of assumptions. This allows PCAT to have catalogs with very low brightness thresholds and identify sources (star candidates) that, while not bright enough to be confidently identified as stars, can influence the sources around them. The multiple catalogs are eventually collapsed into a single one after source influences and other factors have been accounted for.

A Few Levels Deeper

The completeness of different catalogs obtained from the SDSS images relative to the Hubble images. The x-axis is stellar brightness in Hubble magnitudes and the y-axis is completeness, with 1.0 meaning that all the Hubble stars were recovered in the SDSS data and 0.0 meaning no Hubble stars were recovered. DAOPHOT corresponds to the traditional catalog, Portillo et al. corresponds to PCAT applied to a single band, and r+g and r+i refer to the combinations of multiband images used in this work. [Adapted from Feder et al. 2020]

Feder and collaborators wanted to examine whether PCAT could yield better results when it was simultaneously applied to images taken in multiple bands — multiband images — rather than in a single band. In line with some other PCAT studies, they chose to look at M2.

M2 has the advantage of having existing catalogs generated from images taken by the Sloan Digital Sky Survey (SDSS) and the Hubble Space Telescope (HST). Feder and collaborators worked with SDSS images taken with the g, r, and i bands, using the HST catalog as a “complete” catalog to determine how well they were finding stars. They also compared their results against a traditional catalog generated from SDSS images.

Feder and collaborators found that they recovered stars ~0.4 magnitude fainter when using the multiband images instead of the single band ones. They went 1.5 magnitudes fainter than the traditional SDSS catalog and could confidently recover stars brighter than 20th magnitude. They also developed a more reliable method of determining star positions accurately across images.

With a better understanding of what assumptions can be made for images taken by different telescopes, PCAT could be widely applied to astronomical data in the near future. Stay tuned!

Citation

“Multiband Probabilistic Cataloging: A Joint Fitting Approach to Point-source Detection and Deblending,” Richard M. Feder et al 2020 AJ 159 163. doi:10.3847/1538-3881/ab74cf

22 April 2020

Rescuing an Overlooked Planet

In every batch of detections from the Kepler spacecraft, some transit signals get relegated to “false positive” status by an automated vetting pipeline. How do we ensure that real exoplanet detections don’t accidentally get discarded by the pipeline?

The Kepler False Positive Working Group is on the case — and they just rescued quite a find from being relegated to a false-positive fate.

To Be a Planet Candidate

An illustration of some of the planetary systems discovered by the Kepler spacecraft. The stars at the centers of these systems are not pictured. [NASA Ames/UC Santa Cruz]

Since Kepler’s launch in 2009, this hard-working satellite has found signals from thousands of candidate transiting exoplanets. But all transit signals aren’t just immediately declared planet candidates!

The first hint in Kepler data of a potential transiting planet is what’s known as a “Threshold Crossing Event” (TCE). That TCE could either be a true signal from a planet transiting across the face of its host star, or it could be a false positive or false alarm — a signal mimicking a transiting planet that’s instead caused by a background eclipsing binary system, noise in the data, instrumental artifacts, etc.

Early on in the Kepler mission, every TCE was reviewed by a team of scientists and classified as a true planet candidate or a false positive. But as the mission ramped up and data volume grew, scientists turned to an automated pipeline — aptly named the Robovetter — to categorize the TCEs.

The transit signals of Kepler-1649 b (top; previously known) and c (bottom; newly discovered), in the star’s light curve. [Adapted from Vanderburg et al. 2020]

Human vs. Machine

The automated approach has many advantages: we can process larger volumes of data, and the statistical uniformity allows us to make inferences about the sample completeness. But it’s inevitable that the Robovetter will sometimes be wrong, misclassifying a true planet as a false positive.

To address this, the Kepler False Positive Working Group was established to visually inspect all signals the Robovetter classified as false positives and confirm the categorization. This process allows us to improve the Robovetter’s algorithms — and it also opens the door to new discoveries hidden in old data.

Such is the case with Kepler-1649c, a planet candidate that was incorrectly categorized by the Robovetter as a false positive. In a new study led by Andrew Vanderburg (NASA Sagan Fellow at The University of Texas at Austin), a team of scientists presents their rescue of this sneaky planet.

Earth-like Discovery

The locations of Kepler-1649 planets b and c relative to the star’s optimistic (light green) and conservative (dark green) habitable zone. [Vanderburg et al. 2020]

Kepler-1649c is a planet the same size as Earth that orbits around its M-dwarf host star once every ~20 days, placing it firmly in its host star’s habitable zone. Its star also hosts a previously known inner planet that appears to be equivalent to Venus in its size and the amount of flux it receives.

How did the Robovetter miss this important habitable-zone, Earth-like planet? Vanderburg and collaborators suspect that the pipeline was fooled into mistaking Kepler-1649’s location due to the star’s high proper motion. This introduced noise into the inferred light curve, making the Robovetter think the transit signal wasn’t real.

Vanderburg and collaborators point out that there are likely hundreds of undiscovered planets left in the extended Kepler mission data. While automated pipelines do a great job of doing the heavy lifting, the discovery of Kepler-1649c goes to show that there’s value in having a human eye checking results.

Citation

“A Habitable-zone Earth-sized Planet Rescued from False Positive Status,” Andrew Vanderburg et al 2020 ApJL 893 L27. doi:10.3847/2041-8213/ab84e5

17 April 2020

Pulsar Discovery from an Enormous Telescope

Magnetized neutron stars in distant globular clusters are a challenge to detect — but it’s a job made easier by the world’s largest filled-aperture radio telescope. Recent high-sensitivity observations have uncovered an erratic new star system.

Artist’s illustration of a pulsar (left) and its small stellar companion (right), viewed within their orbital plane. [NASA Goddard SFC/Cruz deWilde]

Pulses from Distant Clusters

Pulsars are the compact remnants of dead stars that shine powerful beams of emission into space as they spin. The brightness of these beams and the regular timing of their pulsations makes pulsars valuable targets for observatories; not only can they tell us about stellar evolution and their environments, but they also serve as probes of the interstellar medium, space-time, and more.

Since the discovery of the first pulsar in 1967, we’ve found thousands of these stellar clocks in our galaxy. While many are located relatively nearby in the galactic disk, we’ve also observed a population of pulsars in the distant globular clusters that orbit the Milky Way. These pulsars are a useful tool for probing a very different environment: the dense stellar cores made up of an old population of stars.

Hubble image of the globular cluster M92. [ESA/Hubble]

Until recently, we’d only discovered 156 pulsars in 29 globular clusters; due to these clusters’ large distances (tens to hundreds of thousands of light-years away), it takes very powerful and sensitive radio telescopes to find them using deep surveys. Now, a new observatory has entered the game.

A Powerful Telescope

The Five-hundred-meter Aperture Spherical radio Telescope (FAST), built into the hilly landscape in southwest China, is the world’s largest filled-aperture telescope. Its size dwarfs that of the Arecibo Observatory in Puerto Rico, and its dish has the advantage of being shapable — the panels that make up its surface can be tilted by a computer to change the telescope’s focus.

Comparison of the FAST (bottom) and Arecibo Observatory (top) radio dish profiles at the same scale. [Cmglee]

FAST achieved first light in 2016, and it’s been going undergoing testing and commissioning for the last few years. As of January 2020, the FAST is officially open for business, and we’re now seeing some of the major results coming from this powerful radio observatory.

Among them: the first discovery of an eclipsing binary pulsar in globular cluster M92, as reported in a recent publication led by Zhichen Pan (NAO, Chinese Academy of Sciences).

An Exotic System

Phase-folded pulse data for PSR J1717+4308A, as observed by FAST (left panel) and by the Green Bank Telescope (right panel). Eclipses are visible as breaks in the data. The difference in sensitivity between the two telescopes is starkly evident. [Adapted from Pan et al. 2020]

Pan and collaborators announce the FAST detection of a pulsar with a pulse period of 3.16 milliseconds orbiting around a low-mass companion in a globular cluster that’s about 27,000 light-years away.

This pulsar, PSR J1717+4308A, is in a close (period of 0.20 days) eclipsing orbit with its companion, making it what’s known as a “red-back pulsar”. Radiation from the pulsar has pummeled its companion star, creating a cloud of ionized material that surrounds it and causes the pulsar’s eclipses to vary in duration and timing.

The discovery of this object demonstrates the potential of FAST as a probe of the globular cluster pulsar population. More observations of M92 are planned in the future, as well as observations of dozens of even richer clusters. Keep an eye out for more FAST results as this telescope ramps up operations!

Citation

“The FAST Discovery of an Eclipsing Binary Millisecond Pulsar in the Globular Cluster M92 (NGC 6341),” Zhichen Pan et al 2020 ApJL 892 L6. doi:10.3847/2041-8213/ab799d

15 April 2020

Observations of Betelgeuse’s Dimming from the Stratosphere

This plot of V-band brightness shows Betelgeuse’s regular ~420-day pulsations, as well as the unprecedented dip in recent months. Red filled circles show the times of the three SOFIA/EXES observations compared in this study. [Harper et al. 2020]

The unprecedented dimming of the red supergiant star Betelgeuse has been making headlines since late last year. To find out what’s causing it, an airplane-borne telescope took to the skies.

A Dramatic Decline

In October 2019, Betelgeuse — identifiable as the bright, massive red supergiant lying at the left shoulder of the constellation Orion — began declining in brightness. By February 2020, it had dimmed to less than 40% of its average luminosity, leading some to speculate that this star might be preparing to end its life as a dramatic supernova.

But Betelgeuse doesn’t appear to be going anywhere just yet. In February 2020, the star stopped dimming and started to climb in brightness again — and yet we still don’t know what caused its remarkable drop.

Betelgeuse, shown here in an infrared image from the Herschel Space Observatory, is a luminous red supergiant star located about 700 light-years away. [ESA/Herschel/PACS/L. Decin et al.]

The Role of Red Supergiants

Why do we want to understand what’s happening with Betelgeuse? Red supergiants like this one represent a late evolutionary stage of massive stars. In this phase, strong winds flow off of the star, carrying away mass and populating the surrounding area with enriched stellar material.

But despite the important role these stars play in shaping galaxies and populating them with elements, the red supergiant stage is poorly understood, and there’s a lot we don’t know about the atmosphere, outflows, and timing of a star’s behavior during this phase. By tracking the evolution of Betelgeuse, a conveniently bright and nearby laboratory, we can further explore these processes.

A Telescope in Flight

Scientists have proposed two main explanations for Betelgeuse’s recent dimming: either it’s an intrinsic cooling of the star’s photosphere, or Betelgeuse has thrown off dust that’s now lying between it and us, blocking some of its light.

SOFIA, a modified Boeing 747SP carrying a 2.7-m telescope. [NASA]

Because infrared observations will be critical to exploring these options, NASA-DLR’s Stratospheric Observatory for Infrared Astronomy (SOFIA) planned an extensive campaign to look at Betelgeuse and its environment.

SOFIA consists a telescope mounted on an airplane that flies above 99% of the Earth’s infrared-blocking atmosphere. Observations of Betelgeuse were planned throughout winter/spring 2020 with all the instruments scheduled to fly on SOFIA. Now the first of these results, from the Echelon Cross-Echelle Spectrograph (EXES) instrument, have been published in a new study led by Graham Harper (University of Colorado Boulder).

Going with the Flow

Harper and collaborators explored Betelgeuse’s circumstellar envelope, the sphere of stellar material that flows off of and surrounds the star. In particular, the SOFIA/EXES observations are of two gas emission lines: singly ionized iron, and neutral sulfur. The authors compare observations of these lines from February 2020, when Betelgeuse was at its dimmest, to observations from 2015 and 2017, when Betelgeuse was at its normal brightness.

SOFIA/EXES observations of the ionized iron emission line around Betelgeuse during Cycle 2 (2015; yellow), Cycle 5 (2017; red) and Cycle 7 (February 2020; blue). [Harper et al. 2020]

The team finds that the lines from the different observing cycles are very nearly the same, suggesting that Betelgeuse’s circumstellar flow has not been affected by whatever caused the star to dim — whether that’s changes in the photosphere or the presence of new dust in the sightline to the star. The observations also indicate that the heating from the stellar wind didn’t change during the dimming.

These results are one more piece in the puzzle of Betelgeuse’s strange behavior. And with additional observations from other SOFIA instruments soon to be analyzed, we can anticipate more news to come!

Citation

“SOFIA-EXES Observations of Betelgeuse during the Great Dimming of 2019/2020,” Graham M. Harper et al 2020 ApJL 893 L23. doi:10.3847/2041-8213/ab84e6

13 April 2020

Astronomy Impacts of Satellite Megaconstellations

In May of 2019, SpaceX launched a batch of 60 satellites into low Earth orbit (LEO) in the first of a series of launches designed to populate a “megaconstellation” of satellites called Starlink. A new study now examines how the presence of these satellites — and those of future megaconstellations — will impact optical astronomy.

A Prominent Population

The number of objects (>100 kg) in lower LEO by year; click to enlarge. Before Starlink’s launch, there were fewer than 400 objects in this range. Now, Starlink (cyan) has begun to dominate this naked-eye-visible population. [Adapted from McDowell 2020]

With the goal of providing global internet access, Starlink and similar satellite megaconstellations sound like they should be a good thing. But this project, which is proposed to expand into a network of thousands of LEO satellites, has a drawback: these satellites are both large and orbit at low altitudes — which means they’re visible.

Right now, Starlink consists of 358 satellites orbiting in lower LEO (that’s below an altitude of 600 km); each of these satellites is 260 kg in mass and several meters across. If SpaceX launches the proposed number of satellites, there will eventually be more than 12,000 Starlink satellites — 9,000 of them in lower LEO — and they will dominate the naked-eye population by factors of 4 to 20.

Simulated instantaneous distribution of Starlink satellites. [McDowell et al. 2020]

Looking Through Trails

How will reflections from these satellites impact optical astronomy observatories? Are we doomed to a future in which satellite trails ruin every exposure from ground-based telescopes?

In a new study, scientist Jonathan McDowell (Center for Astrophysics | Harvard & Smithsonian) models the proposed satellite population as a function of latitude, time of year, and time of night. He then explores the density of satellites that will be illuminated and visible to optical observatories around the world.

Evaluating Impact

The number of Starlink satellites illuminated above a given elevation for four cases: at the horizon, 5° above, 10° above, and 30° above. The satellites pose the greatest challenge for observations nearer to the horizon and near twilight. Note that this particular plot is for mid-summer at a low-latitude observatory (30°S): neither the best nor worst case. [Adapted from McDowell 2020]

McDowell finds that there will be a number density of 0.005–0.01 objects per square degree illuminated at elevations relevant to observing. Several factors influence which observations are most affected by these reflected-light sources, including:

Latitude of the observatory and local time of year
In local winter, observatories at low latitudes will have perhaps 6 hours of observing time each night without illuminated satellites visible. At the other extreme, in local summer, intermediate-latitude observatories (like those in much of Europe) will contend with hundreds of illuminated satellites visible above the horizon.
Type of observing program
Certain types of observing campaigns will be more affected. This includes those that rely on long exposures of large fields of view (like many trans-Neptunian object surveys, including those searching for the hypothesized Planet Nine) and those that rely on twilight observations (like near-Earth-asteroid detection programs).

An Uncertain Future

The upshot? Starlink probably doesn’t spell the end of optical astronomy, but McDowell’s work shows that it clearly will have an impact on astronomers’ abilities to conduct useful observations with ground-based telescopes. And with other countries working on their own megaconstellations, we can expect the challenges to astronomy to continue to grow.

Photograph of a batch of Starlink satellites shortly after launch. [Official SpaceX Photos – Starlink Mission]

In response to astronomers’ concerns, SpaceX recently launched a batch of Starlink satellites coated in a special material to reduce their reflectivity. The jury is still out on whether this approach is effective, but one thing is clear: we will certainly benefit from working together to find a solution that helps us to advance our technology while still being able to study and learn from the universe.

Citation

“The Low Earth Orbit Satellite Population and Impacts of the SpaceX Starlink Constellation,” Jonathan C. McDowell 2020 ApJL 892 L36. doi:10.3847/2041-8213/ab8016

10 April 2020

Merger Partners? Maybe.

The Laser Interferometer Gravitational-wave Observatory (LIGO) and the Virgo interferometer have been turning up more and more binary black hole mergers in their observing runs. Do the black holes involved in these mergers have anything in common or are they paired purely by chance?

Binary Black Holes and Where They Come From

The question of how binary black holes (BBHs) form is still wide open, further complicated by the fact that the masses of the black holes involved are higher than expected. Some astronomers have suggested that BBHs are the result of massive stars that were already in binaries, while others have proposed scenarios where black holes in dense stellar populations encounter each other and pair off. Another possibility is that the black holes in BBHs formed as they are in the early universe — skipping existence as a star — and ended up in binaries.

Artist’s illustration of the merger of two black holes in space. [LIGO/T Pyle]

BBH mergers are a good way to study BBHs themselves; properties of the merger components (like mass) are imprinted into the resulting gravitational waves. In their first two observing runs, LIGO and Virgo spotted ten BBH mergers, and the black holes involved appear to have masses ranging from 18 to 84 solar masses.

In a new study, Maya Fishbach and Daniel Holz (The University of Chicago) explored how BBHs pair off in terms of their masses. And they found something interesting — it turns out the black holes in binaries may have more in common with each other than we thought!

Underlying Distributions

Fishbach and Holz attempted to understand BBH pairing through different black hole mass distributions. Broadly speaking, they considered three scenarios:

This plot shows, in blue, the estimated masses without uncertainties for the black holes that LIGO detected in binaries during its first two observing runs. When uncertainties are included, all 10 of LIGO’s detected systems are consistent with having equal component masses. [LIGO-Virgo/Frank Elavsky/Northwestern U.]

The black hole masses come from a distribution that is only constrained by minimum and maximum masses.
The black hole masses come from a distribution that depends on minimum and maximum masses, and the ratio between the masses of the black holes in a BBH.
The black hole masses come from a distribution that depends on minimum and maximum masses, the mass ratio between the BBH components, and the total mass of the BBH.

On modeling and applying these scenarios to the ten available BBH merger observations, Fishbach and Holz came away with two main findings: random pairings are thoroughly disfavored, and black holes in BBHs are five times more likely to be of similar mass than not. They also find that total system mass may not play a big role in BBH pairing.

The BBH formation models that end with black holes of similar mass are usually those that involve massive stellar binaries. This doesn’t rule out other formation mechanisms but Fishbach and Holz’s work suggests that future models may need to account for the mass ratio in BBHs.

Of course, this work is based on only ten observations. However, with more observations from LIGO/Virgo already on the way, astronomers will soon be able to further constrain and eventually solve this puzzle.

Citation

“Picky Partners: The Pairing of Component Masses in Binary Black Hole Mergers,” Maya Fishbach and Daniel E. Holz 2020 ApJL 891 L27. https://doi.org/10.3847/2041-8213/ab7247

8 April 2020

Jets from a Galaxy Collision

Dramatic collisions of galaxies can provide fireworks shows in more ways than one. New observations have now confirmed a long-theorized link between galaxy mergers and the launch of powerful relativistic jets.

Feeding the Fire

We know that nearly every galaxy hosts a supermassive black hole of millions to tens of billions of solar masses. Some, like the one at the center of our own Milky Way, are quiet. But many actively accrete gas, flaring with bright emission across the electromagnetic spectrum. In addition to accreting material, some of these active galactic nuclei (AGN) also fling incoming material back out, forming powerful jets that zip along at velocities close to the speed of light.

When two galaxies collide, gas can be driven to their centers, feeding the supermassive black holes that lurk there. [NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration]

But how does the gas feeding this dramatic AGN activity arrive at the center of the galaxy in the first place? One theory is that violent collisions of galaxies deliver the necessary fuel. In this picture, when two galaxies merge, the turbulent collision feeds gas into the nuclei of the galaxies, causing the AGN to light up and triggering the launch of energetic jets.

One Moment in a Long History

How can we test this theory? Galaxy mergers take billions of years, so watching one in real time isn’t an option. But if we spot galaxies that are mid-merger and that also sport AGN activity with young jets, that would provide reasonably convincing evidence that the jet production is related to the merger.

With this goal in mind, a team of scientists led by Vaidehi Paliya (Deutsches Elektronen-Synchrotron DESY, Germany) went on the hunt for evidence of young AGNs that might also be in the process of colliding — and they found what they were looking for in TXS 2116–077.

A Merger Caught Red-Handed

TXS 2116–077 lies 4.3 billion light-years away. Notably, this young AGN hosts a speeding jet at its heart, pointed close to our line of sight. Because the jet power is relatively low for an AGN, we can observe the accretion environment around TXS 2116–077 without being blinded by the jet’s emission.

Paliya and collaborators used the 8.2-m Subaru Telescope in Hawaii to image TXS 2116–077, revealing that this galaxy is in the process of merging with a nearby companion. Their close separation of just 40,000 light-years indicates that the two galaxies are in a late stage of merger and approaching coalescence.

This infrared image of TXS 2116−077, obtained with the Subaru telescope, reveals the presence of two galaxies interacting with one another. Overplotted contours show radio contours, revealing the compact jet. Click to enlarge. [Paliya et al. 2020]

The authors obtain follow-up data with the 10.4-m Gran Telescopio Canarias and the 4.2-m William Herschel Telescope in Spain, as well as with the Chandra X-ray Observatory. The combined observations show that gravitational interactions between the two galaxies have caused visible disturbances in their morphologies, and both galaxies boast active nuclei.

Old Collision, Young Jet

By modeling the observed stellar populations, Paliya and collaborators estimate that the merger of these galaxies began ~0.5–2.5 billion years ago. The jet, in contrast, is estimated to be only 15,000 years old, based on its approximate length and speed.

The fact that the jet clearly formed after these two galaxies began merging provides strong evidence in favor of mergers as a trigger for AGN accretion and the launch of relativistic jets. Targets like TXS 2116–077 therefore represent ideal sources for studying newly formed jets and their birth environments.

Citation

“TXS 2116−077: A Gamma-ray Emitting Relativistic Jet Hosted in a Galaxy Merger,” Vaidehi S. Paliya et al 2020 ApJ 892 133. doi:10.3847/1538-4357/ab754f

3 April 2020

Peering Into the Atmosphere of the Hottest Planet Known

As the ultra-hot Jupiter KELT-9b blazes across the face of its host star, we have an excellent opportunity to examine its scalding atmosphere. A new study now reports on what we’ve found.

A Passing Glance

In our efforts to learn more about worlds beyond our solar system, atmospheres provide a critical key. Characterizing the atmospheres of exoplanets can provide us with insight into the planets’ compositions and climates, their evolution, and even — with some potential caveats — their habitability.

As a star’s light filters through a planet’s atmosphere on its way to Earth, the atmosphere absorbs certain wavelengths depending on its composition. [European Southern Observatory]

In particular, transiting exoplanets provide us with a unique opportunity. As a planet passes in front of its host star, we briefly observe the star’s light filtering through the planet’s atmosphere. By exploring the spectrum of that light, not only can we identify the presence of specific atoms and molecules in the planet’s atmosphere, but we can also learn more about where they are and what the atmospheric properties are at those locations.

In a new study led by Jake Turner (Cornell University), a team of scientists digs deep into such a transmission spectrum for the exoplanet KELT-9b.

Not Exactly Temperate

KELT-9b is an extreme world. Clocking in with a dayside temperature of more than 4,500 K (~7,600 °F), it is the hottest planet known — hotter than many stars! This ultra-hot Jupiter orbits at a mere 0.035 AU from its scalding A- or B-type host star, whizzing around its host in just 1.5 days.

The intense radiation bombarding KELT-9b almost certainly takes a toll: this energetic light should dissociate molecules into their component atoms and ionize metals in the hot atmosphere, and it may inflate the envelope of hydrogen gas around the planet to the point where the hot gas escapes.

Observed and modeled Hα (top) and Ca II (bottom three) spectral lines in the atmosphere of the ultra-hot Jupiter KELT-9b. [Adapted from Turner et al. 2020]

Turner and collaborators explore the extreme conditions in KELT-9b’s atmosphere with high-resolution transmission spectra taken with the CARMENES instrument on the Calar Alto 3.5-m telescope in Spain.

Detecting Atmospheric Thermometers

The authors find absorption lines indicating the presence of ionized calcium, Ca II, in KELT-9b’s atmospheric spectra; this is just the second time that Ca II has been observed in a hot Jupiter’s atmosphere. They also find prominent Hα absorption — evidence that confirms the existence of an extended envelope of hydrogen surrounding the irradiated planet.

By modeling the spectra they obtain for KELT-9b, Turner and collaborators are able to identify the pressures, altitudes, and temperatures at which these spectral lines form in the atmosphere. They find that the Ca II lines probe the atmosphere at an altitude of about 1.32–1.40 times the planet’s radius. The Hα line provides information from higher up, at 1.44 planetary radii.

Together, these absorption lines act as atmospheric thermometers, providing a picture of KELT-9b’s atmospheric temperature profile and yielding insight into the energy that enters and leaves the planet’s atmosphere.

These results demonstrate the power of this technique, revealing the remarkable wealth of information we can glean from some distant starlight filtered through the atmosphere of an extreme world.

Citation

“Detection of Ionized Calcium in the Atmosphere of the Ultra-hot Jupiter KELT-9b,” Jake D. Turner et al 2020 ApJL 888 L13. doi:10.3847/2041-8213/ab60a9

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