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illustration of a planet orbiting two stars

Can planets form in a tilted disk around a pair of stars? New simulations explore what happens in off-kilter planetary systems.

Seeking Skewed Systems

radio observations of the disk around a protostellar binary system

Radio observations revealed that the young binary system L1551 NE has a misaligned disk. [Adapted from Takakuwa et al. 2017]

Thanks to exoplanet-hunting spacecraft like Kepler, we’ve discovered more than a dozen planets orbiting two stars rather than one. Though all of the circumbinary planets discovered so far orbit close to the same plane as their host stars, theory suggests they could be found perpendicular to the plane of the binary system, and observations of tilted disks around young stars suggest that a wide range of inclination angles is possible.

The lack of planets seen in highly misaligned orbits might mean these planets are rare, but it could also mean that these planets are simply difficult to detect. With misaligned circumbinary systems still hard to come by, we must turn to models to explore what happens in these systems — what determines whether a planet orbiting two stars does so in the same plane as the binary system, perpendicular to it, or somewhere in between?

Modeling Misalignment

Anna Childs and Rebecca Martin (University of Nevada, Las Vegas) approached this question by modeling how the eccentricity of a binary system and the initial tilt of the surrounding disk affect the final orbital parameters of a planetary system. Childs and Martin investigated binary systems with perfectly circular orbits (e=0) and disks inclined by 30 and 60 degrees, as well as binary systems with very elongated orbits (e=0.8) and disks tilted by 60 degrees. In each case, the team initiated their simulations late in the process of planet formation, when the planetary systems are mostly free of gas and contain a host of Moon- and Mars-sized planetesimals.

example simulation results

Simulation results at three points in time for a circular binary system with a 30° inclined disk (top) and an eccentric binary system with a 60° inclined disk (bottom). Click to enlarge. [Childs & Martin 2022]

Their results showed that certain initial configurations generate only coplanar planets, while others churn out only perpendicular planets. In circular binary systems, disks that are tilted by 30 degrees tend to form planets that stick close to the plane of the binary, but if the initial tilt of the disk is cranked up to 60 degrees, collisions and gravitational interactions kick out 84% of the planet-forming material. In contrast, in eccentric binary systems, planets can still form in an extremely tilted disk — but in this case, the resulting planets orbit perpendicular to the binary system.

Giant Planets in the Mix

plot of simulation results

Degree of misalignment from a coplanar or polar orbit for simulations without (left) or with (right) giant planets introduced. The simulations shown are circular and inclined by 30 degrees (C30), circular and inclined by 60 degrees (C60), and eccentric and inclined by 60 degrees (P60). The symbol size and color varies with the planet mass. Click to enlarge. [Adapted from Childs & Martin 2022]

The authors also introduced Jupiter and Saturn analogs into some of their simulations to understand how the presence of giant planets affects the formation of planets in a misaligned disk. Giant planets tended to increase the amount of material kicked out of the planetary systems, but they also increased the rate at which the planetesimals collided, so the few planets remaining at the end of the simulations tended to be more massive.

Notably, almost all of the simulated planets fell into coplanar or perpendicular orbits. If future observations reveal planets at intermediate inclinations, this might mean that those planets followed a different formation pathway from the one explored in this study. Hopefully, it’s just a matter of time before we detect circumbinary planets in misaligned orbits and put our theories to the test!

Citation

“Misalignment of Terrestrial Circumbinary Planets as an Indicator of Their Formation Mechanism,” Anna C. Childs and Rebecca G. Martin 2022 ApJL 927 L7. doi:10.3847/2041-8213/ac574f

Galaxy with a big halo [made of dark matter] surrounding it

Does the spin of a dark matter halo align with the spin of the galaxy it’s situated around? And what can this tell us about the early universe? Hydrodynamical simulations of galaxies in the early universe might help us answer these questions. 

Bullet cluster (two galaxy clusters colliding)

The Bullet Cluster of galaxies. X-rays are shown in pink and the gravitational lensing is shown in blue. This cluster is considered one of the smoking guns for the presence of dark matter. [X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.]

No Longer in the Dark About Dark Matter 

Though the field of dark matter is expansive and exciting these days, astronomers didn’t even know dark matter existed until the 1980s. Now, dark matter can be used to probe everything from the force of gravity to galactic structure and evolution. Studying dark matter also has implications for cosmology and can help us better understand the initial conditions of the universe. Specifically, the alignment between the spins of a galaxy and its dark matter halo can help constrain the dark matter equation of state (which can tell us about the mass of the galaxy and help with predictions of its dynamics).

Taking Dark Matter for a Spin 

Galaxy with arrows on it showing how the galaxy is spinning.

Illustration of the spin of a galaxy. The velocity stays constant with radius, which shows that dark matter is present in the halo. [Adapted from ESO/L. Calçada; CC BY 4.0]

There are two main questions: first, how well do the observable spins of galaxies align with the spins of their dark matter halos, which can’t be measured? Studies exploring this question using numerical simulations have concluded that galaxies and their surrounding dark matter halos can be substantially misaligned. The second, potentially more important question is what this misalignment implies — if the spins are not aligned, does this mean that the motions of a galaxy’s stars are decoupled from changes in the background gravitational field? If so, this implies that we can no longer use the spin alignment of visible matter to probe the background cosmology. A team led by Jounghun Lee at Seoul National University aims to address this second question using hydrodynamical simulations to probe different scenarios. 

Illustrating What’s Happening Inside Galaxies 

Galaxies placed along filaments in the cosmic web

Simulation of the cosmic web. Every point of light is a galaxy and those galaxies are placed along filaments. [Illustris Project]

The team used the IllustrisTNG suite of simulations to model galactic dynamics. This software takes into account everything from star formation rate of the galaxy to feedback from supernovae and growth of black holes to model the physics going on. When the universe was ~9 billion years old, the luminous matter and various forms of stellar feedback like supernovae occurred along filaments in the cosmic web, which coupled the galaxy and dark matter halo spins, allowing us to probe early cosmology. However, going back to when the universe was ~5 billion years old, those filaments hadn’t yet formed and the matter density was fairly uniform, so these matter processes occurred randomly and didn’t have any structure to follow. This led to the uncoupling of the galaxy spin and the dark matter halo spin.

Lee and collaborators also find that properties such as black hole-to-stellar mass ratio, specific star formation rate (rate of creation of stars per unit stellar mass), and average metallicity either correlate or anticorrelate with the angle between the galaxy stellar and dark matter spins. 

Future work will involve finding direct evidence for the scenario of decoupling between the spins of the galaxy and its dark matter halo earlier in the universe’s history, modeling it, and exploring its connection to the initial conditions of the universe.

Citation 

“How Do the Galaxy Stellar Spins Acquire a Peculiar Tidal Connection?,” Jounghun Lee et al 2022 ApJ 927 29. doi:10.3847/1538-4357/ac4bda

hubble image of a galaxy cluster

How can we trace the formation of structure in the early universe? A new article surveys the environments around massive galaxies that existed less than two billion years after the Big Bang to learn more.

Questing for Quasars

image of a distant quasar

A cloud of gas surrounds the distant quasar SDSS J102009.99+104002.7 in this image from ESO’s Very Large Telescope. The name “quasar” is a shortening of “quasi-stellar radio source”, though we now know that only a small fraction of quasars are radio-loud. [ESO/Arrigoni Battaia et al.; CC BY 4.0]

One of the best ways to understand the conditions in the early universe is by studying quasars — extremely bright centers of young galaxies where supermassive black holes are accreting material. Looking back billions of years into the past, quasars appear not to be distributed randomly throughout space, which suggests that the massive galaxies they inhabit might be tracers of underlying dark matter structures. If this is the case, non-quasar-hosting galaxies in the early universe should also be found preferentially close to quasars.

Past studies have explored this hypothesis, but the results have been conflicting. Some studies have found that quasars have an abundance of galaxies in their vicinity, while others have found that there are no more galaxies than to be expected if they were randomly scattered throughout space. There are many potential reasons for this disagreement, including the possibility that dust hides these distant galaxies from the searching eyes of optical telescopes. In a new article, a team led by Cristina García-Vergara (Leiden Observatory, The Netherlands) approached this question in a new way — by using a massive array of radio telescopes to peer through the dust.

ALMA emission line maps and spectra

Sample emission line maps (left column) and spectra extracted from each source’s brightest pixel (right column) for the sources detected in this work. Click to enlarge. [Adapted from García-Vergara et al. 2022]

Long Wavelengths and Large Distances

García-Vergara and collaborators observed the areas surrounding 17 quasars with redshifts of z ~ 4 (roughly 1.6 billion years after the Big Bang) with the Atacama Large Millimeter/submillimeter Array (ALMA) — a collection of 66 radio telescopes working together as one. The team sought emission from a particular spectral line of carbon monoxide, which can signal the presence of a galaxy even if it is so shrouded in dust that it would be invisible at optical wavelengths.

Using a search algorithm, García-Vergara and collaborators identified all the sources of carbon monoxide emission in each quasar’s vicinity, finding a total of nine carbon monoxide-line-emitting galaxies among the 17 fields they searched. As is often important when studying sources billions of light-years away, the team also assessed the likelihood that the sources they detected are actually galaxies in the local universe masquerading as galaxies in the early universe and found it unlikely.

Galactic Neighbors

plot of the sources in relation to their central quasars

Distribution of the nine carbon monoxide sources (filled symbols) for all 17 quasar fields combined. The black diamond at the center indicates the location of the quasar, and the empty symbols indicate the locations of Lyman-alpha emitting galaxies from a previous study. [García-Vergara et al. 2022]

The team then used their observations to estimate how many more galaxies are present in the vicinity of quasars than would be expected if the galaxies were distributed randomly throughout space. Based on this analysis, the team found that there were 17.6 times more carbon monoxide-line-emitting galaxies in the areas surveyed than predicted by a random distribution.

Not only are there more galaxies than expected, they’re also tightly clustered around the quasars. These two pieces of evidence strongly support the idea that quasars are tracers of massive structures forming early in the universe, though the authors note that the statistical significance of their result could be improved by pushing the observations deeper or wider — hopefully the future will bring new observations and a fresh perspective on galaxies in the early universe!

Citation

“ALMA Reveals a Large Overdensity and Strong Clustering of Galaxies in Quasar Environments at z ∼ 4,” Cristina García-Vergara et al 2022 ApJ 927 65. doi:10.3847/1538-4357/ac469d

photograph of the rubin observatory telescope mount assembly as of april 2021

photograph of a mountain ridge with two telescopes

Rubin Observatory will sit atop the Cerro Pachón ridge, joining Gemini south and the Southern Astrophysical Research Telescope. [LSST Project Office; CC BY 4.0]

First light for the Vera C. Rubin Observatory atop Cerro Pachón in Chile is fast approaching — the eagerly awaited facility is slated to begin observations in 2024. The observatory’s 8.36-meter Simonyi Survey Telescope will carry out the Legacy Survey of Space and Time (LSST), a 10-year endeavor that will map the entire sky visible from its location every three days.

Rubin Observatory and LSST are poised to revolutionize astronomy in ways both anticipated and unexpected; through repeated observations, LSST will enable us to detect and monitor transient events like supernovae and gamma-ray bursts, map the Milky Way, probe the nature of dark energy and dark matter, and expand our catalogs of solar system objects by more than an order of magnitude. A new focus issue of the Astrophysical Journal Supplement Series explains how astronomers used science to guide the development of the upcoming survey.

map of the LSST footprint

LSST footprint showing the number of visits as a function of position on the sky. The acronyms refer to the surveys that fall under the LSST umbrella. Click to enlarge. [Bianco et al. 2022]

The survey — which is actually a combination of multiple surveys with specific goals — is notable not only for its breadth and depth, but also its design, which incorporated input from the astronomy community at every step. The articles in the new focus issue, six of which are already published, detail the scientific rationale behind the path the survey will take across the night sky, known as the observing cadence, and explore the potential science gains to will achieve. A few of the topics explored in this focus issue include:

  • Standard candles: RR Lyrae-type variable stars show correlations between the period of their variation, their luminosities, and their colors. These relationships allow RR Lyrae stars to be used as standard candles for determining the distances to other galaxies, so precisely determining the period of their variation is critical. LSST potentially occupies the sweet spot for studying RR Lyrae stars: wide enough coverage to allow for population-wide studies of these important stars, but with short enough cadence to capture subtle changes in the period and amplitude of their variations, the cause of which is still unknown.
  • illustration of a blazar

    Artist’s impression of the jet from an active galactic nucleus directed toward Earth. [NASA/Goddard Space Flight Center Conceptual Image Lab]

    Blazar variability: Blazars — relativistic jets pointed toward Earth that emanate from distant supermassive black holes that are actively accreting material — can vary in brightness on timescales from minutes to years. LSST is expected to observe thousands of known blazars while potentially discovering thousands more, enhancing our understanding of blazar variability as well as the environments they live in.
  • The unknown: Possibly even more enticing than the science advances we expect are those that we can’t even begin to imagine. How can a survey be optimized to discover something that we know nothing about? This issue is tackled by estimating how complete the survey will be in terms of the volume of space explored, the wavelengths covered, and other factors. They key to discovery might be switching things up; survey strategies that vary exposure time and time between observations find more novel sources than those that don’t.

Rubin Observatory is notable not only for the science data it will collect during the planned 10-year duration of LSST — roughly 300 petabytes’ worth (that’s 300,000,000,000,000,000 bytes) — but also what it plans to do with it. Each night, the observatory will release data related to millions of transient and variable objects to enable immediate follow-up of intriguing targets. The 20 terabytes of data collected nightly will be processed, stored, and shared with all scientists in the United States and Chile, as well as anyone affiliated with a number of participating institutions. Two years after its collection, the data will be made available to all. A decade of exceptional data, shared widely with the community and the world? It doesn’t get much better than that!

Citation

Articles in the Rubin LSST Survey Strategy Optimization Focus Issue will be collected here.

“Optimization of the Observing Cadence for the Rubin Observatory Legacy Survey of Space and Time: A Pioneering Process of Community-focused Experimental Design,” Federica B. Bianco et al 2022 ApJS 258 1. doi:10.3847/1538-4365/ac3e72

magnetar

New observations have captured pulses of radiation from a magnetic stellar remnant called a magnetar. What do these observations tell us about how magnetars and other neutron stars generate their beams of emission?

Rare Stellar Remnants

x-ray image of the vela pulsar

The Vela pulsar — seen emitting jets of fast-moving particles in this image — was the first pulsar to be detected at submillimeter wavelengths. Now, the first neutron star to pulsate in this wavelength range has been discovered. [NASA/CXC/Univ of Toronto/M. Durant et al.]

Neutron stars — ultra-dense, city-sized remnants of stars that exploded as supernovae — come in many flavors. Those that emit narrow beams of radio waves that sweep past Earth like the beacon of a lighthouse are called pulsars. Those that have extremely strong magnetic fields — 100 million times more intense than the strongest magnet ever made — are called magnetars. In rare cases, a neutron star can be both a pulsar and a magnetar!

It’s not yet clear how pulsars generate their beams of radio emission. One way to probe the generation mechanism is by studying the emission across a wide range of wavelengths, since some models predict that the emission should increase at a “turn-up” point somewhere between radio and infrared wavelengths. Previous observations have found tantalizing hints of this feature, but it has never been detected definitively. Can a new search at submillimeter wavelengths find the elusive turn-up point?

plot of submillimeter emission

Detection of XTE J1810−197 on 27 February 2020 at a wavelength of 0.85 mm. The panels show the target during a pulse (left) and not during a pulse (right) as well as the difference between the two states (bottom). Click to enlarge. [Adapted from Torne et al. 2022]

A Submillimeter Signal

A team of astronomers led by Pablo Torne (Institute of Millimeter Radio Astronomy, Spain; East Asian Observatory; and Max Planck Institute for Radio Astronomy, Germany) searched for signs of the turn-up point in observations of XTE J1810-197, one of only six neutron stars categorized as both a pulsar and a magnetar.

Torne and collaborators used telescopes across the globe to observe XTE J1810-197 over the course of 15 months. They detected a beam of emission swinging by for a few hundred milliseconds once per rotation period (5.54 seconds) at wavelengths ranging from 0.85 millimeters to 5.0 centimeters, marking the first time pulses from a neutron star have been detected at submillimeter wavelengths. However, they didn’t detect the pulses at 0.45 mm — the shortest wavelength searched in this study. What do these observations imply about the location of the turn-up point?

Where Will the Turn-up Turn Up?

plot of flux density versus frequency

Power-law (top) and broken power-law (bottom) spectral fits for XTE J1810−197. The downward pointing arrows indicate upper limits. [Torne et al. 2022]

Torne and collaborators found that XTE J1810-197’s emission is mostly flat across the wavelength range surveyed, with a potential downturn at longer wavelengths — and no sign of the turn-up point. If a turn-up is present, it might lie in the infrared or in the unexplored 0.37–3.00 cm (10–80 gigahertz) range.

However, searching these wavelength ranges may not be as simple as pointing the right kind of telescope at the target; magnetars are complicated, ever-changing objects that exhibit extremely energetic outbursts at short wavelengths and day-to-day variability across all wavelengths. This high level of variability can make determining the true shape of a neutron star’s spectrum challenging, since it might not be possible to compare measurements made at different times. (For example, astronomers have observed XTE J1810-197 in the infrared once before, but those observations were made when the magnetar was undergoing an explosive outburst.) However, with a little planning, simultaneous observations from radio to infrared could help us track down the turn-up point.

Citation

“Submillimeter Pulsations from the Magnetar XTE J1810-197,” Pablo Torne et al 2022 ApJL 925 L17. doi:10.3847/2041-8213/ac4caa

artist's impression of a gas giant exoplanet

Astronomers have discovered a curious exoplanet with an extremely low bulk density — nearly 15 times less dense than Jupiter and 60 times less dense than Earth. The first spectrum of this planet’s atmosphere gives clues to the cause of this unusual quality.

An Exceptional Exoplanet

black and white photograph of neptune's rings

Voyager 2 captured this shot of Neptune’s rings in 1989. Given its distance from its host star, HIP 41378 would likely have rings that are rocky like Neptune’s rather than icy like Saturn’s. [NASA/JPL]

HIP 41378 f is one of the least dense exoplanets known, floating in at just 0.09 grams per cubic centimeter, but it’s not yet clear why this planet is so loosely packed. HIP 41378 f might be an example of a rare class of exoplanets called super-puffs, which contain far more gas than expected given their masses. On the other hand, the planet’s low bulk density could be just a trick of the light curve — if the planet has rings, its radius might appear artificially large, deflating its calculated bulk density.

Both possibilities are exciting, since we’ve only discovered a handful of super-puff planets, and we’ve yet to definitively detect rings around an exoplanet. Now, a team led by Munazza Alam (Carnegie Earth & Planets Laboratory and Center for Astrophysics | Harvard & Smithsonian) has collected the first near-infrared transmission spectrum of HIP 41378 f’s atmosphere to gain a better understanding of this unusual planet.

transmission spectra and modeled spectra

Transmission spectrum for HIP 41378 f (black circles) compared to model results (colored lines). The three panels show the same data and models over three different wavelength ranges. Click to enlarge. [Alam et al. 2022]

Puffy Planet, Rocky Rings, or Something Else?

Alam and collaborators used the Hubble Space Telescope to measure the light that filters through HIP 41378 f’s atmosphere as the planet makes its 19-hour transit across the face of its parent star. The team found that HIP 41378 f’s spectrum is nearly featureless, lacking the characteristic dips that signal absorption of light by molecules in the atmosphere.

Using one-dimensional atmospheric models, the authors were able to rule out a clear atmosphere rich in hydrogen and helium. However, they found that the planet’s nearly flat spectrum is consistent with multiple scenarios: HIP 41378 f might have an atmosphere exceptionally rich in elements heavier than helium, a layer of haze, or rings. In the ringed planet case, the authors calculated that HIP 41378 f’s true radius would be about 60% smaller than the current estimate, leading to a bulk density of 1.2 grams per cubic centimeter, which is roughly the density of Jupiter and Uranus.

Transit Opportunities

plot of transit times and transit timing variations

Previous and predicted transit times for HIP 41378 f. [Alam et al. 2022]

HIP 41378 f’s lack of spectral features doesn’t mean we can’t learn more about the planet’s atmosphere. Measuring the planet’s transmission spectrum at longer wavelengths might help distinguish between the possibilities, since a hazy atmosphere would induce transits of different depths at different wavelengths, while the presence of rings would produce less variation.

The authors explored the possibility of observing HIP 41378 f with JWST and found that the telescope’s infrared instruments are sensitive enough to distinguish between the competing scenarios. With an orbital period of roughly 1.5 Earth years, there will only be a few opportunities to catch HIP 41378 f transiting in front of its parent star during JWST’s planned five-year mission. Alam and collaborators used the timing of the transit they observed to update the prediction for HIP 41378 f’s future transits; late 2022 and mid 2024 will bring new opportunities to study this enticing faraway world!

Citation

“The First Near-infrared Transmission Spectrum of HIP 41378 f, A Low-mass Temperate Jovian World in a Multiplanet System,” Munazza K. Alam et al 2022 ApJL 927 L5. doi:10.3847/2041-8213/ac559d

Neutron Star Merger

The merger of two neutron stars releases an enormous amount of energy and reconfigures the magnetic field of the whole binary system. How well do we need to know the initial conditions of the system to predict the outcome of the merger? 

When Worlds Collide 

In 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first observation of a neutron star–neutron star merger by detecting the ripples in spacetime released as the two massive objects collided and sent the signal ringing through the cosmos. This detection showed that neutron star–neutron star mergers are powerful enough to emit gravitational waves and allowed us to observe properties of these neutron stars such as their mass and radius. 

Simulations of the magnetic field of the merger (shown as spirals getting tighter over time)

Magnetic field evolution over the course of the merger. Time increases from left to right, showing snapshots at 2, 5, 10, and 20 milliseconds after the merger. Each row is a different simulation, and the darker the area in the image, the more intense the magnetic field. [Aguilera-Miret et al. 2022]

Does the End Point to the Means? 

Though the dynamics of colliding neutron stars are fairly well understood, a few open questions remain, such as how the magnetic field amplifies and reorganizes during the merger. This is important because the amplification and reorganization is necessary for the production of the jet associated with short gamma-ray bursts.

Studying this critical process is difficult, as it involves capturing fluctuations and instabilities on a very small scale and requires precise knowledge of the initial parameters of the system. A team led by Ricard Aguilera-Miret (University of the Balearic Islands / Institute of Space Studies of Catalonia) performed complex simulations to tackle the question of how much the initial magnetic field configuration of the system affects the end product of the merger. 

A plot showing time vs. magnetic energy. The four models stars at different places on the y-axis and converge as time goes on.

The magnetic field energy for various simulations. This shows that no matter what energy the system begins with, the final energy will be roughly the same. [Aguilera-Miret et al. 2022]

Magnetic Merging 

Using supercomputers, the team explored the effect of different initial magnetic configurations on the final magnetic field strengths of the simulated binary neutron star mergers. As they moved time forward, exploring up to 30 milliseconds after the merger, they found that the initial topology of the system does not affect the end product because small-scale turbulence erases any memory of magnetic fields greater than 1012 G within a few milliseconds of the merger. This creates a new conundrum, as it shows that we can’t infer the initial magnetic field of a system by observing it post-merger. 

These simulations show that using a simplified magnetic field model is acceptable in binary neutron star mergers, as long as the magnetic field isn’t too large, because it doesn’t make a difference in the final configuration. Further observations of neutron star–neutron star mergers will provide a test of this theory. 

Citation 

“Universality of the Turbulent Magnetic Field in Hypermassive Neutron Stars Produced by Binary Mergers,” Ricard Aguilera-Miret et al 2022 ApJL 926 L31. doi:10.3847/2041-8213/ac50a7

artist's impression of a star being ripped apart by a black hole

Astronomers may have discovered the fifth instance of a new kind of high-energy outburst seen in distant galaxies. How did they track down the source, and what does this discovery mean for our understanding of these rare eruptions?

Rare Events

Only four galaxies in the universe are currently thought to produce quasi-periodic eruptions: enormous outbursts of X-ray photons that occur roughly every few hours. Two of these sources are associated with galaxies in which the central supermassive black hole is actively consuming material from its surroundings, while the other two sources are quiescent galaxies. Despite their differences, all four sources have eruptions of similar duration, frequency, and energy dependence, which suggests that they share a common origin.

However, the source of these events remains uncertain, and further detections are needed to narrow in on a cause. In today’s article, Joheen Chakraborty (Columbia University and MIT Kavli Institute for Astrophysics and Space Research) and coauthors describe their hunt, discovery, and analysis of a potential fifth quasi-periodic eruptor.

plot of X-ray count rate and amplitude

Count rate and amplitude measured by three detectors on XMM-Newton during the potential quasi-periodic eruptions (left) and during a quiescent phase (right). Note that the y-axis range is different between the two columns. Click to enlarge. [Adapted from Chakraborty et al. 2021]

Slewing Past a Source

Chakraborty and collaborators searched for quasi-periodic eruptions in archival observations from the X-ray Multi-Mirror Mission (XMM-Newton), a space telescope that has been observing the X-ray sky since 2000. Using an algorithm that was originally developed to detect tiny variations in the timing of exoplanet transits, the team found a single convincing source out of nearly 12,000 observations: the galaxy XMMSL1 J0249-041244, referred to as J0249. XMM-Newton first detected this source in 2004 as part of its slew survey, which observes objects that happen to fall en route from one target to the next.

The team’s algorithm identified one and a half X-ray flares from 2006 that resembled the quasi-periodic eruptions seen in other galaxies. That’s not the only interesting thing about this source: it has also been tagged as a potential site of a tidal disruption event, in which a black hole rips apart and consumes a star. Is it possible that tidal disruption events are the underlying cause of quasi-periodic eruptions? The authors dig into the observations of this doubly intriguing galaxy to find out more.

And Then There Were Five

three plots of eruption properties

Comparison of the properties of J0249’s eruptions (purple symbols) to those of the previously detected sources. Click to enlarge. [Adapted from Chakraborty et al. 2021]

In many ways, J0249 behaved similarly to the four previously discovered quasi-periodic eruptors. Its flares were largest in low-energy X-rays and less prominent at higher energies, and its flux was fairly constant outside of the flaring episodes. However, the authors also detected potential dips in the ultraviolet emission during the X-ray flares, which marks the first time a quasi-periodic eruptor candidate has shown hints of variability at longer wavelengths. This might suggest that J0249’s emitting region is larger than that of the previously detected sources, but it may also indicate the presence of an accretion disk that is unrelated to the eruptions.

What does the future hold for J0249? In 2021, the team requested a final XMM-Newton observation long enough to capture 2-3 eruptions, but there was no sign of the flares seen in 2006. This may mean that the eruptions were linked to a tidal disruption event, and they ceased when the star was finally engulfed. With only five sources found so far, there’s plenty we don’t yet know about about quasi-periodic eruptions. Hopefully, further detections will clarify the role that tidal disruption events play in these rare outbursts!

Citation

“Possible X-Ray Quasi-periodic Eruptions in a Tidal Disruption Event Candidate,” Joheen Chakraborty et al 2021 ApJL 921 L40. doi:10.3847/2041-8213/ac313b

illustration of RS Ophiuchi

A new star appears in the constellation Ophiuchus roughly every 15 years when the surface of a white dwarf ignites in a burst of nuclear fusion. Can X-ray observations tell us what happens in the aftermath of these explosions?

Hello Again, RS Oph

six-panel plot showing count rate as a function of location on the sky

Images of RS Oph obtained after processing. The expanding lobes that extend in the east–west direction are clearly visible in the 2009 and the two 2011 observations, which are combined into one for the analysis. The black outlines indicate the location of a known artifact in the imaging system. [Adapted from Montez et al. 2022]

The RS Ophiuchi (RS Oph) binary system, composed of a red giant and a white dwarf, is usually hundreds of times too faint to see with the unaided eye. Occasionally, though, it briefly winks into view. RS Oph is a rare example of a recurrent nova, only a handful of which are known in the Milky Way. Outbursts occur when a white dwarf in a binary system steals enough material from the atmosphere of its puffed-up companion to ignite a brief flash of nuclear fusion on its surface, releasing a thousand times the Sun’s yearly energy output in just a few days.

RS Oph has flared up every 9–21 years since its first recorded explosion in 1898, with the two most recent outbursts occurring in 2006 and 2021. The aftermath of RS Oph’s 2006 eruption has been observed all across the electromagnetic spectrum, revealing an expanding ring of circumstellar material and a massive bipolar outflow. A new article led by Rodolfo Montez Jr. (Center for Astrophysics ∣ Harvard & Smithsonian) introduces X-ray observations from just a few years after the 2006 outburst, giving us an unprecedented view of the system’s expanding outflows.

three panel plot of X-ray count rate

Profiles derived by integrating X-ray images along the declination axis. [Montez et al. 2022]

Expanding Our Knowledge

Montez and collaborators investigated images and spectra taken by the Chandra X-ray Observatory in 2007, 2009, and 2011 to study the X-ray-emitting plasma flowing outward from the white dwarf. They detected jet-like structures spreading out from the system to the east and west, mirroring the structures seen at other wavelengths.

The jets were too close to the RS Oph system to be discernible in 2007, but they grew visibly between 2009 and 2011, traveling across the sky at a rate of 1.1 milliarcseconds per day. The distance to RS Oph is still uncertain, making it difficult to measure the precise velocity of the outflow, but the authors estimate that this apparent movement corresponds to 6,000 kilometers per second — meaning that the material was launched at roughly 2% the speed of light!

No Slowing Down

How did the bipolar jets evolve as they expanded? Based on archival multiwavelength images, the jets appear to have expanded linearly over time, without slowing down. They also appear not to have cooled down; modeling of the X-ray spectra suggests that the outflow maintained a constant temperature of 2 million kelvin.

modeled and observed X-ray spectra

Model output (orange lines) and data (blue symbols) for the 2009 observations (top panel) and the two 2011 observations (bottom panel). [Montez et al. 2022]

The constant velocity and temperature imply that the flow expanded freely without collecting enough gas and dust to slow it down. This suggests that the 2006 outburst expanded into a cavity left by the previous eruption, which occurred in 1985, and that repeated eruptions might generate a series of shells and cavities surrounding the binary.

As data trickle in after RS Oph’s most recent flare-up in August 2021, astronomers are already beginning to discern the presence of new outflows. With luck, the next few years will bring many observations of this intriguing system, allowing astronomers to search for new structures that might arise from the latest outburst.

Citation

“Expanding Bipolar X-Ray Structure After the 2006 Eruption of RS Oph,” R. Montez Jr. et al 2022 ApJ 926 100. doi:10.3847/1538-4357/ac4583

ultraviolet image of a solar filament on the sun

On May 30, 2017, a tendril of twisted plasma fought the Sun’s gravitational pull and escaped into the solar system. What can space- and ground-based observations tell us about this event?

A Filament by Any Other Name

extreme-ultraviolet image of the Sun

A solar prominence extends from the Sun’s surface in the lower left quadrant of this image. If this prominence were viewed against the Sun’s disk, the relatively cool plasma would appear dark against the solar surface. [NASA/STEREO]

Solar physics is ideal for people who love terminology. In what other field can you find sunquakes, switchbacks, and supergranules? Even more intriguing are terms that change based on your perspective: filament and prominence refer to the same structure, just seen from different angles. When viewed against the disk of the Sun, a strand of solar plasma suspended by magnetic field lines is called a filament. When that same structure is silhouetted against the blackness of space, it’s called a prominence.

Sometimes, filaments explode into space just hours after they form, often accompanied by other types of solar outbursts like flares or coronal mass ejections. On other occasions, they can linger quietly for days or months before erupting or slowly sinking back toward the Sun’s surface. Today’s article analyzes an extremely long solar filament that loitered for a week before erupting.

three panel plot showing the velocity of the filament compared to background regions

Evolution of the filament’s line-of-sight velocity over time. Several hours before the eruption (left panel), the filament’s velocity was close to zero. As the eruption progressed (center and right panels), the filament’s outward velocity increased. Click to enlarge. [Wang et al. 2022]

From Quiescent to Eruptive

A team led by Shuo Wang (New Mexico State University) studied the evolution of a 500,000-kilometer-long solar filament that erupted from the Sun in May 2017. Wang and collaborators analyzed spectra and images from telescopes on Earth and in space to understand how the velocity of the filament changed in the hours leading up to its eruption. Understanding how the velocity of an erupting filament evolves over time is important for discerning the mechanism that caused it to erupt as well as predicting when an Earth-directed eruption might hit us.

The team found that the filament didn’t launch into space from a complete standstill, but rather had a (relatively) low upward velocity of 6.3 kilometers per second in the hours before the eruption. Rather than being a coherent structure, the filament contained multiple discrete threads of plasma, which showed their own velocity evolution. As the eruption drew nearer, the velocity distribution of these threads became heavily skewed toward higher velocities. Finally, the filament broke free of the Sun with a velocity of 430 km/s — fast enough to reach Earth’s orbit in just four days.

Launching an Investigation

plot of velocity of the filament as a function of time

Velocity of the filament over time, as derived from the multiple data sets analyzed in this work. The solid and dashed green lines show the exponential and linear fits to the data, respectively. Click to enlarge. [Adapted from Wang et al. 2022]

What caused the eruption of this solar filament? The answer may be hinted at by the filament’s velocity, which Wang and collaborators found to increase exponentially over time. Previous numerical simulations suggest that an exponential increase in velocity could be due to an instability such as the kink instability, which kicks in when a curled rope of plasma becomes twisted too tightly.

The question of what caused the filament’s initial, low-velocity motion away from the Sun may be harder to answer since there is a gap in the observations between the filament’s quiescent phase, when its velocity was zero, and when it began to move upward at 6.3 km/s. Other events have also shown slow upward motion hours before the eruption, so further analysis of other eruptive solar filaments may help us understand why some filaments erupt while others sink back to the surface unperturbed.

Bonus

Check out this video from the authors’ article, which shows the evolution of the filament over the course of 19 hours from two different angles.

Citation

“Velocities of an Erupting Filament,” Shuo Wang et al 2022 ApJ 926 18. doi:10.3847/1538-4357/ac3a04

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