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photograph of the solar corona

Editor’s Note: In these last two weeks of 2023, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume in January.

Coronal Heating as Determined by the Solar Flare Frequency Distribution Obtained by Aggregating Case Studies

Published May 2023

Main takeaway:

With the help of numerous undergraduate physics students, a team led by James Paul Mason (University of Colorado at Boulder and Johns Hopkins University) analyzed an enormous sample of solar flares to determine the flare frequency distribution — the number of flares as a function of flare energy — and assess the importance of nanoflares as a way of heating the Sun’s outer atmosphere. This analysis showed that nanoflares cannot be the only source of heat, and plasma waves must play an important role as well.

Why it’s interesting:

The outer layer of the Sun’s atmosphere, called the corona, is extremely sparse and extremely hot. How exactly the solar corona reaches its million-degree temperature is a matter of great interest, as the corona is the site of significant solar activity and the source of the solar wind. The investigation has closed in on two main mechanisms that convert magnetic energy to heat: tiny solar flares called nanoflares and plasma waves called Alfvén waves. Nanoflares are fleeting bursts of high-energy electromagnetic radiation, and Alfvén waves are oscillations in which energy is transferred back and forth between ions and magnetic fields. Neither of these critical mechanisms can be observed directly.

How the flare frequency distribution helps settle the score:

plot of the flare frequency distribution

The resulting flare frequency distribution and measured slope. [Mason et al. 2023]

The flare frequency distribution is a way of tallying the number of solar flares that occur at a given energy. Previous studies have shown that more powerful solar flares are less common than less powerful solar flares. Even though we can’t see nanoflares, we can guess how common they are — and therefore how much energy they can donate to the solar corona — by measuring the slope of the flare frequency distribution. Mason’s team found that nanoflares aren’t frequent enough to heat the corona to millions of degrees, so Alfvén waves must make up the difference.

Citation

James Paul Mason et al 2023 ApJ 948 71. doi:10.3847/1538-4357/accc89

illustration of a brown dwarf and its magnetic field

Editor’s Note: In these last two weeks of 2023, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume in January.

Periodic Radio Emission from the T8 Dwarf WISE J062309.94–045624.6

Published July 2023

Main takeaway:

observations of the radio emission from the source

Observations of WISE J062309.94–045624.6 from the MeerKAT array. Click to enlarge. [Rose et al. 2023]

A research team led by Kovi Rose (The University of Sydney) detected circularly polarized radio emission from a brown dwarf named WISE J062309.94–045624.6. The radio emission rose and fell with the object’s 1.9-hour rotation period, suggesting that the way the emission is produced is similar to the mechanism that produces aurorae on gas giant planets. WISE J062309.94–045624.6 is the coolest sub-stellar object from which radio emission has been detected.

Why it’s interesting:

One of the many questions about brown dwarfs, which sit in the gap between the smallest stars and the largest planets, is the nature of their magnetic fields. The magnetic fields of stars and sub-stellar objects can be probed through their radio emission. Magnetic fields threaded through extremely hot coronal gas help to power radio emission from stars, but brown dwarfs are too cool to produce radio emission this way. Instead, their rapid rotation helps to generate currents that in turn create aurora-like radio emission.

Why more brown dwarf radio emissions are likely to be detected:

Brown dwarfs are faint at radio wavelengths, but their radio emission is expected to be strongly circularly polarized, making them stand out in surveys of circularly polarized radiation. WISE J062309.94–045624.6 was discovered through a search for strongly circularly polarized radio waves, and future searches with the exquisitely sensitive Square Kilometre Array (currently being constructed) should uncover even more — and even cooler — sources.

Citation

Kovi Rose et al 2023 ApJL 951 L43. doi:10.3847/2041-8213/ace188

Illustration of Earth-like planets transiting an M-dwarf star

Editor’s Note: In these last two weeks of 2023, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume in January.

Terminator Habitability: The Case for Limited Water Availability on M-dwarf Planets

Published March 2023

Main takeaway:

Ana Lobo (University of California, Irvine) and collaborators use global climate models to show that the abundance of water on a planet orbiting an M-dwarf star can determine the fraction of the planet’s surface that maintains habitable temperatures. The study focuses on whether temperatures along the terminator, the line that separates day and night, can remain habitable when daytime temperatures on a tidally locked planet become too hot. Perhaps counterintuitively, planets with more water can have less habitable area in this zone.

Why it’s interesting:

M dwarfs are the smallest, coolest, and most common type of star in our universe. Their cool temperatures result in close-in habitable zones, making for easy detection and characterization; rather than waiting for a once-annual transit of an Earth-like planet around a Sun-like star, habitable-zone M-dwarf planets zip around in days or weeks. However, many if not all of these planets are tidally locked, orbiting with one side permanently facing the host star and one side in permanent shadow. While both the day and night sides of these planets are unlikely to have habitable temperatures, the terminator could be livable.

simulation results showing the surface temperature of each simulated planet

Surface temperatures for aquaplanets (simulation names beginning with “Aq”) and land planets (names starting with “L”). The sub-stellar point is at the center of each image. Click to enlarge. [Lobo et al. 2023]

Why water plays an important role:

On a water-poor tidally locked planet, daytime temperatures can be extremely hot, and nighttime temperatures can be below freezing, leaving just the terminator at a comfortable temperature. Even when water-poor planets are close enough to their host stars that daytime temperatures are too hot to sustain life, there isn’t enough transport of the blazing daytime heat around the planet to render the terminator inhospitable. When more water is splashed in, resulting in an “aquaplanet,” the energy transport increases, extending the uninhabitable region to the terminator.

Citation

Ana H. Lobo et al 2023 ApJ 945 161. doi:10.3847/1538-4357/aca970

Editor’s Note: In these last two weeks of 2023, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume in January.

The Dark Energy Camera Plane Survey 2 (DECaPS2): More Sky, Less Bias, and Better Uncertainties

Published January 2023

Main takeaway:

maps of source density

Map of the source density from DECaPS2. Click to enlarge. [Saydjari et al. 2023]

A team led by Andrew Saydjari (Harvard University; Center for Astrophysics | Harvard & Smithsonian) reports on the second data release of the Dark Energy Camera (DECam) Plane Survey, or DECaPS2. Phases one and two of this survey have covered the entirety of the plane of the Milky Way, and the DECaPS2 catalog contains 3.32 billion sources extracted from 260 hours of observations.

Why it’s interesting:

Situated as we are within the plane of the Milky Way, it’s difficult to study objects within the disk of our galaxy — the galactic plane is crowded with stars and clouded with dust and gas. However, most of the objects in our galaxy lie within this plane, so it’s critical to design surveys to investigate this rich environment. In addition to cataloging billions of objects, the team behind DECaPS developed techniques to identify the faint nebulosity that suffuses the galactic disk and complicates our characterization of objects within it.

What’s next for nebulosity:

Knowing the distribution of faint filamentary gas clouds in our galaxy is important for any study of galactic plane objects, since dust makes objects appear redder than they are, and, if unaccounted for, can skew our understanding of these objects’ properties. Looking ahead, the DECaPS team hopes to use their knowledge of nebulosity to infer the three-dimensional structure of gas and dust in the Milky Way.

Citation

Andrew K. Saydjari et al 2023 ApJS 264 28. doi:10.3847/1538-4365/aca594

image of the newly discovered nebula and the Andromeda Galaxy

Editor’s Note: In these last two weeks of 2023, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume in January.

Discovery of Extensive [O III] Emission Near M31

Published January 2023

Main takeaway:

image of the newly discovered nebular feature next to Andromeda

The discovery image showing the nebulous feature to the left of Andromeda. [Adapted from Drechsler et al. 2023]

Amateur astronomers Marcel Drechsler and Xavier Strottner discovered a glowing arc of gas near the Andromeda Galaxy in images taken by Yann Sainty with a 500.7-nanometer filter, which highlights emission from doubly ionized oxygen atoms. The nature of this filamentary feature is unknown.

Why it’s interesting:

The Andromeda Galaxy is one of the nearest galaxies to the Milky Way, and its proximity and gorgeous spiral structure make it a common target for astrophotographers. Despite the attention paid to this galaxy, the new feature has escaped our attention until now. While the wispy nebula is visible at 500.7 nanometers, it doesn’t show up when an H-alpha filter is used. The object’s faint emission, present in only a narrow wavelength range, likely explains why it’s escaped notice until now.

On its nebulous nature:

The nature of this feature, as well as its origins and whether it’s associated with the Milky Way or Andromeda, remains unknown. Possibilities within the Milky Way include a supernova remnant or a planetary nebula — the ghostly, glowing atmosphere shed by a low- to intermediate-mass star that has ceased to fuse hydrogen in its core — but both of these options lack supporting data, such as the presence of radio or ultraviolet emission from a supernova remnant. Another possibility is that the feature formed through the gravitational interaction between Andromeda and the Milky Way or another galaxy that has been torn apart.

Citation

Marcel Drechsler et al 2023 Res. Notes AAS 7 1. doi:10.3847/2515-5172/acaf7e

Four images of black holes in a line

Editor’s Note: In these last two weeks of 2023, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume in January.

The Image of the M87 Black Hole Reconstructed with PRIMO

Published April 2023

Main takeaway:

three black hole images side by side

Comparison of a black hole image released by the Event Horizon Telescope collaboration (left) with the results from PRIMO (middle and right). [Medeiros et al. 2023]

A team led by Lia Medeiros (Institute for Advanced Study and University of Arizona) reanalyzed observations of the supermassive black hole at the center of the galaxy Messier 87. Using a new algorithm, the team created a much sharper image of the black hole, opening the door for more precise determination of its properties.

Why it’s interesting:

In April 2019, the Event Horizon Telescope collaboration released the first images of a black hole, constructed from data taken at observatories across the planet. The images allowed researchers to study the properties of the black hole and test our theories of gravity. While these first black hole images were a huge triumph, the work didn’t stop there: researchers continued to develop new algorithms to be applied to the complex interferometric data from the Event Horizon Telescope. And while initial analyses of the Event Horizon Telescope data were careful to be model agnostic — that is, not assuming anything about the black hole’s shape or properties — the excellent agreement between the images and our theory-based expectations allowed researchers to adjust their analysis methods to produce more precise results.

What’s special about PRIMO:

The Event Horizon Telescope consists of telescopes across multiple continents, but even with these powerful observatories working in concert, the data set is still considered “sparse,” interferometrically speaking. This presents a challenge for modelers that PRIMO, which refers to principal-component interferometric modeling, meets by filling in the gaps with simulations. PRIMO is trained on a database of more than 30,000 simulated images of accreting supermassive black holes, and this specialized training set enables the creation of images with resolution up to the nominal resolution of the Event Horizon Telescope.

Citation

Lia Medeiros et al 2023 ApJL 947 L7. doi:10.3847/2041-8213/acc32d

A photograph of a bunny suit-clad engineer standing besides 6 large hexagonal mirrors, one of which is coated gold.

As we approach the second anniversary of JWST’s triumphant liftoff and migration into deep space, astronomers are starting to get a handle on how to approach the data it beams back. The unprecedented precision and stability of its images and spectra necessitated the invention of new analysis tools and motivated reexaminations of old ones. Recently, a team of astronomers made significant progress in this effort: using methods usually reserved for large ground-based telescopes, they successfully detected carbon monoxide in an exoplanet’s atmosphere.

One Positive, One Inconclusive

About one year ago, a large collaboration of astronomers published the first analyses of several observations taken with JWST’s Near Infrared Spectrograph of a planet named WASP-39 b. This world, a scorchingly hot Jupiter-sized planet that lives just 11 stellar radii from its host star, was an early target for exoplanet astronomers looking to characterize atmospheres subjected to extreme conditions. One of these observations revealed carbon monoxide (CO) floating in the atmosphere. Another, which focused on a smaller wavelength range, did not see the same statistically significant signal.

This was a puzzling mismatch, and an unfortunate one: the abundance of CO is a key ingredient in models that predict how far away a planet was born from its star, and without a detection in both wavelength ranges, any attempt to constrain that distance would be forced to accept large uncertainties. Thus motivated to check the data again, a team led by Emma Esparza-Borges (Institute of Astrophysics of the Canary Islands) decided to take a new angle of attack to extract this faint, but suspected, signal.

Cross-Correlation

The team’s key insight was that they could pretend that JWST was actually a high-resolution ground-based spectrograph. Historically, the techniques used to analyze an exoplanet’s atmosphere depended on the facility at which the data were taken. Ground-based spectrographs can have much higher resolutions than their space-based counterparts, meaning they can more cleanly resolve narrow spectral features. To determine which molecules are present using a ground-based instrument, astronomers usually construct high-resolution “templates” of different species, then compare these templates to the actual data to see if, and where, they line up the best. This technique is known as cross-correlation, and it has been a workhorse technique for extracting atmosphere compositions for many years.

Six wavelength vs flux plots at various resolutions, all of which describe the absorption of CO and its isotopologues.

Steps detailing the construction of CO templates that the team then compared to the data. Click to enlarge. [Esparza-Borges et al. 2023]

However, at lower resolutions, such a technique is overkill and unlikely to succeed. If the instrument isn’t extremely stable, small jitters will overwhelm the signal and a template likely won’t line up well.

Luckily, JWST is extremely stable: even after staring at a target for many hours, it is unlikely to have drifted by more than a fraction of a pixel, and the temperature is unlikely to have changed by more than a fraction of a fraction of a degree. Realizing this, Esparza-Borges and colleagues decided to take cross-correlation for a spin on JWST data for the first time.

Alarm Triggered

A corner plot of well-constrained, mostly Gaussian looking posterior distributions.

A selection of the final fits to the data. [Adapted from Esparza-Borges et al. 2023]

When the dust settled and their computers stopped churning, the team was left with a 6.6–7.5-sigma detection of CO in WASP-39 b’s atmosphere. This not only resolved a year-old mystery, it also validated a new path for future analysis. The next time an astronomer puzzles over a JWST spectrum, perhaps they’ll reach for the cross-correlation approach.

Citation

“Detection of Carbon Monoxide in the Atmosphere of WASP-39b Applying Standard Cross-correlation Techniques to JWST NIRSpec G395H Data,” Emma Esparza-Borges et al 2023 ApJL 955 L19. doi:10.3847/2041-8213/acf27b

image of the Sun with a 3D effect

Editor’s Note: The Monthly Roundup is a new series that takes a broad view of one astronomical topic each month, exploring it through the lens of 3-5 recent research articles. The first post in this series, published in November, presented JWST observations of the TRAPPIST-1 system.

As we collect more and more high-resolution data of our home star, we find new phenomena that require explanation. This month, we’ll take a look at four research articles that examine different aspects of the Sun, from fine structures to strange flares.

Throwing Turbulence into the Mix

Solar flares are eruptions of high-energy radiation, occurring as frequently as several times a day during the most active part of the solar cycle. Flares are thought to be powered by magnetic reconnection, in which the local solar magnetic field springs into a new configuration and releases magnetic energy. While this model has been hugely successful in explaining various features of solar flares, increasingly precise data have enabled researchers to examine flares more closely, and adjustments to this model may be required to explain the variety of solar flare characteristics across large and small spatial scales.

comparisons of simulated output and real data

Comparison of synthesized X-ray Telescope (XRT) images (a) and observations (b and c). Click to enlarge. [Wang et al. 2023]

Yulei Wang (Nanjing University) and collaborators examined magnetic reconnection in three dimensions using magnetohydrodynamics simulations with high spatial resolution. The team explored turbulent reconnection, in which random, chaotic motions of the solar plasma spur constant reconnection. The simulations showed that turbulent reconnection arises naturally in a solar flare’s current sheet — the surface that separates regions of oppositely directed magnetic field — and generates dynamic structures like the vortices and fingers seen in recent flare observations. What’s more, Wang’s teams model produced small-scale structures not yet seen in observations, though the team expects that even higher-resolution observations by facilities like the Daniel K. Inouye Solar Telescope (DKIST) or the Multi-slit Solar Explorer (MUSE) will reveal them.

extreme-ultraviolet image of the Sun

This 19.3-nanometer image, taken by the Solar Dynamics Observatory on December 11, 2023, shows at the center of the Sun’s disk the remnant of a large coronal hole. [NASA/SDO and the AIA, EVE, and HMI science teams]

Structure Within a Coronal Hole

R. J. Morton and R. Cunningham, both of Northumbria University, examined the small-scale structures seen in observations of a coronal hole by the Solar Dynamics Observatory spacecraft. Coronal holes are most apparent when viewing the Sun at extreme-ultraviolet and X-ray wavelengths, which showcase the Sun’s extremely hot and rarefied upper atmosphere, or corona.

Coronal holes aren’t really holes, but instead areas where the Sun’s magnetic field lines extend into space rather than looping back to the surface. The outstretched magnetic field lines carry plasma out into the solar system, leaving a depleted region of less dense, cooler plasma behind. The cooler plasma emits less radiation and appears as a dark “hole” in the Sun’s bright atmosphere.

image of plumes from a polar coronal hole

Example of plumes associated with a polar coronal hole. [Adapted from Morton and Cunningham 2023]

The plasma within coronal holes forms column-shaped plumes, within which rapidly varying small-scale structures called plumelets have been found. These structures are thought to be connected to jets that form during magnetic reconnection. To learn more about these structures, Morton and Cunningham analyzed images of a coronal hole from 2011. They found that detailed structures are widespread in plumes and the region between plumes, and the structures are faint, contributing just 1% of the total radiation from that region. The structures are tiny, spanning just a few arcseconds, and their size pushes the resolution limit of current observations. Future observations, such as those made by the Parker Solar Probe as it swings close to the Sun, may be able to reveal structures on even smaller scales, illuminating the connection between plume structure and magnetic reconnection.

example of the red wings

Example of the red wings on CII emission lines. The region of interest is marked by the horizontal dashed line. [Adapted from Xu et al. 2023]

Red Wings of a Solar Flare

In March 2022, researchers witnessed a solar flare with some unusual properties. Like many solar flares, this event exhibited two flare ribbons — bright, linear features that appear in the chromosphere, the region between the Sun’s surface and its corona — but it also had an unexpected compact bright region near the ribbons. A team led by Yan Xu (New Jersey Institute of Technology) reported on their investigation of this feature, analyzing images and spectra from several spacecraft. Spectra of the compact emission region show that its emission lines are not symmetrical, but rather have a “wing” that extends in the red (longer-wavelength) direction.

Modeling of these lopsided spectral lines suggested that they were produced by gas plunging downward at 160 kilometers per second — far faster than expected for a typical solar flare. But what could cause the plasma to race downward? Xu’s team suggests that this is an extreme example of a chromospheric condensation, in which plasma in the chromosphere flows downward, propelled by energy output higher in the atmosphere. Though velocities as high as those seen here aren’t unheard of, their rarity and the fact that models of chromospheric condensations struggle to produce such high speeds pose a challenge for future works.

And Now, for Our Space Weather Forecast…

Jihyeon Son (Kyung Hee University) and collaborators applied machine-learning techniques to one of the most pressing problems in solar physics: how to forecast space weather. Space weather refers to the ever-changing plasma environment in our solar system, from the gentle solar wind to powerful coronal mass ejections. By using models to forecast the plasma environment near Earth, researchers explore how conditions near the Sun correlate with conditions near Earth, test their theories of how plasma and magnetic fields behave, and, ultimately, provide a way to predict when damaging space weather will reach us.

Son’s team used a deep-learning model to predict the speed of the solar wind at Earth from extreme-ultraviolet images of the Sun and past measurements of the solar wind speed. Deep learning is a subset of machine learning that uses computing techniques that imitate the way the human brain learns. The team’s model used two neural networks: one to address the extreme-ultraviolet image inputs, and one to handle the past values of the solar wind speed. The two networks are then linked to give a prediction of the solar wind speed at Earth for the next three days.

comparison of model output to another prediction and to data

Predictions of the model used in this work (red line) compared to the output from the model used by the Space Weather Prediction Center (green line) and the actual solar wind speed (gray line). Click to enlarge. [Son et al. 2023]

The team found that the model predicts the solar wind speed well, especially during solar minimum when complicating events like coronal mass ejections are uncommon. Not only is the model able to predict the speed overall, it can also forecast the times at which the speed will abruptly change. The authors suggest that their model can be used to forecast the solar wind speed nearly in real time, and future work will address the relationship between solar wind speed and coronal mass ejections and explore other avenues to improve the model.

Citation

“Three-Dimensional Turbulent Reconnection Within the Solar Flare Current Sheet,” Yulei Wang et al 2023 ApJL 954 L36. doi:10.3847/2041-8213/acf19d

“The Fine-Scale Structure of Polar Coronal Holes,” R. J. Morton and R. Cunningham 2023 ApJ 954 90. doi:10.3847/1538-4357/acea7c

“Extreme Red-Wing Enhancements of UV Lines During the 2022 March 30 X1.3 Solar Flare,” Yan Xu et al 2023 ApJ 958 67. doi:10.3847/1538-4357/acf8c6

“Three-Day Forecasting of Solar Wind Speed Using SDO/AIA Extreme-Ultraviolet Images by a Deep-Learning Model,” Jihyeon Son et al 2023 ApJS 267 45. doi:10.3847/1538-4365/ace59a

X-ray and optical image of light echoes surrounding Circinus X-1

New high-cadence observations of Circinus X-1, a binary system containing an extremely young neutron star, have led researchers to propose a unified model for the system’s complicated X-ray emission.

A Young Neutron Star

composite X-ray, optical, and radio image of Circinus X-1

A composite X-ray, optical, and radio image of Circinus X-1 that shows the surrounding supernova remnant. [X-ray: NASA/CXC/Univ. of Wisconsin-Madison/S.Heinz et al; Optical: DSS; Radio: CSIRO/ATNF/ATCA]

Researchers discovered Circinus X-1, an X-ray source in the southern constellation Circinus, during the flight of a sounding rocket in 1969. With the help of 50 more years of X-ray observations, researchers have found that Circinus X-1 is a binary system containing an extremely young neutron star — likely just 4,600 years old, judging by the surrounding supernova remnant created by the explosion that birthed the neutron star.

As the neutron star steals material from its binary companion, it results in long-term, orders-of-magnitude changes in the system’s X-ray flux, fleeting bursts, and changes over the course of each orbit. To understand the complex changes that take place in this system over each orbit, a research team led by Mayu Tominaga (Japan Aerospace Exploration Agency and The University of Tokyo) turned to high-cadence X-ray monitoring.

X-ray light curves of Circinus X-1

The new high-cadence NICER observations (top panel) show the same phases as the previous lower-cadence data from the Monitor of All-sky X-ray Image (MAXI; bottom panel). Click to enlarge. [Tominaga et al. 2023]

High-Cadence Monitoring

Using the Neutron Star Interior Composition Explorer (NICER), a sensitive X-ray telescope on the International Space Station, Tominaga’s team observed Circinus X-1 roughly every four hours for an entire orbital period, 16.6 days. The object’s light curve over this time shows three distinct phases: stable, dipping, and flaring.

To link these phases to the properties of the binary system, the team modeled spectra from each phase. At first glance, the three phases have completely different spectra, but the team was able to unite the seemingly disparate spectra under a single model in which an accretion disk, emitting light across the electromagnetic spectrum, is periodically blocked by a cloud of neutral gas. Surrounding this whole system is ionized gas.

A Unified Model

diagram of the proposed unifying model

Diagram of the model proposed by the authors. Here, “diskbb” is the blackbody emission from the accretion disk. Click to enlarge. [Tominaga et al. 2023]

In this model, the stable phase arises when light from the accretion disk isn’t blocked by the neutral clouds, and simply trickles through the surrounding gas that is ionized by X-rays from the blazing surface of the neutron star and the superheated gas of the accretion disk. Sustained dips occur when the neutral cloud blocks some of the emission from the disk. Tominaga’s team suggests that this neutral cloud forms where the stream of gas accreted from the companion star meets the disk. Over an orbital period, this cloud swings in and out of our line of sight. As the neutral cloud moves out of the line of sight, the transmission of light through clumpy material trailing the cloud produces the rapid changes seen during the flaring period.

This picture of how the system’s emission is moderated over the course of its orbit explains nearly all of the NICER spectra of Circinus X-1. Tominaga and collaborators hope that this simple model can be used to explain short- and long-term behaviors of the system in other parts of the electromagnetic spectrum as well.

Citation

“X-ray Spectral Variations of Circinus X-1 Observed with NICER Throughout an Entire Orbital Cycle,” Mayu Tominaga et al 2023 ApJ 958 52. doi:10.3847/1538-4357/ad0034

illustration of black hole sizes

A chance observation during a survey of active galactic nuclei opened a new window onto an ultra-luminous X-ray source that may be an intermediate-mass black hole.

Looking for the Missing Black Hole Pieces

illustration of two black holes that merged to form a larger black hole

The Laser Interferometer Gravitational-Wave Observatory (LIGO) announced in 2020 the detection of a merger that resulted in a 142-solar-mass black hole, the largest merger product detected by LIGO to date. [LIGO/Caltech/MIT/R. Hurt (IPAC)]

Astronomers often search for universal superlatives: the smallest, the largest, the oldest, the brightest. But when it comes to black holes, it’s the search for the objects in the middle of parameter space that’s most compelling. Intermediate-mass black holes, which have masses in the 100–105-solar mass range, have proved more elusive than their stellar-mass or supermassive counterparts; it’s not uncommon to detect gravitational waves from the birth of a black hole in this mass range, but electromagnetic detections are less certain.

Intermediate-mass black holes likely announce themselves the same way that stellar-mass and supermassive black holes do: by shining brightly across the electromagnetic spectrum when they consume material from their surroundings. Bright X-ray emission may have exposed the location of an intermediate-mass black hole in the outskirts of the galaxy NGC 5252. The source, named CXO J133815.6+043255, or CXO J1338+04 for short, is thought to be surrounded by swirling metal-poor gas. This hints that CXO J1338+04 could have been the central black hole of a metal-poor dwarf galaxy that merged with NGC 5252.

position of the intermediate-mass black hole candidate relative to the center of its host galaxy

Radio image showing CXO J1338+04’s location relative to the center of NGC 5252. [Adapted from Smith et al. 2023]

A Long-Wavelength Look at CXO J1338+04

Recently, Krista Lynne Smith (Texas A&M University and Southern Methodist University) and collaborators used the Very Large Array to observe the nucleus of NGC 5252 as part of their study of nearby active galactic nuclei: luminous galactic centers powered by accreting supermassive black holes. By chance, their field of view encompassed CXO J1338+04, which shines brightly at the 22 gigahertz frequency of their observations.

Combining their new data with archival radio observations at lower frequencies, Smith’s team measured the slope of CXO J1338+04’s spectral energy distribution and found it to be consistent with expectations for an outflowing jet that could arise from an accreting black hole. The team noted that the observed spectral slope could also match that of a star-forming region, but they find that unlikely, especially since previous studies found active galactic nucleus–like optical emission lines that are hard to explain with star formation.

Further Support

plot of radio to X-ray luminosity ratio

Ratio of radio and X-ray luminosities at two frequencies (red stars) for CXO J1338+04 compared to radio-loud quasars, radio-quiet quasars, and other active galactic nuclei and ultra-luminous X-ray sources. Click to enlarge. [Smith et al. 2023]

Smith’s team investigated the ratio of the target’s radio luminosity to its X-ray luminosity, which can distinguish between different types of objects, such as radio-loud and radio-quiet active galactic nuclei. (As the names suggest, active galactic nuclei are often grouped based on the strength of their radio emission.) CXO J1338+04’s radio-to-X-ray luminosity ratio suggests that it’s a lower-mass version of a radio-loud active galactic nucleus. Radio-loud active galactic nuclei produce radio emission via their powerful outflowing jets, just as is suspected for CXO J1338+04.

Taken together, CXO J1338+04’s spectral slope and radio emission match what’s expected for an accreting intermediate-mass black hole, supporting its inclusion on the small but steadily growing list of promising intermediate-mass black hole candidates.

Citation

“The Nature of the IMBH Candidate CXO J133815.6+043255: High-Frequency Radio Emission,” Krista Lynne Smith et al 2023 ApJ 956 3. doi:10.3847/1538-4357/acf4f8

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