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Two diagrams illustrating the radiation and particles generated by a kilonova

The image above shows the radiation and particles generated by a kilonova, the explosion produced when the remnants of two massive stars collide. Kilonovae made headlines in 2017, when scientists observed gravitational waves and a kilonova from colliding neutron stars for the first time. Recently, a research team led by Haille Perkins (University of Illinois Urbana-Champaign) used information gleaned from the 2017 event to estimate the threat posed by a kilonova’s X-rays, gamma rays, and cosmic rays. Fortunately for us, a kilonova would have to be quite close for life on Earth to be endangered by its X-rays or gamma rays — within about 3 light-years for X-rays or 13 light-years for gamma rays. Even after the initial glow of the collision fades, though, there’s still danger: high-energy charged particles called cosmic rays pose a threat years after the dangerous radiation has passed, and a kilonova’s cosmic rays can be lethal out to 36 light-years away. Luckily, kilonovae are extremely rare, and the danger to life on Earth is minimal. As researchers observe more kilonovae and improve their models, we’ll be able to refine our understanding of how close is too close when it comes to kilonovae, the threat they pose to Earth, and the role they play in determining where in the universe life can form and flourish.


“Could a Kilonova Kill: A Threat Assessment,” Haille M. L. Perkins et al 2024 ApJ 961 170. doi:10.3847/1538-4357/ad12b7

three side-by side images of galaxy clusters and lensed quasar images

three side-by side images of galaxy clusters and lensed quasar images

Hubble Space Telescope images of the three lensed quasars studied in this work. The multiple lensed images of the quasars are indicated by the cyan letters. [Napier et al. 2023]

How fast is the universe expanding? Currently, the measured expansion rate of the universe, described by the Hubble constant, depends on how you measure it, creating a clash of constants called the Hubble tension. The apparent disagreement between the rate measured from fluctuations in the cosmic microwave background — the oldest light in the universe — and the rate obtained from measurements of certain types of exploding stars suggests that either new physics is afoot, or these measurements are muddied by systematic errors.

Finding other ways to measure the Hubble constant may help alleviate the tension. In a recent article, Kate Napier (University of Michigan) and collaborators describe their derivation of the Hubble constant from observations of distant quasars: extremely luminous galactic centers in the early universe that are powered by matter falling onto a supermassive black hole. The quasars in the team’s sample are gravitationally lensed by clusters of galaxies, meaning that their light has been bent and magnified by the immense gravity of the clusters. The resulting value of the Hubble constant is consistent with measurements made from both the cosmic microwave background and supernovae, but the technique isn’t yet precise enough to favor one value over the other. To learn more about the use of lensed quasars for measuring the Hubble constant, be sure to check out the full article linked below.


“Hubble Constant Measurement from Three Large-Separation Quasars Strongly Lensed by Galaxy Clusters,” Kate Napier et al 2023 ApJ 959 134. doi:10.3847/1538-4357/ad045a

results of the Swift Deep Galactic Plane Survey

results of the Swift Deep Galactic Plane Survey

The plane of the Milky Way through the eyes of the Swift Observatory. Click to see a high-resolution version. [O’Connor et al. 2023]

Researchers often discover faint X-ray sources only by chance, when they happen to occupy the same field of view as a bright source under investigation. Recently, as described in a research article led by Brendan O’Connor (Carnegie Mellon University), the team behind the Swift Deep Galactic Plane Survey took a more purposeful approach to finding faint X-ray sources in our galaxy, spending a collective 22 days observing the plane of the Milky Way with the Neil Gehrels Swift Observatory to do just that. Bright galactic X-ray sources tend to be flaring X-ray binaries — neutron stars or black holes that siphon material from a companion star, creating a super-hot accretion disk — and faint X-ray sources are likely cataclysmic variable stars, or magnetars and X-ray binaries in their quiescent state. The images above and to the right give a broad view of the results of the survey, which led to the discovery of 348 previously uncatalogued X-ray sources. The team found many objects near the brightness limit of their survey, suggesting that even more sensitive telescopes are needed to fully understand the X-ray sources in our galaxy. To learn more about the Milky Way’s population of faint X-ray sources, be sure to check out the full article below for more information about the first phase of the Swift Deep Galactic Plane Survey.


“The Swift Deep Galactic Plane Survey (DGPS) Phase I Catalog,” B. O’Connor et al 2023 ApJS 269 49. doi:10.3847/1538-4365/ad0228

up-close image of a solar flare

Solar flares, like the one photographed by the Solar Dynamics Observatory and shown above, are flashes of solar radiation powered by magnetic reconnection. Many solar flares are accompanied by immense eruptions of magnetized plasma called coronal mass ejections. In a recent research article, Maria Kazachenko (University of Colorado Boulder and National Solar Observatory) explored why some solar flares, called eruptive flares, come with a coronal mass ejection, while others, called confined flares, do not. Kazachenko analyzed 480 solar flares, from middle-of-the-pack C-class flares to the most energetic X-class flares, cataloging the thermodynamic and magnetic properties of each flare and noting whether it was eruptive or confined. Confined flares tend to arise from active regions — areas of the solar surface with strong magnetic fields — that are larger and more strongly magnetic, and a smaller fraction of the active region’s magnetic field undergoes magnetic reconnection during a confined flare. For the first time, Kazachenko showed that magnetic reconnection happens more rapidly in confined flares than eruptive flares. To learn more about the properties of confined and eruptive flares, be sure to check out the full research article linked below


“A Database of Magnetic and Thermodynamic Properties of Confined and Eruptive Solar Flares,” Maria D. Kazachenko 2023 ApJ 958 104. doi:10.3847/1538-4357/ad004e

four sunspot umbrae within a single penumbra

Sunspots are dark, relatively cool regions of the Sun where magnetic flux pokes through the Sun’s surface. Sunspots have a two-toned appearance consisting of one or more dark regions, or umbrae (from Latin for “shade”), surrounded by a slightly lighter region called a penumbra. Over the past several decades, scientists have observed oscillations in sunspot umbrae with periods of 3 and 5 minutes. The causes of these oscillations aren’t yet known, though researchers generally agree that their source lies deeper in the Sun’s interior. To learn more about umbral oscillations, a team led by Wei Wu (Chinese Academy of Sciences and University of Chinese Academy of Sciences) analyzed images of sunspots, including the sunspot with four umbrae pictured above. Wu’s team found that the oscillations of umbrae within a shared penumbra are loosely correlated, suggesting that the waves travel horizontally from umbra to umbra, and stronger correlations might indicate a shared source. The team also analyzed observations of the chromosphere and corona above the sunspots and found that sunspot oscillations can, in some cases, travel upward through the Sun’s atmosphere. To learn more about the study of umbral oscillations, be sure to check out the full article linked below.


“Propagation Properties of Sunspots Umbral Oscillations in Horizontal and Vertical Directions,” Wei Wu et al 2023 ApJ 958 10. doi:10.3847/1538-4357/acf457

two images of the pulsar wind nebula named the Cosmic Hand

Some 16,000 light-years from Earth, a ghostly hand glows with X-ray light. The distinctive “Cosmic Hand” is a pulsar wind nebula: an X-ray-emitting cloud powered by charged-particle winds from the spinning remnant of a massive star that exploded as a supernova. In X-ray images, the Cosmic Hand has a thumb and three fingers formed by brighter ridges of emission and a delicate wrist illuminated by the pulsar’s powerful jet. Recently, a team led by Roger Romani (Stanford University) used the Imaging X-ray Polarimetry Explorer (IXPE) to make the first observations of polarized X-ray light from the Cosmic Hand. The images above show the degree of polarization detected and the direction of the derived magnetic field (left image, white and yellow bars atop an X-ray image from the Chandra X-ray Observatory) and the 2–8-kiloelectronvolt brightness (right image, greyscale). The new measurements show that the polarization generally follows the structure of the nebula, especially the thumb and the arched region surrounding the jet. To learn more about the structure and magnetic fields of this pulsar wind nebula, be sure to check out the full article linked below.


“The Polarized Cosmic Hand: IXPE Observations of PSR B1509−58/MSH 15−52,” Roger W. Romani et al 2023 ApJ 957 23. doi:10.3847/1538-4357/acfa02

image of an ultra-diffuse galaxy

Ultra-diffuse galaxies are as large as the Milky Way but contain only a smattering of stars and gas, scarcely enough to fill a dwarf galaxy. Observations suggest that these oddly faint galaxies are common, leaving astronomers puzzling over how they form. To disentangle the history of the ultra-diffuse galaxy UGC 9050-Dw1, Catherine Fielder (Steward Observatory) and collaborators tracked down its globular clusters: spherical collections of hundreds of thousands of stars that can hold clues to a galaxy’s star formation and merger history. The team identified 52 globular clusters (cyan circles) in new observations from the Hubble Space Telescope, shown above. This is an exceptionally large number of globular clusters for a galaxy of UGC 9050-Dw1’s brightness; 20% of the galaxy’s light comes from globular clusters! The clusters are all roughly the same color, which suggests that they are composed of stars of the same age and metallicity. This hints that UGC 9050-Dw1’s globular clusters formed in a single burst of star formation, which could have been triggered by a merger. Fielder and collaborators favor the merger of dwarf galaxies as an explanation for UGC 9050-Dw1’s unusual properties, adding yet another possible formation mechanism for ultra-diffuse galaxies to the list. To learn more, be sure to check out the full research article linked below.


“The Disturbed and Globular-Cluster-Rich Ultradiffuse Galaxy UGC 9050-Dw1,” Catherine E. Fielder et al 2023 ApJL 954 L39. doi:10.3847/2041-8213/acf0c3

X-ray image of sources in NGC 4214

An X-ray view of the dwarf starburst galaxy NGC 4214 created from Chandra X-ray Observatory Data. [Adapted from Lin et al. 2023]

Ten million light-years from Earth, a tiny galaxy that glitters with new stars houses a rare kind of binary system. The binary, cataloged as CXOU J121538.2+361921 and referred to simply as X-1, is the brightest X-ray point source in its home galaxy. The system’s bright X-ray light is the result of a star having its atmosphere stolen by a compact companion, like a black hole or a neutron star, creating an extremely hot accretion disk. In a recent research article, a team led by Zikun Lin (Key Laboratory of Optical Astronomy, Chinese Academy of Sciences; University of Chinese Academy of Sciences) used data from the Chandra X-ray Observatory and the Hubble Space Telescope to learn more about this unusual system. The Chandra data, shown to the right, allowed the team to confirm that the binary components eclipse each other every 3.6 hours. The Hubble data, shown above, allowed them to identify the optical counterpart of the X-ray binary for the first time: a blazingly hot blue star that is likely a massive, evolved star with powerful winds and a metal-rich atmosphere. In some tens of millions of years, this star and its companion — the team suspects a black hole, though a neutron star can’t yet be ruled out — will lose their orbital energy to gravitational waves and coalesce in a spectacular cosmic explosion.


“On the Short-period Eclipsing High-mass X-Ray Binary in NGC 4214,” Zikun Lin et al 2023 ApJ 954 46. doi:10.3847/1538-4357/ace770

map of the surface of the minor planet Vesta, with the surface subdivided into dozens of different regions based on origin and composition

hybrid map of Vesta's surface

The final hybrid map, showing Vesta’s polar regions in the top two views and equatorial regions at the bottom. Click for high-resolution version. [Yingst et al. 2023]

In 2011, the Dawn spacecraft began its 14-month survey of the minor planet Vesta, giving us the clearest look yet at Vesta’s surface. Dawn collected data on Vesta’s surface color, roughness, and composition, and mapped its large-scale surface features. While these observations presented an excellent opportunity to study one of the largest objects in the asteroid belt, it also presented a challenge: how do you combine different types of data to maximize the amount of information presented? Using data from the Dawn mission, Aileen Yingst (Planetary Science Institute) and collaborators first made two separate maps that divided Vesta’s surface into dozens of regions based on 1) the color of the surface and 2) large-scale surface features like craters. To combine these maps into one hybrid map, shown above and to the right, the team used a decision tree to determine how to combine the different data sets to characterize each region. The resulting map includes 18 types of terrain, distinguished by both the type of landform (craters, highlands, and more) and the color. For more details about this mapping method and what it revealed about Vesta’s surface, be sure to read the full research article linked below.


“A Geologic Map of Vesta Produced Using a Hybrid Method for Incorporating Spectroscopic and Morphologic Data,” R. Aileen Yingst et al 2023 Planet. Sci. J. 4 157. doi:10.3847/PSJ/acebe9

images of the newly discovered Einstein cross system

As the light from a distant galaxy travels toward us, it sometimes encounters a region of spacetime warped by a massive galaxy in its path. If the alignment between the foreground galaxy and the background galaxy is just right, we’re treated to a spectacular sight: multiple images of the background galaxy arrayed around the foreground galaxy — a phenomenon called gravitational lensing. If the foreground galaxy is elliptical, the images of the background galaxy form a cross known as an Einstein Cross, as is the case with a system newly confirmed by Aleksandar Cikota (Gemini Observatory/NSF’s NOIRLab) and collaborators. To confirm the gravitationally lensed nature of the system, the team demonstrated via spectroscopy that the four images were of the same galaxy. The golden central galaxy is an elliptical behemoth whose light has been traveling to us for nearly 6 billion years, while the lensed starburst galaxy is far more distant, giving us a glimpse of when the universe was just 18.5% of its current age. To learn more about this new addition to the short list of known Einstein Crosses, be sure to check out the full article linked below.


“DESI-253.2534+26.8843: A New Einstein Cross Spectroscopically Confirmed with Very Large Telescope/MUSE and Modeled with GIGA-Lens,” Aleksandar Cikota et al 2023 ApJL 953 L5. doi:10.3847/2041-8213/ace9da

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