ST/ Glimpse of inner depths of an active galaxy

Paradigm

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Paradigm

36 min readNov 9, 2022

Space biweekly vol.64, 28th October — 9th November

TL;DR

Scientists have found evidence of high-energy neutrino emission from NGC 1068, also known as Messier 77, an active galaxy in the constellation Cetus and one of the most familiar and well-studied galaxies to date.
Astronomers have discovered the closest-known black hole to Earth. This is the first unambiguous detection of a dormant stellar-mass black hole in the Milky Way. Its close proximity to Earth, a mere 1600 light-years away, offers an intriguing target of study to advance our understanding of the evolution of binary systems.
Twilight observations have enabled astronomers to spot three near-Earth asteroids (NEA) hiding in the glare of the Sun. These NEAs are part of an elusive population that lurks inside the orbits of Earth and Venus. One of the asteroids is the largest object that is potentially hazardous to Earth to be discovered in the last eight years.
A signature in the X-ray light emitted by a highly magnetized dead star known as a magnetar suggests the star has a solid surface with no atmosphere.
A new study finds the original crust on Mars is more complex, and evolved, than previously thought. Researchers have determined the Martian crust has greater concentrations of the chemical element silicon, which may mean Mars’ original surface may have been similar to Earth’s first crust.
The first observations of a mass-accreting black hole from the Imaging X-Ray Polarimetry Explorer (IXPE) mission reveal new details about the configuration of extremely hot matter in the region immediately surrounding it. Researchers are using measurements of the polarization of X-rays to test and refine models that describe how black holes swallow matter, becoming some of the most luminous sources of light — including X-rays — in the universe.
Seismologists have developed a new method to scan the deep interior of planets in our solar system to confirm whether they have a core at the heart of their existence.
Bizarre quantum properties of black holes — including their mind-bending ability to have different masses simultaneously — have been confirmed by physicists.
An Earth-like planet orbiting an M dwarf — the most common type of star in the universe — appears to have no atmosphere at all. This discovery could cause a major shift in the search for life on other planets.
NASA’s Double Asteroid Redirection Test (DART) spacecraft crashed into Dimorphos, a moonlet of the near-Earth asteroid Didymos, at 14,000 miles per hour. Prior to the impact, engineers and scientists performed an experiment to study the cratering process that produces the mass of ejected materials and measures the subsequent momentum enhancement of the impact.
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The global smart space market size is projected to grow from USD 9.4 billion in 2020 to USD 15.3 billion by 2025, at a Compound Annual Growth Rate (CAGR) of 10.2% during the forecast period. The increasing venture capital funding and growing investments in smart space technology to drive market growth.

Analysts at Morgan Stanley and Goldman Sachs have predicted that economic activity in space will become a multi-trillion-dollar market in the coming decades. Morgan Stanley’s Space Team estimates that the roughly USD 350 billion global space industry could surge to over USD 1 trillion by 2040.

Source: Satellite Industry Association, Morgan Stanley Research, Thomson Reuters. *2040 estimates

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Evidence for neutrino emission from the nearby active galaxy NGC 1068

by R. Abbasi, M. Ackermann, J. Adams, J. A. Aguilar, M. Ahlers, M. Ahrens, et al in Science

For the first time, an international team of scientists have found evidence of high-energy neutrino emission from NGC 1068, also known as Messier 77, an active galaxy in the constellation Cetus and one of the most familiar and well-studied galaxies to date. First spotted in 1780, this galaxy, located 47 million light-years away from us, can be observed with large binoculars.

The detection was made at the National Science Foundation-supported IceCube Neutrino Observatory, a massive neutrino telescope encompassing 1 billion tons of instrumented ice at depths of 1.5 to 2.5 kilometers below Antarctica’s surface near the South Pole. This unique telescope, which explores the farthest reaches of our universe using neutrinos, reported the first observation of a high-energy astrophysical neutrino source in 2018. The source, TXS 0506+056, is a known blazar located off the left shoulder of the Orion constellation and 4 billion light-years away.

At a distance of 47 light-years, the spiral galaxy NGC 1068 is a relatively close neighbor to our Milky Way. NASA / ESA / A. van der Hoeven.

“One neutrino can single out a source. But only an observation with multiple neutrinos will reveal the obscured core of the most energetic cosmic objects,” says Francis Halzen, a professor of physics at the University of Wisconsin-Madison and principal investigator of IceCube. He adds, “IceCube has accumulated some 80 neutrinos of teraelectronvolt energy from NGC 1068, which are not yet enough to answer all our questions, but they definitely are the next big step towards the realization of neutrino astronomy.”

Unlike light, neutrinos can escape in large numbers from extremely dense environments in the universe and reach Earth largely undisturbed by matter and the electromagnetic fields that permeate extragalactic space. Although scientists envisioned neutrino astronomy more than 60 years ago, the weak interaction of neutrinos with matter and radiation makes their detection extremely difficult. Neutrinos could be key to our queries about the workings of the most extreme objects in the cosmos.

High-resolution scan around the most significant location.

“Answering these far-reaching questions about the universe that we live in is a primary focus of the U.S. National Science Foundation,” says Denise Caldwell, director of NSF’s Physics Division.

As is the case with our home galaxy, the Milky Way, NGC 1068 is a barred spiral galaxy, with loosely wound arms and a relatively small central bulge. However, unlike the Milky Way, NGC 1068 is an active galaxy where most radiation is not produced by stars but due to material falling into a black hole millions of times more massive than our Sun and even more massive than the inactive black hole in the center of our galaxy. NGC 1068 is an active galaxy — a Seyfert II type in particular — seen from Earth at an angle that obscures its central region where the black hole is located. In a Seyfert II galaxy, a torus of nuclear dust obscures most of the high-energy radiation produced by the dense mass of gas and particles that slowly spiral inward toward the center of the galaxy.

“Recent models of the black hole environments in these objects suggest that gas, dust, and radiation should block the gamma rays that would otherwise accompany the neutrinos,” says Hans Niederhausen, a postdoctoral associate at Michigan State University and one of the main analyzers of the paper. “This neutrino detection from the core of NGC 1068 will improve our understanding of the environments around supermassive black holes.”

Multimessenger spectral energy distribution of NGC 1068.

NGC 1068 could become a standard candle for future neutrino telescopes, according to Theo Glauch, a postdoctoral associate at the Technical University of Munich (TUM), in Germany, and another main analyzer.

“It is already a very well-studied object for astronomers, and neutrinos will allow us to see this galaxy in a totally different way. A new view will certainly bring new insights,” says Glauch.

These findings represent a significant improvement on a prior study on NGC 1068 published in 2020, according to Ignacio Taboada, a physics professor at the Georgia Institute of Technology and the spokesperson of the IceCube Collaboration.

“Part of this improvement came from enhanced techniques and part from a careful update of the detector calibration,” says Taboada. “Work by the detector operations and calibrations teams enabled better neutrino directional reconstructions to precisely pinpoint NGC 1068 and enable this observation. Resolving this source was made possible through enhanced techniques and refined calibrations, an outcome of the IceCube Collaboration’s hard work.”

The IceCube Lab sits atop a 1-billion-ton network of sensing equipment and ice at the South Pole. Using the powerful neutrino telescope, researchers have identified a new source of astrophysical neutrinos coming from the galaxy NGC 1068. Martin Wolf, IceCube/NSF.

The improved analysis points the way toward superior neutrino observatories that are already in the works.

“It is great news for the future of our field,” says Marek Kowalski, an IceCube collaborator and senior scientist at Deutsches Elektronen-Synchrotron, in Germany. “It means that with a new generation of more sensitive detectors there will be much to discover. The future IceCube-Gen2 observatory could not only detect many more of these extreme particle accelerators but would also allow their study at even higher energies. It’s as if IceCube handed us a map to a treasure trove.”

With the neutrino measurements of TXS 0506+056 and NGC 1068, IceCube is one step closer to answering the century-old question of the origin of cosmic rays. Additionally, these results imply that there may be many more similar objects in the universe yet to be identified.

“The unveiling of the obscured universe has just started, and neutrinos are set to lead a new era of discovery in astronomy,” says Elisa Resconi, a professor of physics at TUM and another main analyzer.
“Several years ago, NSF initiated an ambitious project to expand our understanding of the universe by combining established capabilities in optical and radio astronomy with new abilities to detect and measure phenomena like neutrinos and gravitational waves,” says Caldwell. “The IceCube Neutrino Observatory’s identification of a neighboring galaxy as a cosmic source of neutrinos is just the beginning of this new and exciting field that promises insights into the undiscovered power of massive black holes and other fundamental properties of the universe.”

Polarized x-rays from a magnetar

by Roberto Taverna, Roberto Turolla, Fabio Muleri, Jeremy Heyl, et ai in Science

The study led by researchers at the University of Padova, uses data from a NASA satellite, the Imaging X-ray Polarimetry Explorer (IXPE), which was launched last December. The satellite, a collaboration between NASA and the Italian Space Agency, provides a new way of looking at X-ray light in space by measuring its polarisation — the direction of the light waves’ wiggle.

The team looked at IXPE’s observation of magnetar 4U 0142+61, located in the Cassiopeia constellation, about 13,000 light years away from Earth. This was the first time polarised X-ray light from a magnetar had been observed.

Magnetars are neutron stars — very dense remnant cores of massive stars that have exploded as supernovae at the ends of their lives. Unlike other neutron stars, they have an immense magnetic field — the most powerful in the universe. They emit bright X-rays and show erratic periods of activity, with the emission of bursts and flares which can release in just one second an amount of energy millions of times greater than our Sun emits in one year. They are believed to be powered by their ultra-powerful magnetic fields, 100 to 1,000 times stronger than standard neutron stars.

Normalized, background-subtracted Stokes parameters Q/I and U/I for x-ray emission from 4U 0142+61.

The research team found a much lower proportion of polarised light than would be expected if the X-rays passed through an atmosphere. (Polarised light is light where the wiggle is all in the same direction — that is, the electric fields vibrate only in one way. An atmosphere acts as a filter, selecting only one polarisation state of the light.)

The team also found that, for particles of light at higher energies, the angle of polarisation — the wiggle — flipped by exactly 90 degrees compared to light at lower energies, following what theoretical models would predict if the star had a solid crust surrounded by an external magnetosphere filled with electric currents.

Co-lead author Professor Silvia Zane (UCL Mullard Space Science Laboratory), a member of the IXPE science team, said: “This was completely unexpected. I was convinced there would be an atmosphere. The star’s gas has reached a tipping point and become solid in a similar way that water might turn to ice. This is a result of the star’s incredibly strong magnetic field.
“But, like with water, temperature is also a factor — a hotter gas will require a stronger magnetic field to become solid. “A next step is to observe hotter neutron stars with a similar magnetic field, to investigate how the interplay between temperature and magnetic field affects the properties of the star’s surface.”

Phase-dependent x-ray flux and polarization properties.

Lead author Dr Roberto Taverna, from the University of Padova, said: “The most exciting feature we could observe is the change in polarisation direction with energy, with the polarisation angle swinging by exactly 90 degrees. “This is in agreement with what theoretical models predict and confirms that magnetars are indeed endowed with ultra-strong magnetic fields.”

Quantum theory predicts that light propagating in a strongly magnetised environment is polarised in two directions, parallel and perpendicular to the magnetic field. The amount and direction of the observed polarisation bear the imprint of the magnetic field structure and of the physical state of matter in the vicinity of the neutron star, providing information inaccessible otherwise. At high energies, photons (particles of light) polarised perpendicularly to the magnetic field are expected to dominate, resulting in the observed 90-degree polarisation swing.

Professor Roberto Turolla, from the University of Padova, who is also an honorary professor at the UCL Mullard Space Science Laboratory, said: “The polarisation at low energies is telling us that the magnetic field is likely so strong to turn the atmosphere around the star into a solid or a liquid, a phenomenon known as magnetic condensation.”

The solid crust of the star is thought to be composed of a lattice of ions, held together by the magnetic field. The atoms would not be spherical, but elongated in the direction of the magnetic field. It is still a subject of debate whether or not magnetars and other neutron stars have atmospheres. However, the new paper is the first observation of a neutron star where a solid crust is a reliable explanation.

Professor Jeremy Heyl of the University of British Columbia (UBC) added: “It is also worth noting that including quantum electrodynamics effects, as we did in our theoretical modelling, gives results compatible with the IXPE observation. Nevertheless, we are also investigating alternative models to explain the IXPE data, for which proper numerical simulations are still lacking.”

An Evolved Early Crust Exposed on Mars Revealed Through Spectroscopy

by V. Payré, M. R. Salvatore, C. S. Edwards in Geophysical Research Letters

Early crust on Mars may be more complex than previously thought — and it may even be similar to our own planet’s original crust.

The Martian surface is uniformly basaltic, a product of billions of years of volcanism and flowing lava on the surface that eventually cooled. Because Mars did not undergo full-scale surface remodeling like the shifting of continents on Earth, scientists had thought Mars’ crustal history was a relatively simple tale. But in a new study, researchers found locations in the Red Planet’s southern hemisphere with greater concentrations of silicon, a chemical element, than what would be expected in a purely basaltic setting. The silica concentration had been exposed by space rocks that slammed into Mars, excavating material that was embedded miles below the surface, and revealing a hidden past.

“There is more silica in the composition that makes the rocks not basalt, but what we call more evolved in composition,” says Valerie Payré, assistant professor in the Department of Earth and Environmental Sciences at the University of Iowa and the study’s corresponding author. “That tells us how the crust formed on Mars is definitely more complex than what we knew. So, it’s more about understanding that process, and especially what it means for how Earth’s crust first formed.”

Images of Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) feldspar-rich outcrops.

Scientists believe Mars formed about 4.5 billion years ago. Exactly how the Red Planet came into being is a mystery, but there are theories. One idea is that Mars formed via a titanic collision of rocks in space that, with its intense heat, spawned an entirely liquefied state, also known as a magma ocean. The magma ocean gradually cooled, the theory goes, yielding a crust, like a layer of skin, that would be singularly basaltic. Another theory is that the magma ocean was not all-encompassing, and that parts of the first crust on Mars had a different origin, one that would show silica concentrations different from basaltic.

Payré and her research partners analyzed data gathered by the Mars Reconnaissance Orbiter for the planet’s southern hemisphere, which previous research had indicated was the oldest region. The researchers found nine locations — such as craters and fractures in the terrain — that were rich in feldspar, a mineral associated with lava flows that are more silicic than basaltic.

“This was the first clue,” Payré says. “It is because the terrains are feldspar-rich that we explored the silica concentrations there.”

Feldspar had been found previously in other regions on Mars, but further analysis showed the chemical composition in those areas was more basaltic. That did not deter the researchers, who turned to another instrument, called THEMIS, which can detect silica concentrations through infrared wavelength reflections from the Martian surface. With data from THEMIS, the team determined the terrain at their chosen locations was more silicic than basaltic. Adding further credence to their observations, meteorites such as Erg Chech 002, discovered in the Sahara and dating roughly to the birth of the solar system, show similar silicic and other mineral compositions that the team observed in the nine locations on Mars.

Silica versus alkali diagram. Intermediate and felsic rocks analyzed by the Curiosity rover and igneous clasts from Northwest Africa 7533 and its paired meteorites are shown with green patches and dark green squares, respectively.

The researchers also dated the crust to about 4.2 billion years, which would make it the oldest crust found on Mars to date. Payré says she was mildly surprised at the discovery.

“There have been rovers on the surface that have observed rocks that were more silicic than basaltic,” she says. “So, there were ideas that the crust could be more silicic. But we never knew, and we still don’t know, how the early crust was formed, or how old it is, so it’s kind of a mystery still.”

While Mars’ crustal origin remains shrouded, Earth’s crustal history is even less clear, as any vestiges of our planet’s original crust have been long erased due to the shifting of continental plates for billions of years. Still, the finding may offer insights into Earth’s origins.

“We don’t know our planet’s crust from the beginning; we don’t even know when life first appeared,” Payré says. “Many think the two could be related. So, understanding what the crust was like a long time ago could help us understand the whole evolution of our planet.”

Polarized x-rays constrain the disk-jet geometry in the black hole x-ray binary Cygnus X-1

by Henric Krawczynski, Fabio Muleri, Michal Dovčiak, Alexandra Veledina, et al in Science

Researchers’ recent observations of a stellar-mass black hole called Cygnus X-1 reveal new details about the configuration of extremely hot matter in the region immediately surrounding the black hole.

Matter is heated to millions of degrees as it is pulled toward a black hole. This hot matter glows in X-rays. Researchers are using measurements of the polarization of these X-rays to test and refine models that describe how black holes swallow matter, becoming some of the most luminous sources of light — including X-rays — in the universe.

The new measurements from Cygnus X-1 represent the first observations of a mass-accreting black hole from the Imaging X-Ray Polarimetry Explorer (IXPE) mission, an international collaboration between NASA and the Italian Space Agency (ASI). Cygnus X-1 is one of the brightest X-ray sources in our galaxy, consisting of a 21 solar mass black hole in orbit with a 41 solar mass companion star.

“Previous X-ray observations of black holes only measured the arrival direction, arrival time and energy of the X-rays from hot plasma spiraling toward the black holes,” said lead author Henric Krawczynski, the Wayman Crow Professor of Physics in Arts & Sciences at Washington University in St. Louis and a faculty fellow in the university’s McDonnell Center for the Space Sciences. “IXPE also measures their linear polarization, which carries information about how the X-rays were emitted — and if, and where, they scatter off material close to the black hole.”

No light, not even light from X-rays, can escape from inside the event horizon of a black hole. The X-rays detected with IXPE are emitted by the hot matter, or plasma, in a 2,000-km diameter region surrounding the 60-km diameter event horizon of the black hole. Combining the IXPE data with concurrent observations from NASA’s NICER and NuSTAR X-ray observatories in May and June 2022 allowed the authors to constrain the geometry — i.e., shape and location — of the plasma.

Energy-dependent x-ray polarization of Cyg X-1.

The researchers found that the plasma extends perpendicular to a two-sided, pencil-shaped plasma outflow, or jet, imaged in earlier radio observations. The alignment of the direction of the X-ray polarization and the jet lends strong support to the hypothesis that the processes in the X-ray bright region close to the black hole play a crucial role in launching the jet.

The observations match models predicting that the corona of hot plasma either sandwiches the disk of matter spiraling toward the black hole or replaces the inner portion of that disk. The new polarization data rule out models in which the black hole’s corona is a narrow plasma column or cone along the jet axis. The scientists noted that a better understanding of the geometry of the plasma around a black hole can reveal much about the inner workings of black holes and how they accrete mass.

“These new insights will enable improved X-ray studies of how gravity curves space and time close to black holes,” Krawczynski said.

Related to the Cygnus X-1 black hole specifically, “IXPE observations reveal that the accretion flow is seen more edge-on than previously thought,” explained co-author Michal Dovčiak at the Astronomical Institute of the Czech Academy of Sciences.

“This may be a signature of a misalignment of the equatorial plane of the black hole and the orbital plane of the binary,” or the paired duo of the black hole and its companion star, clarified co-author Alexandra Veledina from the University of Turku. “The system may have acquired that misalignment when the black hole progenitor star exploded.”
“The IXPE mission uses X-ray mirrors fabricated at NASA’s Marshall Space Flight Center and focal plane instrumentation provided by a collaboration of ASI, the National Institute for Astrophysics (INAF) and the National Institute for Nuclear Physics,” said co-author Fabio Muleri of INAF-IAPS. “Beyond Cygnus X-1, IXPE is being used to study a wide range of extreme X-ray sources, including mass accreting neutron stars, pulsars and pulsar wind nebulae, supernova remnants, our galactic center and active galactic nuclei. We’ve found a lot of surprises, and we’re having a lot of fun.”

Comparison of the observed 2–8 keV polarization degree and angle with model predictions.

A second paper in the same issue of Science was led by Roberto Taverna at the University of Padova and describes the IXPE detection of highly polarized X-rays from the magnetar 4U 0142+61.

“We are thrilled to be part of this new wave of scientific discovery in astrophysics,” Krawczynski said.

A Sun-like star orbiting a black hole

by Kareem El-Badry, Hans-Walter Rix, Eliot Quataert, et al in Monthly Notices of the Royal Astronomical Society

Astronomers using the International Gemini Observatory, operated by NSF’s NOIRLab, have discovered the closest-known black hole to Earth. This is the first unambiguous detection of a dormant stellar-mass black hole in the Milky Way. Its close proximity to Earth, a mere 1600 light-years away, offers an intriguing target of study to advance our understanding of the evolution of binary systems.

Black holes are the most extreme objects in the Universe. Supermassive versions of these unimaginably dense objects likely reside at the centers of all large galaxies. Stellar-mass black holes — which weigh approximately five to 100 times the mass of the Sun — are much more common, with an estimated 100 million in the Milky Way alone. Only a handful have been confirmed to date, however, and nearly all of these are ‘active’ — meaning they shine brightly in X-rays as they consume material from a nearby stellar companion, unlike dormant black holes which do not.

Astronomers using the Gemini North telescope on Hawai’i, one of the twin telescopes of the InternationalGemini Observatory, operated by NSF’s NOIRLab, have discovered the closest black hole to Earth, which the researchers have dubbed Gaia BH1. This dormant black hole is about 10 times more massive than the Sun and is located about 1600 light-years away in the constellation Ophiuchus, making it three times closer to Earth than the previous record holder, an X-ray binary in the constellation of Monoceros. The new discovery was made possible by making exquisite observations of the motion of the black hole’s companion, a Sun-like star that orbits the black hole at about the same distance as the Earth orbits the Sun.

Artist’s impression of the closest black hole to Earth and its Sun-like companion star.

“Take the Solar System, put a black hole where the Sun is, and the Sun where the Earth is, and you get this system,” explained Kareem El-Badry, an astrophysicist at the Center for Astrophysics | Harvard & Smithsonianand the Max Planck Institute for Astronomy, and the lead author of the paper describing this discovery. “While there have been many claimed detections of systems like this, almost all these discoveries have subsequently been refuted. This is the first unambiguous detection of a Sun-like star in a wide orbit around a stellar-mass black hole in our Galaxy.”

Though there are likely millions of stellar-mass black holes roaming the Milky Way Galaxy, those few that have been detected were uncovered by their energetic interactions with a companion star. As material from a nearby star spirals in toward the black hole, it becomes superheated and generates powerful X-rays and jets of material. If a black hole is not actively feeding (i.e., it is dormant) it simply blends in with its surroundings.

“I’ve been searching for dormant black holes for the last four years using a wide range of datasets and methods,” said El-Badry. “My previous attempts — as well as those of others — turned up a menagerie of binary systems that masquerade as black holes, but this is the first time the search has borne fruit.”

The team originally identified the system as potentially hosting a black hole by analyzing data from the European Space Agency’s Gaia spacecraft. Gaia captured the minute irregularities in the star’s motion caused by the gravity of an unseen massive object. To explore the system in more detail, El-Badry and his team turned to the Gemini Multi-Object Spectrograph instrument on Gemini North, which measured the velocity of the companion star as it orbited the black hole and provided precise measurement of its orbital period. The Gemini follow-up observations were crucial to constraining the orbital motion and hence masses of the two components in the binary system, allowing the team to identify the central body as a black hole roughly 10 times as massive as our Sun.

“Our Gemini follow-up observations confirmed beyond reasonable doubt that the binary contains a normal star and at least one dormant black hole,” elaborated El-Badry. “We could find no plausible astrophysical scenario that can explain the observed orbit of the system that doesn’t involve at least one black hole.”

The team relied not only on Gemini North’s superb observational capabilities but also on Gemini’s ability to provide data on a tight deadline, as the team had only a short window in which to perform their follow-up observations.

“When we had the first indications that the system contained a black hole, we only had one week before the two objects were at the closest separation in their orbits. Measurements at this point are essential to make accurate mass estimates in a binary system,” said El-Badry. “Gemini’s ability to provide observations on a short timescale was critical to the project’s success. If we’d missed that narrow window, we would have had to wait another year.”

Astronomers’ current models of the evolution of binary systems are hard-pressed to explain how the peculiar configuration of Gaia BH1 system could have arisen. Specifically, the progenitor star that later turned into the newly detected black hole would have been at least 20 times as massive as our Sun. This means it would have lived only a few million years. If both stars formed at the same time, this massive star would have quickly turned into a supergiant, puffing up and engulfing the other star before it had time to become a proper, hydrogen-burning, main-sequence star like our Sun.

It is not at all clear how the solar-mass star could have survived that episode, ending up as an apparently normal star, as the observations of the black hole binary indicate. Theoretical models that do allow for survival all predict that the solar-mass star should have ended up on a much tighter orbit than what is actually observed.

This could indicate that there are important gaps in our understanding of how black holes form and evolve in binary systems, and also suggests the existence of an as-yet-unexplored population of dormant black holes in binaries.

A Deep and Wide Twilight Survey for Asteroids Interior to Earth and Venus

by Scott S. Sheppard, David J. Tholen, Petr Pokorný, Marco Micheli, et al in The Astronomical Journal

Twilight observations with the US Department of Energy-fabricated Dark Energy Camera at Cerro Tololo Inter-American Observatory in Chile, a Program of NSF’s NOIRLab, have enabled astronomers to spot three near-Earth asteroids (NEA) hiding in the glare of the Sun. These NEAs are part of an elusive population that lurks inside the orbits of Earth and Venus. One of the asteroids is the largest object that is potentially hazardous to Earth to be discovered in the last eight years.

An international team using the Dark Energy Camera (DECam) mounted on the Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory in Chile, a Program of NSF’s NOIRLab, has discovered three new near-Earth asteroids (NEAs) hiding in the inner Solar System, the region interior to the orbits of Earth and Venus. This is a notoriously challenging region for observations because asteroid hunters have to contend with the glare of the Sun.

By taking advantage of the brief yet favorable observing conditions during twilight, however, the astronomers found an elusive trio of NEAs. One is a 1.5-kilometer-wide asteroid called 2022 AP7, which has an orbit that may someday place it in Earth’s path. The other asteroids, called 2021 LJ4 and 2021 PH27, have orbits that safely remain completely interior to Earth’s orbit. Also of special interest to astronomers and astrophysicists, 2021 PH27 is the closest known asteroid to the Sun. As such, it has the largest general-relativity effects of any object in our Solar System and during its orbit its surface gets hot enough to melt lead.

Known NEOs plotted with semimajor axis vs. eccentricity. The new discoveries from this survey are shown by big blue circles.

“Our twilight survey is scouring the area within the orbits of Earth and Venus for asteroids,” said Scott S. Sheppard, an astronomer at the Earth and Planets Laboratory of the Carnegie Institution for Science and the lead author of the paper describing this work. “So far we have found two large near-Earth asteroids that are about 1 kilometer across, a size that we call planet killers.”
“There are likely only a few NEAs with similar sizes left to find, and these large undiscovered asteroids likely have orbits that keep them interior to the orbits of Earth and Venus most of the time,” said Sheppard. “Only about 25 asteroids with orbits completely within Earth’s orbit have been discovered to date because of the difficulty of observing near the glare of the Sun.”

Finding asteroids in the inner Solar System is a daunting observational challenge. Astronomers have only two brief 10-minute windows each night to survey this area and have to contend with a bright background sky resulting from the Sun’s glare. Additionally, such observations are very near to the horizon, meaning that astronomers have to observe through a thick layer of Earth’s atmosphere, which can blur and distort their observations.

The R.A. and decl. of a hypothetical population of low-eccentricity, low-inclination, stable Venus co-orbital asteroids (Pokorny & Kuchner 2019) as they would appear on 2020 September 30 at 23:30 UT.

Discovering these three new asteroids despite these challenges was possible thanks to the unique observing capabilities of DECam. The state-of-the-art instrument is one of the highest-performance, wide-field CCD imagers in the world, giving astronomers the ability to capture large areas of sky with great sensitivity. Astronomers refer to observations as ‘deep’ if they capture faint objects. When hunting for asteroids inside Earth’s orbit, the capability to capture both deep and wide-field observations is indispensable. DECam was funded by the US Department of Energy (DOE) and was built and tested at DOE’s Fermilab.

“Large areas of sky are required because the inner asteroids are rare, and deep images are needed because asteroids are faint and you are fighting the bright twilight sky near the Sun as well as the distorting effect of Earth’s atmosphere,” said Sheppard. “DECam can cover large areas of sky to depths not achievable on smaller telescopes, allowing us to go deeper, cover more sky, and probe the inner Solar System in ways never done before.”

As well as detecting asteroids that could potentially pose a threat to Earth, this research is an important step toward understanding the distribution of small bodies in our Solar System. Asteroids that are further from the Sun than Earth are easiest to detect. Because of that these more-distant asteroids tend to dominate current theoretical models of the asteroid population. Detecting these objects also allows astronomers to understand how asteroids are transported throughout the inner Solar System and how gravitational interactions and the heat of the Sun can contribute to their fragmentation.

The orbit of the newly discovered Atira asteroid 2021 PH27, which has the smallest semimajor axis of any known asteroid.

“Our DECam survey is one of the largest and most sensitive searches ever performed for objects within Earth’s orbit and near to Venus’s orbit,” said Sheppard. “This is a unique chance to understand what types of objects are lurking in the inner Solar System.”
“After ten years of remarkable service, DECam continues to yield important scientific discoveries while at the same time contributing to planetary defense, a crucial service that benefits all humanity,” said Chris Davis, NSF Program Director for NOIRLab.

Momentum Enhancement from a 3 cm Diameter Aluminum Sphere Striking a Small Boulder Assembly at 5.4 km s−1

by James D. Walker, Sidney Chocron, Donald J. Grosch, Simone Marchi, Amanda M. Alexander in The Planetary Science Journal

On September 26, NASA’s Double Asteroid Redirection Test (DART) spacecraft crashed into Dimorphos, a moonlet of the near-Earth asteroid Didymos, at 14,000 miles per hour. Prior to the impact, Southwest Research Institute engineers and scientists performed an experiment to study the cratering process that produces the mass of ejected materials and measures the subsequent momentum enhancement of the impact. The internally funded experiment, which used a more realistic target than those previously explored.

NASA not only tracks near-Earth asteroids (NEAs) that could pose a possible impact threat to our home planet but is also exploring technology to deflect the path of a small NEA. Only a small orbital change would be needed to change an object’s trajectory so that it passes safely by Earth, as long as the change is applied sufficiently far in advance of the time of impact. Changing the momentum of an asteroid through a direct collision offers a one-two punch: the direct momentum transfer of the impacting projectile, pushing it forward, and the asteroid’s recoil from the debris erupting from the impact crater, also known as crater ejecta. The ejecta transfers momentum, propelling the target away in an “action-reaction” fashion, much like a rocket launches when high-speed gas erupts from the rear of the vehicle.

“One big question we faced was what the asteroid would actually look like and what its composition would be. Whether we can learn something from small-scale laboratory experiments is an issue of major interest to us,” said Dr. James D. Walker, director of SwRI’s Engineering Dynamics department and the study’s lead author.

Photograph of the stones in the wooden frame before (left) and after (right) the cement to hold them in place was poured.

Walker is a member of the DART Investigation Team alongside his co-authors, Dr. Sidney Chocron, Donald J. Grosch and Dr. Simone Marchi.

The DART mission spacecraft launched from Earth in November 2021. On September 26, it was deliberately crashed into the moonlet Dimorphos to assess whether a spacecraft could deflect an asteroid on a collision course with Earth. Dimorphos orbits the asteroid Didymos, a near-Earth object that has been classified as a potentially hazardous asteroid. DART is designed to nudge the orbit of the moonlet around Didymos.

SwRI’s large two-stage light gas gun, which is capable of launching projectiles at speeds up to seven kilometers per second, was used to launch a projectile at an object representing the moonlet. Because Dimorphos was thought to be a “rubble pile” asteroid made up of pieces of rock bound together by gravity, the moonlet was represented by a collection of rocks and stones, in this case held together by cement.

“We fired an aluminum sphere, which represented the DART space probe, using the two-stage light gas gun at the target at 5.44 kilometers per second, which is approaching the expected 6.1 kilometers per second of the DART impact,” Walker said. “Our experiment measured a momentum transfer to the target of 3.4 times the incoming momentum of the aluminum sphere projectile. The number 3.4 is referred to by scientists as the Greek letter beta of the impact. Hence the crater ejecta provided an additional 240% of momentum to deflect the body, beyond that provided by the projectile itself.”

A sequence of photographs of the impact from a high-speed camera (left to right, top to bottom).

The experiment aimed to study the cratering process and measure the momentum enhancement that would result from the collision. Crucially, the rubble pile was not held in place but was hung vertically as a pendulum to measure the momentum enhancement, or recoil, created by the impact ejecta.

“It’s important to understand the amount of recoil,” co-author Dr. Simone Marchi said. “It all boils down to the amount of momentum that has been transferred to the target from the impact, and there was a significant amount of recoil and ejecta material.”

By measuring the momentum, the SwRI team could then extract important information that could assess the difficulty of deflecting asteroids in space. In this latest experiment, the momentum enhancement was higher than what was witnessed in the team’s prior experiments. A higher recoil suggests it would be easier to deflect the asteroid.

In the weeks following the impact, NASA announced that DART had been successful in nudging the moonlet. Walker is now looking forward to seeing what else can be learned from the mission, including the momentum transfer of the event in space.

“It will take a while to compute the data, in part because it involves estimating the mass of the moonlet, which is unknown,” he said. “Once there’s an agreement on the mass, then the measurement of the change in the moonlet’s orbit will tell us the momentum transfer. We have a speculative body that we’ve impacted and what we’d really like to know is how size affected things. It will be a challenge to determine that.”

Scanning for planetary cores with single-receiver intersource correlations

by Sheng Wang, Hrvoje Tkalčić in Nature Astronomy

Seismologists from The Australian National University (ANU) have developed a new method to scan the deep interior of planets in our solar system to confirm whether they have a core at the heart of their existence.

The scanning method, which works in a similar way to an ultrasound scan using sound waves to generate images of a patient’s body, requires only a single seismometer on a planet’s surface in order to work. It can also be used to confirm the size of a planet’s core. Using the ANU model to scan the entirety of Mars’ interior, the researchers confirmed the Red Planet has a large core at its centre — a theory first confirmed by a team of scientists in 2021. Study co-author Professor Hrvoje Tkalčić, from ANU, said based on data collected using the ANU technique, the researchers determined that the Martian core, which is smaller than Earth’s, is about 3,620 kilometres in diameter.

“Our research presents an innovative method using a single instrument to scan the interior of any planet in a way that’s never been done before,” he said.

Confirming the existence of a planetary core, which the researchers refer to as the “engine room” of all planets, can help scientists learn more about a planet’s past and evolution. It can also help scientists determine at what point in a planet’s history a magnetic field formed and ceased to exist. The core plays an active role in sustaining a planet’s magnetic field. In the case of Mars, it could help explain why, unlike Earth, the Red Planet no longer has a magnetic field — something that is critical to sustaining all life forms.

“Modelling suggests that the Martian core is liquid and while it is made up of mostly iron and nickel, it could also contain traces of lighter elements such as hydrogen and sulphur. These elements can alter the ability of the core to transport heat,” lead author Dr Sheng Wang, who is also from ANU, said.
“A magnetic field is important because it shields us from cosmic radiation, which is why life on Earth is possible.”

Selections of source mechanisms and source pairs for constructing global inter-source correlograms.

Using a single seismometer on Mars’ surface, the ANU team measured specific types of seismic waves. The seismic waves, which were triggered by marsquakes, give off a spectrum of signals, or “echoes,” that change over time as they reverberate throughout the Martian interior. These seismic waves pierce through and bounce off the Martian core.

Professor Tkalčić said researchers are interested in the “late” and “weaker” signals that can survive hours after they are emitted from quakes, meteoroid impacts and other sources.

“Although these late signals appear to be noisy and not useful, the similarity between these weak signals recorded at various locations on Mars manifests itself as a new signal that reveals the presence of a large core in the Red Planet’s heart,” Professor Tkalčić said.
“We can determine how far these seismic waves travel to reach the Martian core but also the speed at which they travel through Mars’ interior. This data helps us make estimations about the size of Mars’ core.”

The researchers say their method of using a single seismometer to confirm the presence of a planetary core is also a “cost-effective solution.”

“There is a single seismic station on Mars. There were four of them on the Moon in 1970s. The situation of having a limited number of instruments is unlikely to change in the coming decades or even this century due to high cost,” Dr Wang said. “We need an approach right now to use only a single seismometer to study planetary interiors.”

The researchers hope this new ANU-developed technique involving a single seismometer could be used to help scientists learn more about our other planetary neighbours, including the moon.

“The US and China plan to send seismometers to the moon, and Australia also has ambitions to participate in future missions, so there’s potential for further studies using new and more sophisticated instruments,” Professor Tkalčić said.
Dr Wang said: “Although there are many studies on planetary cores, the images we have of planetary interiors are still very blurry. But with new instruments and methods like ours we’ll be able to get sharper images which will help us answer questions such as how big the cores are and whether they take a solid or liquid form. “Our method could even be used to analyse the Jupiter moons and the outer solar system planets that are solid.”

To carry out their research, ANU scientists used data collected from a seismometer attached to NASA’s InSight lander, which has been collecting information about marsquakes, Martian weather and the planet’s interior since touching down on Mars in 2018.

Quantum Signatures of Black Hole Mass Superpositions

by Joshua Foo, Cemile Senem Arabaci, Magdalena Zych, Robert B. Mann in Physical Review Letters

Bizarre quantum properties of black holes — including their mind-bending ability to have different masses simultaneously — have been confirmed by University of Queensland physicists.

A UQ-led team of theoretical physicists, headed by PhD candidate Joshua Foo, ran calculations that reveal surprising black hole quantum phenomena.

“Black holes are an incredibly unique and fascinating feature of our universe,” Mr Foo said. “They’re created when gravity squeezes a vast amount of matter incredibly densely into a tiny space, creating so much gravitational pull that even light cannot escape.
“It’s a phenomenon that can be triggered by a dying star. “But, until now, we haven’t deeply investigated whether black holes display some of the weird and wonderful behaviours of quantum physics. “One such behaviour is superposition, where particles on a quantum scale can exist in multiple states at the same time.

Transition probability of the detector as a function of √MB/MA.

“This is most commonly illustrated by Schrödinger’s cat, which can be both dead and alive simultaneously. “But, for black holes, we wanted to see whether they could have wildly different masses at the same time, and it turns out they do.
“Imagine you’re both broad and tall, as well as short and skinny at the same time — it’s a situation which is intuitively confusing since we’re anchored in the world of traditional physics. “But this is reality for quantum black holes.”

To reveal this, the team developed a mathematical framework allowing us to “place” a particle outside a theoretical mass-superposed black hole. Mass was looked at specifically, as it is a defining feature of a black hole, and as it is plausible that quantum black holes would naturally have mass superposition. Research co-supervisor, Dr Magdalena Zych, said that the research in fact reinforces conjectures raised by pioneers of quantum physics.

“Our work shows that the very early theories of Jacob Bekenstein — an American and Israeli theoretical physicist who made fundamental contributions to the foundation of black hole thermodynamics — were on the money,” she said. He postulated that black holes can only have masses that are of certain values, that is, they must fall within certain bands or ratios — this is how energy levels of an atom works, for example.

“Our modelling showed that these superposed masses were, in fact, in certain determined bands or ratios — as predicted by Bekenstein. “We didn’t assume any such pattern going in, so the fact we found this evidence was quite surprising.
“The universe is revealing to us that it’s always more strange, mysterious and fascinating than most of us could have ever imagined.”

GJ 1252b: A Hot Terrestrial Super-Earth with No Atmosphere

by Ian J. M. Crossfield, Matej Malik, Michelle L. Hill, Stephen R. Kane, et al in The Astrophysical Journal Letters

An Earth-like planet orbiting an M dwarf — the most common type of star in the universe — appears to have no atmosphere at all. This discovery could cause a major shift in the search for life on other planets.

Because M-dwarfs are so ubiquitous, this discovery means a large number of planets orbiting these stars may also lack atmospheres and therefore are unlikely to harbor living things. The work that led to the revelations about the no-atmosphere planet, named GJ 1252b, are detailed in the Astrophysical Journal Letters. This planet orbits its star twice during the course of a single day on Earth. It is slightly larger than Earth, and it is much closer to its star than Earth is to the sun, making GJ 1252b intensely hot as well as inhospitable.

“The pressure from the star’s radiation is immense, enough to blow a planet’s atmosphere away,” said Michelle Hill, UC Riverside astrophysicist and study co-author.

Earth also loses some of its atmosphere over time because of the sun, but volcanic emissions and other carbon cycling processes make the loss barely noticeable by helping replenish what is lost. However, in greater proximity to a star, a planet cannot keep replenishing the amount being lost. In our solar system, this is the fate of Mercury. It does have an atmosphere, but one that is extremely thin, made up of atoms blasted off its surface by the sun. The extreme heat of the planet causes these atoms to escape into space.

Spitzer 4.5 μm photometry and secondary eclipse fit (top) and residuals to the fit (bottom). The photometry is shown after detrending for systematic effects, combining the photometry from all 10 eclipse visits, and binning down to a two-minute cadence.

To determine that GJ 1252b lacks an atmosphere, astronomers measured infrared radiation from the planet as its light was obscured during a secondary eclipse. This type of eclipse occurs when a planet passes behind a star and the planet’s light, as well as light reflected from its star, is blocked.

The radiation revealed the planet’s scorching daytime temperatures, estimated to reach 2,242 degrees Fahrenheit — so hot that gold, silver, and copper would all melt on the planet. The heat, coupled with assumed low surface pressure, led the researchers to believe there’s no atmosphere.

Even with a tremendous amount of carbon dioxide, which traps heat, the researchers concluded GJ 1252b would still not be able to hold on to an atmosphere. “The planet could have 700 times more carbon than Earth has, and it still wouldn’t have an atmosphere. It would build up initially, but then taper off and erode away,” said Stephen Kane, UCR astrophysicist and study co-author.

Joint constraints on Bond albedo and the heat redistribution parameter, assuming that GJ 1252b radiates as a blackbody at its 4.5 μm brightness temperature.

M dwarf stars tend to have more flares and activity than the sun, further reducing the likelihood that planets closely surrounding them could hold on to their atmospheres.

“It’s possible this planet’s condition could be a bad sign for planets even further away from this type of star,” Hill said. “This is something we’ll learn from the James Webb Space Telescope, which will be looking at planets like these.”

There are 5,000 stars in Earth’s solar neighborhood, most of them M dwarfs. Even if planets orbiting them can be ruled out entirely, there are still roughly 1,000 stars similar to the sun that could be habitable.

“If a planet is far enough away from an M dwarf, it could potentially retain an atmosphere. We cannot conclude yet that all rocky planets around these stars get reduced to Mercury’s fate,” Hill said. “I remain optimistic.”