Neutron-Star Collision Shakes Space-Time and Lights Up the Sky
On Aug. 17, the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detected something new. Some 130 million light-years away, two super-dense neutron stars, each as small as a city but heavier than the sun, had crashed into each other, producing a colossal convulsion called a kilonova and sending a telltale ripple through space-time to Earth.
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LIGO’s pair of ultrasensitive detectors in Louisiana and Washington state made history two years ago by recording the gravitational waves coming from the collision of two black holes — a discovery that earned the experiment’s architects the Nobel Prize in Physics this month. Three more signals from black hole collisions followed the initial discovery.
Yet black holes don’t give off light, so making any observations of these faraway cataclysms beyond the gravitational waves themselves was unlikely. Colliding neutron stars, on the other hand, produce fireworks. Astronomers had never seen such a show before, but now LIGO was telling them where to look, which sent teams of researchers like Berger’s scurrying to capture the immediate aftermath of the collision across the full range of electromagnetic signals. In total, more than 70 telescopes swiveled toward the same location in the sky.
They struck the motherlode. In the days after Aug. 17, astronomers made successful observations of the colliding neutron stars with optical, radio, X-ray, gamma-ray, infrared and ultraviolet telescopes. The enormous collaborative effort, detailed today in dozens of papers appearing simultaneously in Physical Review Letters, Nature, Science, Astrophysical Journal Letters and other journals, has not only allowed astrophysicists to piece together a coherent account of the event, but also to answer longstanding questions in astrophysics.
Planetary Society-funded telescopes help find ring around Haumea, a distant dwarf planet
Haumea is one of four known dwarf planets beyond Neptune; the others are Pluto, Eris and Makemake. The frigid world is an ellipsoid about as wide as Pluto, shaped roughly like a flattened egg or river stone. This study found its long axis to be about 1704 km. It has two known moons: Hiʻiaka and Namaka.
We know giant planets like Jupiter, Saturn, Uranus and Neptune have rings, but thus far, we’ve only found them around two small worlds. Chariklo is about 250 kilometers wide, and has two rings, while Chiron, about the same size, is also suspected to have a ring. Both Chariklo and Chiron are Centaurs, small worlds orbiting the Sun between the asteroid belt and Kuiper belt, crisscrossing the giant planets’ orbits.
With today’s announcement, Haumea becomes the first, small, non-Centaur known to have a ring, and the farthest ring world we’ve found in our solar system.
Missing baryons in the cosmic web revealed by the Sunyaev-Zel'dovich effect
Observations of galaxies and galaxy clusters in the local universe can account for only 10% of the baryon content inferred from measurements of the cosmic microwave background and from nuclear reactions in the early Universe. Locating the remaining 90% of baryons has been one of the major challenges in modern cosmology. Cosmological simulations predict that the ‘missing baryons’ are spread throughout filamentary structures in the cosmic web, forming a low density gas with temperatures of 105−107 K. Previous attempts to observe this warm-hot filamentary gas via X-ray emission or absorption in quasar spectra have proven difficult due to its diffuse and low-temperature nature. Here we report a 5.1σ detection of warm-hot baryons in stacked filaments through the thermal Sunyaev-Zel'dovich (SZ) effect, which arises from the distortion in the cosmic microwave background spectrum due to ionised gas. The estimated gas density in these 15 Megaparsec-long filaments is approximately 6 times the mean universal baryon density, and overall this can account for ∼30% of the total baryon content of the Universe. This result establishes the presence of ionised gas in large-scale filaments, and suggests that the missing baryons problem may be resolved via observations of the cosmic web.
Doing SETI Better by Understanding Ourselves
One of the reasons SETI is hard is that we don’t know exactly what we are looking for, and part of that difficulty is that we still aren’t sure of who we are. It seems counter-intuitive, but in order to be good at looking for aliens, we have to become experts at understanding ourselves.
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For all of these reasons, SETI needs to include the social sciences, especially anthropology, to help practitioners identify where they are stuck “inside their brains” and get out. Anthropologists are trained to spot these sorts of cultural myopias and avoid them, from the books we read, to the language we use to describe our future and alien species.
Red-edge position of habitable exoplanets around M-dwarfs
One of the possible signs of life on distant habitable exoplanets is the red-edge, which is a rise in the reflectivity of planets between visible and near-infrared (NIR) wavelengths. Previous studies suggested the possibility that the red-edge position for habitable exoplanets around M-dwarfs may be shifted to a longer wavelength than that for Earth. We investigated plausible red-edge position in terms of the light environment during the course of the evolution of phototrophs. We show that phototrophs on M-dwarf habitable exoplanets may use visible light when they first evolve in the ocean and when they first colonize the land. The adaptive evolution of oxygenic photosynthesis may eventually also use NIR radiation, by one of two photochemical reaction centers, with the other center continuing to use visible light. These “two-color” reaction centers can absorb more photons, but they will encounter difficulty in adapting to drastically changing light conditions at the boundary between land and water. NIR photosynthesis can be more productive on land, though its evolution would be preceded by the Earth-type vegetation. Thus, the red-edge position caused by photosynthetic organisms on habitable M-dwarf exoplanets could initially be similar to that on Earth and later move to a longer wavelength.
Observatory detects extragalactic cosmic rays hitting the Earth
Fifty years ago, scientists discovered that the Earth is occasionally hit by cosmic rays of enormous energies. Since then, they have argued about the source of those ultra-high-energy cosmic rays—whether they came from our galaxy or outside the Milky Way.
The answer lies in a galaxy or galaxies far, far away, according to a report published Sept. 22 in Science by the Pierre Auger Collaboration, which includes University of Chicago scientists. The internationally run observatory in Argentina, co-founded by the late UChicago Nobel laureate James Cronin, has been collecting data on such cosmic rays for a more than a decade.
The collaboration found that the rate of such cosmic particles, whose energies are a million times greater than that of the protons accelerated in the Large Hadron Collider, is about six percent greater from one side of the sky than the other, in a direction where the distribution of galaxies is relatively high.
NASA’s Hubble Captures Blistering Pitch-Black Planet
NASA’s Hubble Space Telescope has observed a planet outside our solar system that looks as black as fresh asphalt because it eats light rather than reflecting it back into space. This light-eating prowess is due to the planet’s unique capability to trap at least 94 percent of the visible starlight falling into its atmosphere.
The oddball exoplanet, called WASP-12b, is one of a class of so-called “hot Jupiters,” gigantic, gaseous planets that orbit very close to their host star and are heated to extreme temperatures. The planet’s atmosphere is so hot that most molecules are unable to survive on the blistering day side of the planet, where the temperature is 4,600 degrees Fahrenheit. Therefore, clouds probably cannot form to reflect light back into space. Instead, incoming light penetrates deep into the planet’s atmosphere where it is absorbed by hydrogen atoms and converted to heat energy.
Almost out of view from our fair planet, rotating around the Sun’s western edge giant active region AR2673 lashed out with another intense solar flare followed by a large coronal mass ejection on September 10. The flare itself is seen here at the right in an extreme ultraviolet image from the sun-staring Solar Dynamics Observatory. This intense flare was the fourth X-class flare from AR2673 this month. The active region’s most recent associated coronal mass ejection collided with Earth’s magnetosphere 2 days later. Say farewell to the mighty AR2673, for now. For the next two weeks, the powerful sunspot group will be on the Sun’s far side.
UCLA physicists propose new theories of black holes from the very early universe
A long-standing question in astrophysics is whether the universe’s very first black holes came into existence less than a second after the Big Bang or whether they formed only millions of years later during the deaths of the earliest stars.
Alexander Kusenko, a UCLA professor of physics, and Eric Cotner, a UCLA graduate student, developed a compellingly simple new theory suggesting that black holes could have formed very shortly after the Big Bang, long before stars began to shine. Astronomers have previously suggested that these so-called primordial black holes could account for all or some of the universe’s mysterious dark matter and that they might have seeded the formation of supermassive black holes that exist at the centers of galaxies. The new theory proposes that primordial black holes might help create many of the heavier elements found in nature.
New Activity of Repeating FRB 121102
Andrew Siemion, who heads up the Breakthrough Listen initiative and is director of the Berkeley SETI Research Center, sent out a message to astronomers on August 29 noting recent activity from the radio source FRB 121102. The heightened activity had been noted by Breakthrough Listen postdoctoral researcher Vishal Gajjar. You’ll recall that Fast Radio Bursts (FRBs) are powerful but extremely short-duration radio pulses whose sources generally remain unknown.
What tags FRB 121102 as especially interesting is that it is the only FRB known to repeat. In fact, more than 150 bursts have been observed coming from the dwarf galaxy 3 billion light years from Earth that is thought to be its place of origin.
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The current active state of FRB 121102 could prove quite useful, allowing us to measure this source of FRBs at the highest precision yet. The 15 new pulses Gajjar observed show emissions at higher frequencies than have previously been seen. The brightest FRB 121102 emission occurred around 7 GHz. We’re a long way from being able to figure out what causes FRBs, but the Breakthrough Listen instrumentation is giving us the best look at the frequency spectrum of this powerful source yet.
Historical Observations Reveal Ancient Nova
In 1437, Korean royal astronomers observed a new star appearing in the constellation Scorpius. “A guest star began to be seen between the second and third stars of Wei,” they wrote in the Sejong Sillok, a chronicle of the reign of King Sejong who ruled Korea from 1418 to 1464. The star faded from sight after 14 days.
What the puzzled subjects of King Sejong witnessed was a classical nova – the outburst of a white dwarf in a close binary system when it collects sufficient hydrogen from its companion star.
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A team led by Michael Shara (American Museum of Natural History) with help from Richard Stephenson (Durham University), a historian specializing in Asian astronomical records, followed the indications from the Sejong Sillok to look for the stellar system responsible for the guest star, now called Nova Scorpii 1437. “It was the best-located classical nova in over 2,000 years of records by Chinese, Korean and Japanese astronomers,” Shara says. “We expected it to be faint, so a fairly precise location was essential if we were to have any realistic chance of recovering it.”
Shara’s team went to the records of the Digital Access to a Sky Century at Harvard (DASCH) project, which is digitizing the collection of 500,000 photographic sky patrol plates taken by Harvard astronomers between 1885 and 1993.
There, they found the object they were looking for in an image taken in 1923 with the 24-inch Bruce Doublet telescope at the Harvard Observatory station in Arequipa, Peru. Slightly off target from their interpretation of the Korean records, Shara found a shell-shaped structure — presumably gas ejected by the explosion, which has been expanding for the last 580 years. Inside the shell, they found a cataclysmic variable that new imaging confirms is the origin of Nova Scorpii 1437. It’s now simmering unsteadily with faint brightness — around 16th or 17th magnitude. They explain their findings in the August 31th Nature.
Where Is the Flux Going? The Long-Term Photometric Variability of Boyajian’s Star
We present ~800 days of photometric monitoring of Boyajian’s Star (KIC 8462852) from the All-Sky Automated Survey for Supernovae (ASAS-SN) and ~4000 days of monitoring from the All Sky Automated Survey (ASAS). We show that from 2015 to the present the brightness of Boyajian’s Star has steadily decreased at a rate of 6.3 +/- 1.4 mmag yr^-1, such that the star is now 1.5% fainter than it was in February 2015. Moreover, the longer time baseline afforded by ASAS suggests that Boyajian’s Star has also undergone two brightening episodes in the past 11 years, rather than only exhibiting a monotonic decline. We analyze a sample of ~1000 comparison stars of similar brightness located in the same ASAS-SN field and demonstrate that the recent fading is significant at >99.4% confidence. The 2015-2017 dimming rate is consistent with that measured with Kepler data for the time period from 2009 to 2013. This long-term variability is difficult to explain with any of the physical models for the star’s behavior proposed to date.
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