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Mid May Astronomy Bulletin

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A TIMELINE OF GREAT AURORA STORMS
Spaceweather.com

A new historical study shows that great aurora storms occur every 40 to 60 years. This kind of historical research is not easy. Hundreds of years ago, most people had never even heard of the aurora borealis. When the lights appeared, they were described as "fog," "vapours", "spirits"--almost anything other than "auroras. Making a timeline 500 years long requires digging through unconventional records such as personal diaries, ship's logs, local weather reports--often in languages that are foreign to the researchers. The study defined a 'Great Storm' simply as one in which auroras were visible to the unaided eye at or below 30 degrees magnetic latitude. Visual sightings were key. The human eye is a sensor we've had in common with observers since the beginning of recorded history. Pre-modern scientists didn't have satellites or magnetometers to measure solar storms, but they could look up at the night sky. In all, the team tallied 14 examples of storms where many people saw auroras within 30 degrees of the equator. There's a whole cluster of sightings in Sept. 1770. The Great Storm of 1770 appears to be a 500-year event. There were low-latitude auroras for 9 nights in a row. During the 1770 storm, extremely bright red auroras blanketed Japan and parts of China. Captain James Cook himself saw the display from the HMS Endeavour near Timor Island, south of Indonesia. Drawings have been found of the instigating sunspot; it is twice the size of the sunspot that caused the infamous Carrington Event of 1859. The timeline suggests that this was not "just another Great Storm"; something exceptional happened in 1770 that researchers still don't fully understand. Today's senior space weather researchers were taught that Great Storms are rare. The Carrington Event was long thought to be a singular event, alone in the historical record. Recent studies are finding otherwise. Just last month the US Geological Survey published a paper showing that extreme geomagnetic storms recur every ~45 years or so--a result in accord with the new findings. The last Great Storm in the timeline occurred 32 years ago. Soon, it will be time for another.


LUNAR CRATER RADIO TELESCOPE
NASA

The early-stage NASA concept could see robots hang wire mesh in a crater on the Moon’s far side, creating a radio telescope to help probe the dawn of the Universe. After years of development, the Lunar Crater Radio Telescope (LCRT) project has been awarded $500,000 to support additional work as it enters Phase II of NASA’s Innovative Advanced Concepts (NIAC) program. While not yet a NASA mission, the LCRT describes a mission concept that could transform humanity’s view of the cosmos. The LCRT’s primary objective would be to measure the long-wavelength radio waves generated by the cosmic Dark Ages – a period that lasted for a few hundred million years after the Big Bang, but before the first stars blinked into existence. Cosmologists know little about this period, but the answers to some of science’s biggest mysteries may be locked in the long-wavelength radio emissions generated by the gas that would have filled the Universe during that time.

Radio telescopes on Earth can’t probe this mysterious period because the long-wavelength radio waves from that time are reflected by a layer of ions and electrons at the top of our atmosphere, a region called the ionosphere. Random radio emissions from our noisy civilization can interfere with radio astronomy as well, drowning out the faintest signals. But on the Moon’s far side, there’s no atmosphere to reflect these signals, and the Moon itself would block Earth’s radio chatter. The lunar far side could be prime real estate to carry out unprecedented studies of the early Universe. Radio telescopes on Earth cannot see cosmic radio waves at about 10 metres or longer because of our ionosphere, so there’s a whole region of the Universe that we simply cannot see,. To be sensitive to long radio wavelengths, the LCRT would need to be huge. The idea is to create an antenna over 1 kilometre) wide in a crater over 3 kilometres wide. The biggest single-dish radio telescopes on Earth – like Five-hundred-metre Aperture Spherical Telescope (FAST) in China and the now-inoperative 305-metre-wide Arecibo Observatory in Puerto Rico – were built inside natural bowl-like depressions in the landscape to provide a support structure. This class of radio telescope uses thousands of reflecting panels suspended inside the depression to make the entire dish’s surface reflective to radio waves. The receiver then hangs via a system of cables at a focal point over the dish, anchored by towers at the dish’s perimeter, to measure the radio waves bouncing off the curved surface below. But despite its size and complexity, even FAST is not sensitive to radio wavelengths longer than about 4.3 metres.


VOLCANOES ON MARS COULD BE ACTIVE
University of Arizona

Evidence of recent volcanic activity on Mars shows that eruptions could have taken place in the past 50,000 years. Most volcanism on the Red Planet occurred between 3 and 4 billion years ago, with smaller eruptions in isolated locations continuing perhaps as recently as 3 million years ago. But, until now, there was no evidence to indicate Mars could still be volcanically active. Using data from satellites orbiting Mars, researchers discovered a previously unknown volcanic deposit. The volcanic eruption produced an 8-mile-wide, smooth, dark deposit surrounding a 20-mile-long volcanic fissure. Further investigation showed that the properties, composition and distribution of material match what would be expected for a pyroclastic eruption -- an explosive eruption of magma driven by expanding gasses, not unlike the opening of a shaken can of soda. The majority of volcanism in the Elysium Planitia region and elsewhere on Mars consists of lava flowing across the surface, similar to recent eruptions in Iceland. Although there are numerous examples of explosive volcanism on Mars, they occurred long ago. However, this deposit appears to be different. The site of the recent eruption is about 1,600 kilometres from NASA's InSight lander, which has been studying seismic activity on Mars since 2018. Two Marsquakes, the Martian equivalent of earthquakes, were found to originate in the region around the Cerberus Fossae, and recent work has suggested the possibility that these could be due to the movement of magma deep underground. A volcanic deposit such as this one also raises the possibility for habitable conditions below the surface of Mars in recent history.

Similar volcanic fissures in this region were the source of enormous floods, perhaps as recently as 20 million years ago, as groundwater erupted out onto the surface. The youngest volcanic eruption on Mars happened only 10 kilometres from the youngest large-impact crater on the planet -- a 10 kilometres -wide crater named Zunil. Several studies have found evidence that large quakes on Earth can cause magma stored beneath the surface to erupt. The impact that formed the Zunil crater on Mars would have shaken the Red Planet just like an earthquake. While the more dramatic giant volcanoes elsewhere on Mars -- such as Olympus Mons, the tallest mountain in the solar system -- tell a story of the planet's ancient dynamics, the current hotspot of Martian activity seems to be in the relatively featureless plains of the planet's Elysium region. The volcanic deposit described in this study, along with ongoing seismic rumbling in the planet's interior detected by InSight and possible evidence for releases of methane plumes into the atmosphere detected by NASA's MAVEN orbiter, suggest that Mars is far from a cold, inactive world.


VOYAGER 1 DETECTS PLASMA 'HUM’:
Cornell University

Voyager 1 -- one of two sibling NASA spacecraft launched 44 years ago and now the most distant human-made object in space -- still works and zooms toward infinity. The craft has long since zipped past the edge of the solar system through the heliopause -- the solar system's border with interstellar space -- into the interstellar medium. Now, its instruments have detected the constant drone of interstellar gas (plasma waves). It's very faint and monotone, because it is in a narrow frequency bandwidth. This work allows scientists to understand how the interstellar medium interacts with the solar wind and how the protective bubble of the solar system's heliosphere is shaped and modified by the interstellar environment. Launched in September 1977, the Voyager 1 spacecraft flew by Jupiter in 1979 and then Saturn in late 1980. Travelling at about 38,000 mph, Voyager 1 crossed the heliopause in August 2012. After entering interstellar space, the spacecraft's Plasma Wave System detected perturbations in the gas. But, in between those eruptions -- caused by our own roiling Sun -- researchers have uncovered a steady, persistent signature produced by the tenuous near-vacuum of space. Astronomers believe there is more low-level activity in the interstellar gas than scientists had previously thought, which allows researchers to track the spatial distribution of plasma -- that is, when it's not being perturbed by solar flares. Voyager 1 left Earth carrying a Golden Record created by a committee chaired by the late Carl Sagan, as well as mid-1970s technology. To send a signal to Earth, it took 22 watts, according to NASA's Jet Propulsion Laboratory. The craft has almost 70 kilobytes of computer memory and -- at the beginning of the mission -- a data rate of 21 kilobits per second. Due to the 14-billion-mile distance, the communication rate has since slowed to 160-bits-per-second, or about half a 300-baud rate.


HEAVIEST KNOWN NEUTRON STAR
Physics

A satellite experiment has revealed that the heaviest known neutron star is unexpectedly large, which suggests that the matter in the star’s inner core is less “squeezable” than some models predict. Neutron stars are “cosmic zombies” —corpses of massive stars that collapsed in violent explosions after running out of fuel. By studying these ultra-dense objects, researchers hope to understand the behaviour of matter under extreme conditions, which might help in cracking some of the biggest mysteries in physics. NASA’s Neutron star Interior Composition Explorer (NICER), an x-ray telescope on the International Space Station, has measured the size of the heaviest known neutron star. The surprisingly large radius measured for this star implies a stiffer-than-expected state of matter in the core, disfavouring models that predict a “squishy” centre. Neutron stars are the densest observable objects in the cosmos—packing twice the mass of the Sun in a sphere as wide as a large city. In the outer core of the star, the large pressure breaks up nuclei into nucleons and crushes protons and electrons together, leaving behind a sea of mostly neutrons. Researchers are unsure, however, about what happens in the inner core of the star. Do neutrons persist or decompose into their quark constituents? Do these quarks interact to form exotic particles? Since no laboratory experiment can reproduce neutron star conditions, the only option for studying this exceptional state of matter is to observe neutron stars themselves—inferring what’s going on inside the star from basic properties such as mass and size. Such measurements, however, are no easy feat. Up until today, about two thousand neutron stars have been discovered, but only a handful of them have been sized up, typically by monitoring the x-ray emission of gas surrounding the star.

NICER has developed a unique sizing method applicable to rapidly rotating neutron stars known as pulsars. As pulsars rotate, hot spots on their surface emit x rays that scan the cosmos like lighthouse beams. The experiment monitors the pulsar’s oscillatory x-ray brightness, “time stamping” the arrival of each x-ray photon with a precision of about 100 ns. The path that these photons take is distorted by the gravitational warping of spacetime around the star, allowing some hot spots to remain visible even as they rotate to the far side of the star. From the x-ray time-stamped data, the researchers reconstruct the gravitational potential and, in turn, infer the star size. The combination of x-ray spectroscopy capabilities with timing is a unique feature of NICER, which allows researchers to fully exploit information on the star’s spin to constrain its properties. The NICER Collaboration first used this method in 2019 to measure PSR J0030, a pulsar 1000 light years from Earth. Weighing 1.4 solar masses, J0030 was found to have a diameter of about 26 km. In the new measurement, the collaboration turned to the most massive known neutron star, PSR J0740, in the “giraffe” constellation. Nearly 4 times more distant than J0030, J0740 is 20 times fainter and was thus a “stretch goal for the experiment.”. But its mass (2.1 solar masses) makes this pulsar “so exceptional” that the team decided to devote a significant amount of time to measuring it. The collaboration tasked the data analysis to two independent teams, who used different assumptions on, for instance, the x-ray background in the sky and the instrument calibration. The teams came up with similar values for the most likely diameter of the star: 25 and 27 km, respectively, both close to that of the previously measured, lighter pulsar. The size measurement will allow researchers to vet different options for the star interior. In some models, neutrons break apart into free-roaming quarks, which leads to a squishy, compressible core. These models deliver the counterintuitive prediction that neutron stars should become smaller as their mass increases. Other models, in which some neutrons persist, or the quarks interact strongly, predict a harder-to-compress form of matter. The similar diameters found for J0030 and J0740 “strongly disfavours the squishiest models. Together, the two measurements of J0740 and J0030 offer another thought-provoking conclusion. In textbooks, pulsars are depicted as perfectly symmetric magnetic dipoles, with their hot spots at each of the pulsar’s poles. For both stars investigated by NICER, however, the hot spots appear to lie on the same hemisphere, suggesting a much more complex and asymmetric field configuration. Having seen this asymmetry in the first two stars where the hot spots have been mapped, this beautiful dipole cartoon of pulsars is likely wrong.


SMALL GALAXIES AND EVOLUTION OF UNIVERSE
University of Minnesota

A new study shows that high-energy light from small galaxies may have played a key role in the early evolution of the Universe. The research gives insight into how the Universe became reionized, a problem that astronomers have been trying to solve for years. After the Big Bang, when the Universe was formed billions of years ago, it was in an ionized state. This means that the electrons and protons floated freely throughout space. As the Universe expanded and started cooling down, it changed to a neutral state when the protons and electrons combined into atoms, akin to water vapour condensing into a cloud. Now however, scientists have observed that the Universe is back in an ionized state. A major endeavour in astronomy is figuring out how this happened. Astronomers have theorized that the energy for reionization must have come from galaxies themselves. But, it's incredibly hard for enough high energy light to escape a galaxy due to hydrogen clouds within it that absorb the light, much like clouds in the Earth's atmosphere absorb sunlight on an overcast day. Astrophysicists may have found the answer to that problem. Using data from the Gemini telescope, the researchers have observed the first ever galaxy in a "blow-away" state, meaning that the hydrogen clouds have been removed, allowing the high energy light to escape. The scientists suspect that the blow-away was caused by many supernovas, or dying stars, exploding in a short period of time. The galaxy, named Pox 186, is so small that it could fit inside the Milky Way. The researchers suspect that its compact size, coupled with its large population of stars -- which amount to a hundred thousand times the mass of the Sun -- made the blow-away possible. The findings confirm that a blow-away is possible, furthering the idea that small galaxies were primarily responsible for the reionization of the Universe and giving more insight into how the Universe became what it is today.


HYDROGEN-FREE SUPERNOVA
RAS

A curiously yellow pre-supernova star has caused astrophysicists to re-evaluate what’s possible at the deaths of our Universe’s most massive stars. At the end of their lives, cool, yellow stars are typically shrouded in hydrogen, which conceals the star’s hot, blue interior. But this yellow star, located 35 million light years from Earth in the Virgo galaxy cluster, was mysteriously lacking this crucial hydrogen layer at the time of its explosion. If a star explodes without hydrogen, it should be extremely blue — really, really hot. It’s almost impossible for a star to be this cool without having hydrogen in its outer layer. Astronomers looked at every single stellar model that could explain a star like this, and every single model requires that the star had hydrogen, which, from its supernova, they knew it did not. The team used the Young Supernova Experiment, which uses the Pan-STARRS telescope at Hawaii to catch supernovae right after they explode. After the Young Supernova Experiment spotted supernova 2019yvr in the relatively nearby spiral galaxy NGC 4666, the team used deep space images captured by NASA’s Hubble Space Telescope, which fortunately already observed this section of the sky two and a half years before the star exploded.

The Hubble images show the source of the supernova, a massive star imaged just a couple of years before the explosion. Several months after the explosion however, the team discovered that the material ejected in the star’s final explosion seemed to collide with a large mass of hydrogen. This led the team to hypothesize that the progenitor star might have expelled the hydrogen within a few years before its death. Astronomers have suspected that stars undergo violent eruptions or death throes in the years before we see supernovae. This star’s discovery provides some of the most direct evidence ever found that stars experience catastrophic eruptions, which cause them to lose mass before an explosion. If the star was having these eruptions, then it likely expelled its hydrogen several decades before it exploded. In the new study, the team also presents another possibility: a less massive companion star might have stripped away hydrogen from the supernova’s progenitor star. However, the team will not be able to search for the companion star until after the supernova’s brightness fades, which could take up to a decade. Unlike its normal behaviour right after it exploded, the hydrogen interaction revealed it’s kind of this oddball supernova.


SUPERNOVAE TWINS ENHANCE PRECISION COSMOLOGY
Lawrence Berkeley National Laboratory

Cosmologists have found a way to double the accuracy of measuring distances to supernova explosions—one of their tried-and-true tools for studying the mysterious dark energy that is making the Universe expand faster and faster. The results from the Nearby Supernova Factory (SNfactory) collaboration will enable scientists to study dark energy with greatly improved precision and accuracy, and provide a powerful crosscheck of the technique across vast distances and time. The findings will also be central to major upcoming cosmology experiments that will use new ground and space telescopes to test alternative explanations of dark energy. Supernovae were used in 1998 to make the startling discovery that the expansion of the Universe is speeding up, rather than slowing down as had been expected. This acceleration—attributed to the dark energy that makes up two-thirds of all the energy in the Universe—has since been confirmed by a variety of independent techniques as well as with more detailed studies of supernovae. The discovery of dark energy relied on using a particular class of supernovae, Type Ia. These supernovae always explode with nearly the same intrinsic maximum brightness. Because the observed maximum brightness of the supernova is used to infer its distance, the small remaining variations in the intrinsic maximum brightness limited the precision with which dark energy could be tested. Despite 20 years of improvements by many groups, supernovae studies of dark energy have until now remained limited by these variations.

The new results announced by the SNfactory come from a multi-year study devoted entirely to increasing the precision of cosmological measurements made with supernovae. Measurement of dark energy requires comparisons of the maximum brightnesses of distant supernovae billions of light-years away with those of nearby supernovae "only" 300 million light-years away. The team studied hundreds of such nearby supernovae in exquisite detail. Each supernova was measured a number of times, at intervals of a few days. Each measurement examined the spectrum of the supernova, recording its intensity across the wavelength range of visible light. An instrument custom-made for this investigation, the SuperNova Integral Field Spectrometer, installed at the University of Hawaii 2.2-meter telescope at Maunakea, was used to measure the spectra. Several years ago, physicists made a discovery key to today's results. Looking at a multitude of spectra taken by the SNfactory, they found that in quite a number of instances, the spectra from two different supernovae looked very nearly identical. Among the 50 or so supernovae, some were virtually identical twins. When the wiggly spectra of a pair of twins were superimposed, to the eye there was just a single track. The current analysis builds on this observation to model the behaviour of supernovae in the period near the time of their maximum brightness. The new work nearly quadruples the number of supernovae used in the analysis. This made the sample large enough to apply machine-learning techniques to identify these twins, leading to the discovery that Type Ia supernova spectra vary in only three ways. The intrinsic brightnesses of the supernovae also depend primarily on these three observed differences, making it possible to measure supernova distances to the remarkable accuracy of about 3%. Just as important, this new method does not suffer from the biases that have beset previous methods, seen when comparing supernovae found in different types of galaxies. Since nearby galaxies are somewhat different than distant ones, there was a serious concern that such dependence would produce false readings in the dark energy measurement. Now this concern can be greatly reduced by measuring distant supernovae with this new technique. Conventional measurement of supernova distances uses light curves—images taken in several colours as a supernova brightens and fades. Instead, a spectrum was used of each supernova. These are so much more detailed, and with machine-learning techniques it then became possible to discern the complex behaviour that was key to measuring more accurate distances.


TESS TELESCOPE SPOTS BRIGHT GAMMA-RAY BURST
Southern Methodist University

NASA has a long tradition of unexpected discoveries, and the space programme's TESS mission is no different. Astrophysicists have discovered a particularly bright gamma-ray burst using a NASA telescope designed to find exoplanets -- those occurring outside our solar system -- particularly those that might be able to support life. It's the first time a gamma-ray burst has been found this way. Gamma-ray bursts are the brightest explosions in the Universe, typically associated with the collapse of a massive star and the birth of a black hole. They can produce as much radioactive energy as the Sun will release during its entire 10-billion-year existence. The blast -- called GRB 191016A -- happened on Oct. 16 and its location and duration were also determined. The findings prove this TESS telescope is useful not just for finding new planets, but also for high-energy astrophysics. The GRB 191016A had a peak magnitude of 15.1, which means it was 10,000 times fainter than the faintest stars we can see with the naked eyes. That may sound quite dim, but the faintness has to do with how far away the burst occurred. It is estimated that light from GRB 191016A's galaxy had been travelling 11.7 billion years before becoming visible in the TESS telescope. Most gamma ray bursts are dimmer -- closer to 160,000 times fainter than the faintest stars. The burst reached its peak brightness sometime between 1,000 and 2,600 seconds, then faded gradually until it fell below the ability of TESS to detect it some 7000 seconds after it first went off.

This gamma-ray burst was first detected by a NASA's satellite called Swift-BAT, which was built to find these bursts. But because GRB 191016A occurred too close to the Moon, the Swift-BAT couldn't do the necessary follow-up it normally would have to learn more about it until hours later. TESS happened to be looking at that same part of the sky. That was sheer luck, as TESS turns its attention to a new strip of the sky every month. While exoplanet researchers at a ground-base for TESS could tell right away that a gamma-ray burst had happened, it would be months before they got any data from the TESS satellite on it. But since their focus was on new planets, these researchers asked if any other scientists at a TESS conference in Sydney, Australia were interested in doing more digging on the blast. TESS is an optical telescope that collects light curves on everything in its field of view, every half hour. Light curves are a graph of light intensity of a celestial object or region as a function of time. Smith analyzed three of these light curves to be able to determine how bright the burst was. Data from ground-based observatories and the Swift gamma-ray satellite were used to determine the burst's distance and other qualities about it.

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