Topic: Early August Astronomy Bulletin

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ROSETTA COMET MAY BE A CONTACT BINARY

NASA

ESA's Rosetta probe is approaching Comet 67P/Churyumov-Gerasimenko for a historic mission to orbit and land on the comet's nucleus. As Rosetta approaches the comet (it is now less than 9,000 km away), the form of the nucleus is coming into focus, as an irregular shape.  There were hints of that in previous images, and it has become clear that this is no ordinary comet. Like its name, it seems that comet C-G is in two parts. Such dual objects, known as 'contact binaries' in comet and asteroid terminology, are not uncommon. Indeed, comet Tuttle is thought to be such an object: radio imaging by the ground- based Arecibo telescope in Puerto Rico in 2008 suggested that it consists of two roughly spherical objects. Comet Hartley 2, imaged during the EPOXI fly-by in 2011, was revealed as bone-shaped, with two distinct halves separated by a smooth region. In addition, observations of asteroid 25143 Itokawa by the Hayabusa mission, combined with ground-based data, showed an asteroid comprised of two sections of highly contrasting densities.

Different views exist as to how such objects form. One theory is that they could arise when two comets merged after a low-velocity collision during the Solar System's formation, when small building blocks of rocky and icy debris coalesced to create planets. Or it might be the other way around -- a single comet could be distorted into a curious shape by the strong gravitational pull of a large object like Jupiter or the Sun; after all, at least some comets are rubble piles with little internal strength, as directly witnessed in the fragmentation of comet Shoemaker-Levy 9 and the subsequent impacts into Jupiter 20 years ago. Or Comet C-G may once have been a much rounder object that became highly asymmetrical as a result of ice evaporation. One could also speculate that the striking dichotomy of the comet's morphology is the result of a near-catastrophic impact event. Another idea is that a large outburst event may have weakened one side of the comet so much that it simply gave away, crumbling into space. With not long to go before the August 6 rendezvous, some of the questions may soon be answered.

EXO-PLANET WITH LONGEST KNOWN YEAR

Harvard-Smithsonian Center for Astrophysics

Astronomers have discovered a transiting exo-planet with the longest known 'year'. Kepler-421b circles its star once every 704 days. For comparison, Mars orbits the Sun in 780 days. Most of the 1,800-plus exo-planets discovered to date are much closer to their stars and have much shorter orbital periods. Finding Kepler-421b was a stroke of luck, since the further a planet is from its star, the less likely it is to transit it. Kepler-421b is a Uranus-sized planet orbiting an orange K-type star that is cooler and dimmer than the Sun at a distance of about 110 million miles, where its temperature is about -90 C.

The Kepler spacecraft stared at the same patch of sky for 4 years, watching for stars that dim as planets cross in front of them. Kepler detected only two transits of Kepler-421b, owing to the length of its orbital period. The planet's orbit places it beyond the "snow line" -- the dividing line between rocky and gas planets. Beyond the snow line, water condenses into ice grains that stick together to build gas-giant planets. The snow line is a crucial distance in planet-formation theory, and astronomers think that all gas giants must have formed beyond that distance. Since some gas-giant planets have been found extremely close to their stars, in orbits lasting days or even hours, theorists believe that many exo-planets migrate inward early in their history. Kepler-421b shows that such migration is not necessary and it could have formed right where we see it now. The host star, Kepler-421, is located about 1,000 light-years away in the direction of the constellation Lyra.

FERMI FINDS A 'TRANSFORMER' PULSAR

NASA

In 2013 June, an exceptional binary-star system containing a rapidly spinning neutron star underwent a dramatic change in behaviour never before observed. The pulsar's radio beacon vanished, while at the same time the system brightened fivefold in gamma rays, the most energetic form of light, according to measurements by the Fermi gamma-ray space telescope. The system seemed suddenly to switch from a lower-energy state to a higher-energy one. The change appears to reflect an erratic interaction between the pulsar and its companion, a presumably rare transitional phase in the history of that binary. The system, known as AY Sextantis, located about 4,400 light-years away in the constellation Sextans, pairs a 1.7-millisecond pulsar named PSR J1023+0038 -- J1023 for short -- with a star of about one-fifth the mass of the Sun. The stars complete an orbit in only 4.8 hours, which places them so close together that the pulsar will gradually evaporate its companion. When a massive star collapses and explodes as a supernova, its crushed core may survive as a compact remnant called a neutron star or pulsar, an object in which more mass than the Sun's is packed into a sphere only about 13 km across. Young isolated neutron stars rotate tens of times each second and generate beams of radio waves, visible light, X-rays and gamma rays that astronomers observe as pulses whenever the beams sweep past the Earth. Pulsars also generate powerful outflows, or winds, of high-energy particles moving near the speed of light. The power for all the emissions comes from the pulsar's rapidly spinning magnetic field, and over time, as the pulsars slow down, the emissions fade.

More than 30 years ago, astronomers discovered another type of pulsar revolving in 10 milliseconds or less, with rotational speeds up to 43,000 rpm. While young pulsars usually appear in isolation, more than half of millisecond pulsars occur in binary systems, which suggested an explanation for their rapid spin. Astronomers have long suspected that millisecond pulsars were spun up through the transfer and accumulation of matter from their companion stars, so they are sometimes referred to as recycled pulsars. During the initial mass-transfer stage, such a system would qualify as a low-mass X-ray binary, with a slower-spinning neutron star emitting X-ray pulses as hot gas raced towards its surface. A billion years later, when the flow of matter comes to a halt, the system would be classified as a spun-up millisecond pulsar with radio emissions powered by a rapidly  rotating magnetic field. In an effort to understand J1023's spin and orbital evolution, the system was regularly monitored in radio using the Lovell telescope in England and the Westerbork one in Holland.  The observations showed that the pulsar's radio signal had turned off, and prompted the search for an associated change in its gamma-ray properties. A few months before that, astronomers found a much more distant system that alternated between radio and X-ray states in a matter of weeks. Located in M28, a globular star cluster about 19,000 light-years away, a pulsar known as PSR J1824-2452I underwent an X-ray outburst in 2013 March and April. As the X-ray emission dimmed in early May, the pulsar's radio beam emerged. While J1023 reached much higher energies and is considerably closer, the two binaries are otherwise quite similar. What is thought to be happening is the final stage of the spin-up process for those pulsars. 

In J1023, the stars are close enough for a stream of gas to flow from the ordinary star towards the pulsar. The pulsar's rapid rotation and intense magnetic field are responsible for both the radio beam and its powerful pulsar wind. When the radio beam is detectable, the pulsar wind holds back the companion's gas stream, preventing it from approaching too closely. But now and then the stream surges, pushing its way closer to the pulsar and establishing an accretion disc. Gas in the disc becomes compressed and heated, reaching temperatures hot enough to emit X-rays. Next, material along the inner edge of the disc quickly loses orbital energy and falls towards the pulsar. When it has fallen to an altitude of about 80 km, processes involved in creating the radio beam are either shut down or, more likely, obscured. The inner edge of the disc probably fluctuates considerably at that altitude. Some of it may become accelerated outwards at nearly the speed of light, forming dual particle jets firing in opposite directions -- a phenomenon more typically associated with accreting black holes. Shock waves within and along the periphery of the jets are a probable source of the bright gamma-ray emission detected by Fermi.

SEVEN DWARF GALAXIES DISCOVERED WITH NEW TELESCOPE

Yale University

Yale University astronomers, using a new type of telescope made by stitching together telephoto lenses, recently discovered seven celestial surprises while observing a nearby spiral galaxy. The previously unseen galaxies may yield important insights into dark matter and galaxy evolution, while possibly signalling the discovery of a new class of objects in space. For now, scientists know they have found a septuplet of new galaxies that were previously overlooked because of their diffuse nature. The ghostly galaxies were found in the first observations from the 'home-made' telescope. The discovery came quickly, in a relatively small section of sky. The robotic telescope was designed in a collaboration between Yale and the University of Toronto. Their 'Dragonfly' telephoto array uses eight telephoto lenses with special coatings that suppress internally scattered light. That makes the telescope particularly adept at detecting the low surface brightness of the very diffuse newly discovered galaxies. The telescope was named Dragonfly because the lenses resemble the compound eye of an insect.

The Yale scientists hope to determine whether the seven newly found dwarf galaxies are orbiting around the M101 spiral galaxy, or if they are really located much closer or farther away, and appear in the same direction as M101 just by chance. The possibilities are intriguing enough for the team to have been granted the opportunity to use the Hubble space telescope for further study.

SPIRAL BRIDGE OF YOUNG STARS LINKING ANCIENT GALAXIES

Space Telescope Science Institute (STScI)

The Hubble telescope has photographed a structure 100,000 light-years long, which resembles a corkscrew-shaped string of pearls and winds round the cores of two colliding galaxies. The unusual structure of the star spiral may yield new insights into the formation of stellar superclusters that result from merging galaxies and gas dynamics in that rarely-seen process. Researchers have long known that the 'beads on a string' phenomenon is seen in the arms of spiral galaxies and in tidal bridges between interacting galaxies. However, this particular supercluster arrangement has never been seen before in giant merging elliptical galaxies. Young, blue super star clusters are evenly spaced along the chain through the galaxies at separations of 3,000 light-years. The pair of elliptical galaxies is embedded deep inside the dense cluster of galaxies known as SDSS J1531+3414. The cluster's powerful gravity warps the images of background galaxies into blue streaks and arcs that give the illusion of being inside the cluster, the effect known as gravitational lensing.

The observation is part of a Hubble programme to observe 23 massive clusters that create powerful gravitational lensing effects on the sky. The clusters were first catalogued in the Sloan Digital Sky Survey (SDSS), a project to create a detailed three-dimensional maps of the Universe. The team discovered the bizarre string of stellar superclusters by chance, while reviewing images as they came in from Hubble. The underlying physical processes that give rise to the 'string of pearls' structure are related to the Jeans instability, a physics phenomenon that occurs when the internal pressure of an interstellar gas cloud is not strong enough to prevent gravitational collapse of a region filled with matter, resulting in star formation.  The process is analogous to that which causes a column of water falling from a rain cloud to disrupt, and rain to fall in drops rather than in continuous streams. Scientists are currently trying to understand the star chain's origin. One possibility is that the cold molecular gas fuelling the burst of star formation may have been native to the two merging galaxies. Another possibility is a so-called 'cooling flow' scenario, where gas cools from the ultra-hot (10-million-degree) atmosphere of plasma that surrounds the galaxies, forming pools of cold molecular gas that start to form stars. A third possibility is that the cold gas fuelling the chain of star formation was compressed by a shock wave created when the two giant elliptical galaxies collided.

A HOT SPOT FOR POWERFUL COSMIC RAYS

University of Utah

An observatory run by the University of Utah found a hot spot beneath the Plough emitting a disproportionate number of the highest-energy cosmic rays. The discovery moves physics another step towards identifying the sources of the most energetic particles in the Universe. Many astrophysicists suspect that ultra-high-energy cosmic rays are generated by active galactic nuclei, or AGNs, in which material is sucked into a supermassive black hole at the centre of a galaxy, while other material is ejected in a beam-like jet known as a blazar. Another possibility is that the highest-energy cosmic rays come from some supernovae (exploding stars) that emit gamma-ray bursts. Lower-energy cosmic rays come from the Sun, other stars and exploding stars, but the source or sources of the most energetic cosmic rays remains uncertain. Ultra-high-energy cosmic rays (those above with energies above 10*18 electron volts) come from beyond our galaxy, but 90% of them must come from within 100 megaparsecs because powerful cosmic rays from greater distances are greatly weakened by interaction with the cosmic microwave background radiation -- the faint afterglow of the Big Bang.

The Telescope Array uses two methods to detect and measure cosmic rays. At three locations spread across the desert, sets of mirrors called fluorescence detectors watch the skies for faint blue flashes created when incoming cosmic rays hit nitrogen gas molecules in the atmosphere. The collisions create a cascade of other collisions with atmospheric gases, resulting in 'air showers' of particles that are detected by 523 table-like scintillation detectors spaced over 300 square miles of desert. In the new study, 507 of the scintillation detectors were used to study the ultra-high-energy cosmic rays. The fluorescence detectors helped to determine the energy and chemical make-up of the cosmic-ray particles. The new study by the Telescope Array research team looked at ultra-high-energy cosmic rays above 5.7 times 10*18 electron volts. The high cutoff was picked because the highest-energy cosmic rays are bent the least by magnetic fields in space -- bending that obscures the directions from which they came and thus the directions of their sources. Those very powerful cosmic rays were recorded by the Telescope Array between 2008 and 2013. During the five years, only 72 such cosmic rays were detected, confirmed and analyzed for their energy and source direction. 19 of the 72 came from the direction of the hot spot, compared with only 4.5 that would have been expected if the cosmic rays came randomly from all parts of the sky. The hot spot is a 40-degree-diameter circle representing 6% of the northern sky. It is centred in the southwest corner of Ursa Major (which includes the Plough) at right ascension 146.6 degrees and declination 43.2 degrees. It is near the 'supergalactic plane' -- the rather flattened Virgo supercluster of galaxies. Our Milky Way galaxy is on the outskirts of the supercluster. Observations by the Pierre Auger cosmic-ray observatory in Argentina provide evidence for a weaker Southern Hemisphere hot spot.

A STAND-IN FOR THE FIRST STARS

Goddard Space Flight Center

Astronomers analyzing a long-lasting blast of high-energy light observed in 2013 report finding features strikingly similar to those expected from an explosion from the Universe's earliest stars. If that interpretation is correct, the outburst validates ideas about a recently identified class of gamma-ray bursts and serves as a stand-in for what future observatories may see as the last acts of the first stars. Gamma-ray bursts (GRBs) are the most luminous explosions in the Universe. The blasts emit outbursts of gamma rays and X-rays, and produce rapidly fading afterglows that can be observed in visible-light, infrared and radio wavelengths. On average, the Swift satellite, Fermi gamma-ray space telescope and other spacecraft detect about one GRB each day. Shortly after 04:11 UT on 2013 Sept. 25, Swift triggered on a spike of gamma rays from a source in the constellation Fornax. The spacecraft automatically alerted observatories around the world that a new burst -- designated GRB 130925A, after the date -- was in progress and turned its X-ray telescope towards the source. Other satellites also detected the rising tide of high-energy radiation, including Fermi, the Russian Konus instrument on the Wind spacecraft, and the INTEGRAL observatory. 

The burst was eventually localized to a galaxy 3.9 billion light-years away. Astronomers have observed thousands of GRBs over the past 50 years. Until recently, they were classified into two groups, short and long, based on the duration of the gamma-ray signal. Short bursts, lasting only two seconds or less, are thought to represent a merger of compact objects in a binary system, with the most likely suspects being neutron stars and black holes. Long GRBs may last anywhere from several seconds to several minutes, with typical durations between 20 and 50 seconds. They are thought to be associated with the collapse of a star many times the Sun's mass and the resulting birth of a new black hole. GRB 130925A, by contrast, produced gamma rays for 1.9 hours. Observations by Swift's X-ray telescope showed an intense and highly variable X-ray afterglow that exhibited strong flares for six hours, after which it finally began the steady fade-out usually seen in long GRBs. GRB 130925A is a member of a rare and newly recognized class of ultra-long bursts, but what really sets it apart is its unusual X-ray afterglow, which provides the strongest case yet that ultra-long GRBs come from blue supergiant stars. Astronomers think Wolf-Rayet stars best explain the origin of long GRBs. Born with more than 25 times the Sun's mass, they are so hot that they drive away their outer hydrogen envelopes through an outflow called a stellar wind. By the time the star collapses, its outer atmosphere is almost gone and its physical size is comparable to the Sun's. A black hole forms in the star's core and matter falling towards it powers jets that burrow through the star.  The jets continue operating for a few tens of seconds -- the time- scale of long GRBs.

Because ultra-long GRBs last hundreds of times longer, the source star must have a correspondingly greater physical size. The most likely suspect, astronomers say, is a blue supergiant, a hot star with about 20 times the Sun's mass that retains its deep hydrogen atmosphere,making it roughly 100 times the Sun's diameter. Better yet, blue supergiants containing only a very small fraction of elements heavier than helium -- metals, in astronomical parlance -- could be substantially larger. A star's metal content controls the strength of its stellar wind, and that in turn determines how much of its hydrogen atmosphere it retains before collapse. For the largest blue supergiants, the hydrogen envelope would take hours to fall into the black hole, providing a sustained fuel source to power ultra-long GRBs. The researchers note that radio observations of the GRB afterglow show that it displayed nearly constant brightness over a period of four months. Its extremely slow decline suggests that the explosion's blast wave was moving practically unimpeded through space, which means that the environment around the star must have been largely free of material cast off by a stellar wind. The burst's long-lived X-ray flaring proved a more puzzling feature to explain, requiring observations from Swift, NASA's Chandra X-ray Observatory and ESA's XMM-Newton satellite to sort out. As the high-energy jet bores through the collapsing star, its leading edge rams into cooler stellar gas and heats it. That gas flows down the sides of the jet, surrounding it in a hot X-ray-emitting sheath. Because the jet travels a greater distance through a blue supergiant, that cocoon becomes much more massive than is possible in a Wolf-Rayet star.  While the cocoon should expand rapidly as it leaves the star, the X-ray evidence indicates that it remained intact. The scientific team suggests that magnetic fields may have suppressed the flow of hot gas across the cocoon, keeping it confined close to the jet. The astronomers conclude that the best explanation for the unusual properties of GRB 130925A is that it heralded the demise of a metal- poor blue supergiant, a model they suggest may characterize the entire ultra-long class.