Category: Astronomy

The stars life sequence, ending with the formation of a black hole. Image credit: Nicolle Rager Fuller/NSF Click to enlarge
Just a few hundred millions years after the Big Bang, a massive star exhausted its fuel, collapsed as a black hole, and exploded as a gamma ray burst. The radiation from this catastrophic event has only now reached Earth, and astronomers are using it to peer back to the earliest moments of the Universe. The burst, named GRB 050904, was observed by NASA’s Swift satellite on September 4, 2005. One unusual thing about this burst is that it lasted for 500 seconds – most are over in a fraction of that time.

It came from the edge of the visible universe, the most distant explosion ever detected.

In this week’s issue of Nature, scientists at Penn State University and their U.S. and European colleagues discuss how this explosion, detected on 4 September 2005, was the result of a massive star collapsing into a black hole.

The explosion, called a gamma-ray burst, comes from an era soon after stars and galaxies first formed, about 500 million to 1 billion years after the Big Bang. The universe is now 13.7 billion years old, so the September burst serves as a probe to study the conditions of the early universe.

“This was a massive star that lived fast and died young,” said David Burrows, senior scientist and professor of astronomy and astrophysics at Penn State, a co-author on one of the three reports about this explosion published this week in Nature. “This star was probably quite different from the kind we see today, the type that only could have existed in the early universe.”

The burst, named GRB 050904 after the date it was spotted, was detected by NASA’s Swift satellite, which is operated by Penn State. Swift provided the burst coordinates so that other satellites and ground-based telescopes could observe the burst. Bursts typically last only 10 seconds, but the afterglow will linger for a few days.

GRB 050904 originated 13 billion light years from Earth, which means it occurred 13 billion years ago, for it took that long for the light to reach us. Scientists have detected only a few objects more than 12 billion light years away, so the burst is extremely important in understanding the universe beyond the reach of the largest telescopes.

“Because the burst was brighter than a billion suns, many telescopes could study it even from such a huge distance,” said Burrows, whose analysis focuses mainly on Swift data from its three telescopes, covering a range of gamma-rays, X-rays, and ultraviolet/optical wavelengths, respectively. Burrows is the lead scientist for Swift’s X-ray telescope.

The Swift team found several unique features in GRB 050904. The burst was long–lasting about 500 seconds–and the tail end of the burst exhibited multiple flares. These characteristics imply that the newly created black hole didn’t form instantly, as some scientists have thought, but rather it was a longer, chaotic event.

Closer gamma-ray bursts do not have as much flaring, implying that the earliest black holes may have formed differently from ones in the modern era, Burrows said. The difference could be because the first stars were more massive than modern stars. Or, it could be the result of the environment of the early universe when the first stars began to convert hydrogen and helium (created in the Big Bang) into heavier elements.

GRB 050904, in fact, shows hints of newly minted heavier elements, according to data from ground-based telescopes. This discovery is the subject of a second Nature article by a Japanese group led by Nobuyuki Kawai at the Tokyo Institute of Technology.

GRB 050904 also exhibited time dilation, a result of the vast expansion of the universe during the 13 billion years that it took the light to reach us on Earth. This dilation results in the light appearing much redder than when it was emitted in the burst, and it also alters our perception of time as compared to the burst’s internal clock.

These factors worked in the scientists’ favor. The Penn State team turned Swift’s instruments onto the burst about 2 minutes after the event began. The burst, however, was evolving as if it were in slow motion and was only about 23 seconds into the bursting. So scientists could see the burst at a very early stage.

Only one other object–a quasar–has been discovered at a greater distance. Yet, whereas quasars are supermassive black holes containing the mass of billions of stars, this burst comes from a single star. The detection of GRB 050904 confirms that massive stars mingled with the oldest quasars. It also confirms that even more explosions of distant stars–perhaps from the first stars, theorists say–can be studied through a combination of observations with Swift and other world-class telescopes.

“We designed Swift to look for faint bursts coming from the edge of the universe,” said Neil Gehrels of NASA Goddard Space Flight Center in Greenbelt, Maryland, Swift’s principal investigator. “Now we’ve got one and it’s fascinating. For the first, time we can learn about individual stars from near the beginning of time. There are surely many more out there.”

Swift was launched in November 2004 and was fully operational by January 2005. Swift carries three main instruments: the Burst Alert Telescope, the X-ray Telescope, and the Ultraviolet/Optical Telescope. Swift’s gamma-ray detector, the Burst Alert Telescope, provides the rapid initial location, was built primarily by the NASA Goddard Space Flight Center in Greenbelt and Los Alamos National Laboratory, and was constructed at GSFC. Swift’s X-Ray Telescope and UV/Optical Telescope were developed and built by international teams led by Penn State and drew heavily on each institution’s experience with previous space missions. The X-ray Telescope resulted from Penn State’s collaboration with the University of Leicester in England and the Brera Astronomical Observatory in Italy. The Ultraviolet/Optical Telescope resulted from Penn State’s collaboration with the Mullard Space Science Laboratory of the University College-London. These three telescopes give Swift the ability to do almost immediate follow-up observations of most gamma-ray bursts because Swift can rotate so quickly to point toward the source of the gamma-ray signal.

Original Source: PSU News Release

Artist’s concept symbollically represents early universe organic complex compounds. Image credit: NASA/JPL Click to enlarge
By studying distant galaxies with the Spitzer space telescope, researchers have come to the conclusion that the intense radiation of infant galaxies was very destructive to life’s building blocks. Shortly after the Big Bang, these young galaxies blazed in star formation, but they had very few organic molecules – which are quite common in older galaxies. Even through these organic molecules will be forming in young stars, their intense radiation destroys them again.

The components of life may have been under attack in the hostile environments of the universe’s first galaxies, say astronomers using NASA’s Spitzer Space Telescope.

A science team led by graduate student Yanling Wu of Cornell University, Ithaca, N.Y., recently came to this conclusion after studying the formation and destruction of polycyclic aromatic hydrocarbons molecules (PAHs) in more than 50 blue compact dwarf (BCD) galaxies. These organic molecules, comprised of hydrogen and carbon, are believed by many scientists to be among the building blocks for life.

“One of the outstanding problems in astronomy today is whether complex organic molecules of hydrogen and carbon, similar to those responsible for life on Earth, are present in the early universe,” says Wu.

According to Wu, mature massive galaxies like our Milky Way formed from the merging of smaller galaxies, probably about the size of nearby BCD galaxies. Since current technology is not sensitive enough to easily identify and study in detail the universe’s first galaxies, astronomers must infer the physical properties of the early structures by observing similar nearby galaxies like BCDs.

“We believe that BCD galaxies are similar to the universe’s first galaxies because they are infant galaxies, actively forming stars, and are not very chemically polluted,” said Wu.

Because most atomic elements other than hydrogen and helium are born from the death of stars, astronomers suspect that in the first few million years after the big bang galaxies were not “chemically polluted” with elements other than hydrogen and helium. In astronomy, these relatively unpolluted galaxies are said to have low metallicity.

The BCD galaxies’ blue colors tell astronomers that these structures are actively forming massive stars. By logically combining the galaxy’s blue color with the fact that it is low in metals, astronomers can infer that this is a young galaxy.

In her research, Wu found that nearby BCD galaxies with lowest metallicity also had little or no PAHs. As the galaxies became more chemically polluted, more traces of PAHs were found. She notes that this phenomenon makes sense because heavy metal elements like carbon are formed from the death of stars, and some of these galaxies may just be too “young” to have produced enough carbon to create PAHs.

However, in some of the BCD galaxies where the conditions allow for the formation of PAHs, Wu found that those molecules were being destroyed by intense ultraviolet radiation from the young massive stars.

“Because BCD galaxies are metal poor and very compact, the intense ultraviolet radiation from young stars will destroy PAH molecules even if they are formed,” says Wu. “The threshold for when these PAH molecules stop being destroyed is still uncertain.”

“This leads to an interesting paradox, where the young stars responsible for the formation of PAHs may also be the main culprit of their destruction,” adds co-author Dr. Vassilis Charmandaris, of the University of Greece, Heraklion.

The organic PAHs were detected using Spitzer’s Infrared Spectrometer (IRS).

“Yanling has made significant progress in a research area first opened by International Space Observatory ,” says Dr. Jim Houck of Cornell University. Houck is Wu’s academic advisor and a co-author of the paper. He is also the Principal Investigator for Spitzer’s IRS instrument and played a vital role in its creation.

“With Spitzer, Yanling is able to extend BCDs observations to a much larger sample; the new results provide a glimpse into the formation of galaxies in the early Universe,” he adds.

Wu’s paper will be published in a March issue of Astrophysical Journal. For more information on this discovery please listen to the podcast interview with Yanling Wu.

Original Source: Spitzer Space Telescope

The current understanding of a pulsar. Image credit: Jodrell Bank Observatory. Click to enlarge
Astronomers have discovered a very unusual pulsar that seems to switch off from time to time. It looks like a normal pulsar for about a week, blasting out radio waves, and then goes silent for about a month. This pulsar is slowing down its rate of rotation, but this deceleration increases when it’s active. This braking mechanism is related to the powerful radio emissions. During its active phase, a wind of particles is spewed off, stealing some of its rotational energy.

Astronomers using the 76-m Lovell radio telescope at the University of Manchester’s Jodrell Bank Observatory have discovered a very strange pulsar that helps explain how pulsars act as ‘cosmic clocks’ and confirms theories put forward 37 years ago to explain the way in which pulsars emit their regular beams of radio waves – considered to be one of the hardest problems in astrophysics. Their research, now published in Science Express, reveals a pulsar that is only ‘on’ for part of the time. The strange pulsar is spinning about its own axis and slows down 50% faster when it is ‘on’ compared to when it is ‘off’.

Pulsars are dense, highly magnetized neutron stars that are born in a violent explosion marking the death of massive stars. They act like cosmic lighthouses as they project a rotating beam of radio waves across the galaxy. Dr Michael Kramer explains, “Pulsars are a physicist’s dream come true. They are made of the most extreme matter that we know of in the Universe, and their highly stable rotation makes them super-precise cosmic clocks – but, embarrassingly, we do not know how these clocks work. This discovery goes a long way towards solving this problem.”

The current understanding of a pulsar. The central neutron star is highly magnetised and emits a radio beam along its magnetic axis, which is inclined to the rotation axis. The strong magnetic field eventually leads to the extraction of particles from the surface, filling the surrounding, so-called magnetosphere with plasma. The size of the magnetosphere is given by the distance where plasma co-rotation reaches the speed of light, the so-called light-cylinder. The plasma creating the radio emission eventually leaves the light cylinder as a pulsar wind, which provides a torque onto the pulsar, contributing about 50% to its observed slow-down in rotation.

The research team, led by Dr Kramer, found a pulsar that is only periodically active. It appears as a normal pulsar for about a week and then “switches off” for about one month before emitting pulses again. The pulsar, called PSR B1931+24, is unique in this behaviour and affords astronomers an opportunity to compare its quiet and active phases. As it is quiet the majority of the time, it is difficult to detect, suggesting that there may be many other similar objects that have, so far, escaped detection.

Prof Andrew Lyne points out that, “After the discovery of pulsars, theoreticians proposed that strong electric fields rip particles out of the neutron star surface into a surrounding magnetised cloud of plasma called the magnetosphere – but, for nearly 40 years, there had been no way to test whether our basic understanding was correct.”

The University of Manchester astronomers were delighted when they found that this pulsar slows down more rapidly when the pulsar is on than when it is off. Dr Christine Jordan points out the importance of this discovery, “We can clearly see that something hits the brakes when the pulsar is on.”

This breaking mechanism must be related to the radio emission and the processes creating it and the additional slow-down can be explained by a wind of particles leaving the pulsar’s magnetosphere and carrying away rotational energy. “Such a braking effect of the pulsar wind was expected but now, finally, we have observational evidence for it” adds Dr Duncan Lorimer.

The amount of braking can be related to the number of charges leaving the pulsar magnetosphere. Dr Kramer explains their surprise when it was found that the resulting number was within 2% of the theoretical predictions. “We were really shocked when we saw these numbers on our screens. Given the pulsar’s complexity, we never really expected the magnetospheric theory to work so well.”

Prof Lyne summarized the result: “It is amazing that, after almost 40 years, we have not only found a new, unusual, pulsar phenomenon but also a very unexpected way to confirm some fundamental theories about the nature of pulsars.”

Original Source: PPARC News Release

Andromeda Galaxy taken in ultraviolet. Image credit: GALEX Click to enlarge
Astronomers have long believed that the Andromeda galaxy had a different upbringing from our own Milky Way, but now it seems we aren’t so different after all. An international team of researchers have completed a survey of the metal content in Andromeda’s halo, and found that it’s relatively metal poor – just like the Milky Way. If both galaxies have the same amount of metal in their halos, that means they probably evolved in similar ways; both got started half a billion years after the Big Bang and grew from a collection of protogalactic fragments.

For the last decade, astronomers have thought that the Andromeda galaxy, our nearest galactic neighbor, was rather different from the Milky Way. But a group of researchers have determined that the two galaxies are probably quite similar in the way they evolved, at least over their first several billion years.

In an upcoming issue of the Astrophysical Journal, Scott Chapman of the California Institute of Technology, Rodrigo Ibata of the Observatoire de Strasbourg, and their colleagues report that their detailed studies of the motions and metals of nearly 10,000 stars in Andromeda show that the galaxy’s stellar halo is “metal-poor.” In astronomical parlance, this means that the stars lying in the outer bounds of the galaxy are pretty much lacking in all the elements heavier than hydrogen.

This is surprising, says Chapman, because one of the key differences thought to exist between Andromeda and the Milky Way was that the former’s stellar halo was metal-rich and the latter’s was metal-poor. If both galaxies are metal-poor, then they must have had very similar evolutions.

“Probably, both galaxies got started within a half billion years of the Big Bang, and over the next three to four billion years, both were building up in the same way by protogalactic fragments containing smaller groups of stars falling into the two dark-matter haloes,” Chapman explains.

While no one yet knows what dark matter is made of, its existence is well established because of the mass that must exist in galaxies for their stars to orbit the galactic centers the way they do. Current theories of galactic evolution, in fact, assume that dark-matter wells acted as a sort of “seed” for today’s galaxies, with the dark matter pulling in smaller groups of stars as they passed nearby. What’s more, galaxies like Andromeda and the Milky Way have each probably gobbled up about 200 smaller galaxies and protogalactic fragments over the last 12 billion years.

Chapman and his colleagues arrived at the conclusion about the metal-poor Andromeda halo by obtaining careful measurements of the speed at which individual stars are coming directly toward or moving directly away from Earth. This measure is called the radial velocity, and can be determined very accurately with the spectrographs of major instruments such as the 10-meter Keck-II telescope, which was used in the study.

Of the approximately 10,000 Andromeda stars for which the researchers have obtained radial velocities, about 1,000 turned out to be stars in the giant stellar halo that extends outward by more than 500,000 light-years. These stars, because of their lack of metals, are thought to have formed quite early, at a time when the massive dark-matter halo had captured its first protogalactic fragments.

The stars that dominate closer to the center of the galaxy, by contrast, are those that formed and merged later, and contain heavier elements due to stellar evolution processes.

In addition to being metal-poor, the stars of the halo follow random orbits and are not in rotation. By contrast, the stars of Andromeda’s visible disk are rotating at speeds upwards of 200 kilometers per second.

According to Ibata, the study could lead to new insights on the nature of dark matter. “This is the first time we’ve been able to obtain a panoramic view of the motions of stars in the halo of a galaxy,” says Ibata. “These stars allow us to weigh the dark matter, and determine how it decreases with distance.”

In addition to Chapman and Ibata, the other authors are Geraint Lewis of the University of Sydney; Annette Ferguson of the University of Edinburgh; Mike Irwin of the Institute of Astronomy in Cambridge, England; Alan McConnachie of the University of Victoria; and Nial Tanvir of the University of Hertfordshire.

Original Source: Caltech News Release

Shock wave in Stephen’s Quintet captured by Spitzer. Image credit: NASA/JPL-Caltech. Click to enlarge
This photograph, taken by the Spitzer space telescope and a ground-based telescope in Spain, shows the Stephan’s Quintet galaxy cluster, with one of the largest shockwaves ever seen in the Universe. The green arc in the photograph is the point which two galaxies are colliding. There are actually 5 galaxies in this photograph, but two have been so beaten up, all that’s left are their bright centers. The galaxies are located 300 million light-years away in the Pegasus constellation.

This false-color composite image of the Stephan’s Quintet galaxy cluster clearly shows one of the largest shock waves ever seen (green arc), produced by one galaxy falling toward another at over a million miles per hour. It is made up of data from NASA’s Spitzer Space Telescope and a ground-based telescope in Spain.

Four of the five galaxies in this image are involved in a violent collision, which has already stripped most of the hydrogen gas from the interiors of the galaxies. The centers of the galaxies appear as bright yellow-pink knots inside a blue haze of stars, and the galaxy producing all the turmoil, NGC7318b, is the left of two small bright regions in the middle right of the image. One galaxy, the large spiral at the bottom left of the image, is a foreground object and is not associated with the cluster.

The titanic shock wave, larger than our own Milky Way galaxy, was detected by the ground-based telescope using visible-light wavelengths. It consists of hot hydrogen gas. As NGC7318b collides with gas spread throughout the cluster, atoms of hydrogen are heated in the shock wave, producing the green glow.

Spitzer pointed its infrared spectrograph at the peak of this shock wave (middle of green glow) to learn more about its inner workings. This instrument breaks light apart into its basic components. Data from the instrument are referred to as spectra and are displayed as curving lines that indicate the amount of light coming at each specific wavelength.

The Spitzer spectrum showed a strong infrared signature for incredibly turbulent gas made up of hydrogen molecules. This gas is caused when atoms of hydrogen rapidly pair-up to form molecules in the wake of the shock wave. Molecular hydrogen, unlike atomic hydrogen, gives off most of its energy through vibrations that emit in the infrared.

This highly disturbed gas is the most turbulent molecular hydrogen ever seen. Astronomers were surprised not only by the turbulence of the gas, but by the incredible strength of the emission. The reason the molecular hydrogen emission is so powerful is not yet completely understood.

Stephan’s Quintet is located 300 million light-years away in the Pegasus constellation.

This image is composed of three data sets: near-infrared light (blue) and visible light called H-alpha (green) from the Calar Alto Observatory in Spain, operated by the Max Planck Institute in Germany; and 8-micron infrared light (red) from Spitzer’s infrared array camera.

Original Source: Spitzer Space Telescope

Artist’s illustration represents tightly-wound magnetic field confining jet. Image credit: NRAO/AUI/NSF. Click to enlarge
Radio astronomers have uncovered a dying star with twin jets of material confined by a powerful magnetic field. The star is located about 8,500 light-years away from Earth in the constellation of Aquila, and it’s in the process of forming a planetary nebula. Many stars like this produce elongated nebulae, where the star’s outer envelope is pushed away and channeled into tight jets. The jets come out in a corkscrew shape, which means that the star is slowly rotating.

Molecules spewed outward from a dying star are confined into narrow jets by a tightly-wound magnetic field, according to astronomers who used the National Science Foundation’s Very Long Baseline Array (VLBA) radio telescope to study an old star about 8,500 light-years from Earth.

The star, called W43A, in the constellation Aquila, is in the process of forming a planetary nebula, a shell of brightly-glowing gas lit by the hot ember into which the star will collapse. In 2002, astronomers discovered that the aging star was ejecting twin jets of water molecules. That discovery was a breakthrough in understanding how many planetary nebulae are formed into elongated shapes.

“The next question was, what is keeping this outpouring of material confined into narrow jets? Theoreticians suspected magnetic fields, and we now have found the first direct evidence that a magnetic field is confining such a jet,” said Wouter Vlemmings, a Marie Curie Fellow working at the Jodrell Bank Observatory of the University of Manchester in England.

“Magnetic fields previously have been detected in jets emitted by quasars and protostars, but the evidence was not conclusive that the magnetic fields were actually confining the jets. These new VLBA observations now make that direct connection for the very first time,” Vlemmings added.

By using the VLBA to study the alignment, or polarization, of radio waves emitted by water molecules in the jets, the scientists were able to determine the strength and orientation of the magnetic field surrounding the jets.

“Our observations support recent theoretical models in which magnetically-confined jets produce the sometimes-complex shapes we see in planetary nebulae,” said Philip Diamond, also of Jodrell Bank Observatory.

During their “normal” lives, stars similar to our Sun are powered by the nuclear fusion of hydrogen atoms in their cores. As they near the end of their lives they begin to blow off their outer atmospheres and eventually collapse down to a white dwarf star about the size of Earth. Intense ultraviolet radiation from the white dwarf causes the gas thrown off earlier to glow, producing a planetary nebula. Astronomers believe that W43A is in the transition phase that will produce a planetary nebula. That transition phase, they say, is probably only a few decades old, so W43A offers the astronomers a rare opportunity to watch the process.

While the stars that produce planetary nebulae are spherical, most of the nebulae themselves are not. Instead, they show complex shapes, many elongated. The earlier discovery of jets in W43A showed one mechanism that could produce the elongated shapes. The latest observations will help scientists understand the mechanisms producing the jets.

The water molecules the scientists observed are in regions nearly 100 billion miles from the old star, where they are amplifying, or strengthening, radio waves at a frequency of 22 GHz. Such regions are called masers, because they amplify microwave radiation the same way a laser amplifies light radiation.

The earlier observations showed that the jets are coming out from the star in a corkscrew shape, indicating that whatever is squirting them out is slowly rotating.

Vlemmings and Diamond worked with Hiroshi Imai of Kagoshima University in Japan. The astronomers reported their work in the March 2 issue of the scientific journal Nature.

The VLBA is a system of ten radio-telescope antennas, each with a dish 25 meters (82 feet) in diameter and weighing 240 tons. From Mauna Kea on the Big Island of Hawaii to St. Croix in the U.S. Virgin Islands, the VLBA spans more than 5,000 miles, providing astronomers with the sharpest vision of any telescope on Earth or in space. Dedicated in 1993, the VLBA has an ability to see fine detail equivalent to being able to stand in New York and read a newspaper in Los Angeles.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

Original Source: NRAO News Release

Model image of cepheid L Carinae. Image credit: Click to enlarge
The European Southern Observatory’s Very Large Telescope Interferometer has uncovered three Cepheid variable stars surrounded by a cocoon of hot gas. Cepheids are known to pulse in brightness at a regular rate, and used by astronomers to calculate relatively nearby distances. As a Cepheid pulses, the velocity of its photosphere changes dramatically. It could be that this envelope is stellar material left behind as the star grows and shrinks.

Using ESO’s Very Large Telescope Interferometer (VLTI) at Cerro Paranal, Chile, and the CHARA Interferometer at Mount Wilson, California, a team of French and North American astronomers has discovered envelopes around three Cepheids, including the Pole star. This is the first time that matter is found surrounding members of this important class of rare and very luminous stars whose luminosity varies in a very regular way. Cepheids play a crucial role in cosmology, being one of the first “steps” on the cosmic distance ladder.

The southern Cepheid L Carinae was observed with the VINCI and MIDI instrument at the VLTI, while Polaris (the Pole Star) and Delta Cephei (the prototype of its class) were scrutinised with FLUOR on CHARA, located on the other side of the equator. FLUOR is the prototype instrument of VINCI. Both were built by the Paris Observatory (France).

For most stars, the observations made with the interferometers follow very tightly the theoretical stellar models. However, for these three stars, a tiny deviation was detected, revealing the presence of an envelope.

“The fact that such deviations were found for all three stars, which however have very different properties, seems to imply that envelopes surrounding Cepheids are a widespread phenomenon”, said Pierre Kervella, one of the lead authors.

The envelopes were found to be 2 to 3 times as large as the star itself. Although such stars are rather large – about fifty to several hundreds of solar radii – they are so far away that they can’t be resolved by single telescopes. Indeed, even the largest Cepheids in the sky subtend an angle of only 0.003 arc second. To observe this is similar to viewing a two-storey house on the Moon.

Astronomers have thus to rely on the interferometric technique, which combines the light of two or more distant telescopes, thereby providing the angular resolution of a unique telescope as large as the separation between them. With the VLTI, it is possible to achieve a resolution of 0.001 arc second or less.

“The physical processes that have created these envelopes are still uncertain, but, in analogy to what happens around other classes of stars, it is most probable that the environments were created by matter ejected by the star itself”, said Antoine Merand, lead-author of the second paper describing the results.

Cepheids pulsate with periods of a few days. As a consequence, they go regularly through large amplitude oscillations that create very rapid motions of its apparent surface (the photosphere) with velocities up to 30 km/s, or 108 000 km/h! While this remains to be established, there could be a link between the pulsation, the mass loss and the formation of the envelopes.

Original Source: ESO News Release

The radio pulsar PSR B1259-63. Image credit: ESA Click to enlarge
ESA astronomers have witnessed something very unusual; a pulsar crashing through a ring of gas surrounding a companion star. As the pulsar passed through ring, it lit up the area in gamma and X-rays, visible to ESA’s XMM-Newton observatory. This companion star is several times more massive than our own Sun, and rotates so quickly that it’s constantly spewing material out into a ring of gas. The pulsar goes through this ring twice during its 3.4-year elliptical orbit

Astronomers have witnessed a never-seen-before event in observations by ESA’s XMM-Newton spacecraft – a collision between a pulsar and a ring of gas around a neighbouring star.

The rare passage, which took the pulsar plunging into and through this ring, illuminated the sky in gamma- and X-rays.

It has revealed a remarkable new insight into the origin and content of ‘pulsar winds’, which has been a long-standing mystery. The scientists described the event as a natural but ‘scaled-up’ version of the well-known Deep Impact satellite collision with Comet Tempel 1.

Their final analysis is based on a new observation from XMM-Newton and a multitude of archived data which will lead to a better understanding of what drives well-known ‘pulsar nebulae’, such as the colourful Crab and Vela pulsars.

“Despite countless observations, the physics of pulsar winds have remained an enigma,” said lead author Masha Chernyakova, of the Integral Science Data Centre, Versoix, Switzerland.

“Here we had the rare opportunity to see pulsar wind clashing with stellar wind. It is analogous to smashing something open to see what’s inside.”

A pulsar is a fast-spinning core of a collapsed star that was once about 10 to 25 times more massive than our Sun. The dense core contains about a solar mass compacted in a sphere about 20 kilometres across.

The pulsar in this observation, called PSR B1259-63, is a radio pulsar, which means most of the time it emits only radio waves. The binary system lies in the general direction of the Southern Cross about 5000 light-years away.

Pulsar wind comprises material flung away from the pulsar. There is ongoing debate about how energetic the winds are and whether these winds consist of protons or electrons. What Chernyakova’s team has found, although surprising, ties in neatly with other recent observations.

The team observed PSR B1259-63 orbiting a ‘Be’ star named SS 2883, which is bright and visible to amateur astronomers. ‘Be’ stars, so named because of certain spectral characteristics, tend to be a few times more massive than our Sun and rotate at astonishing speeds.

They rotate so fast that their equatorial region bulges and they become flattened spheres. Gas is consistently flung off such a star and settles into an equatorial ring around the star, with an appearance somewhat similar to the planet Saturn and its rings.

The pulsar plunges into the Be star’s ring twice during its 3.4-year elliptical orbit; but the plunges are only a few months apart, just before and after ‘periastron’, the point when the two objects in orbit are closest to each other. It is during the plunges that X-rays and gamma rays are emitted, and XMM-Newton detects the X-rays.

“For most of the 3.4-year orbit, both sources are relatively dim in X-rays and it is not possible to identify characteristics in the pulsar wind,” said co-author Andrii Neronov. “As the two objects draw closer together, sparks begin to fly.”

The new XMM-Newton data was collected nearly simultaneously with a HESS observation. HESS, the High Energy Stereoscopic System, is a new ground-based gamma-ray telescope in Namibia.

Announced last year, the HESS observation was puzzling in that the gamma-ray emission fell to a minimum at periastron and had two maximums, just before and after the periastron, the opposite of what scientists were expecting.

The XMM-Newton observation supports the HESS observation by showing how the maximums were generated by the double plunging into the Be star’s ring. By combining these two observations with radio observations from the last periastron event, the scientists now have a complete picture of this system.

Tracing the rise and fall of X-rays and gamma rays day after day as the pulsar dug through the Be star’s disk, the scientists could conclude that the wind of electrons at an energy level of 10-100 MeV is responsible for the observed X-ray light. (1 MeV represents one million electron volts.)

Although 10-100 MeV is energetic, this is about 1000 times less than the expected energy level of 100 TeV. Even more puzzling is the multi-TeV gamma-ray emission, which, although surely emanating from the 10-100 TeV wind electrons, seems to be produced differently to how it was thought before.

“The only fact that is crystal clear at the moment is that this is the pulsar system to watch if we want to understand pulsar winds,” said Chernyakova.

“Never have we seen pulsar wind in such detail. We are continuing with theoretical models now. We have some good explanation of the radio-to-TeV-gamma-ray behaviour of this funny system, but it is still ‘under construction.'”

Original Source: ESA Portal

The strange cosmic explosion that occured on February 18th. Image credit: SDSS/Swift Click to enlarge
The Swift satellite, whose mission control center is in State College, has detected a cosmic explosion that has sent scientists around the world scrambling to telescopes to document this startling event. Gamma-ray radiation from the source, detected on 18 February and lasting about half an hour, appears to be a precursor to a supernova, which is the death throes of a star much more massive than the Sun. “The observations indicate that this is an incredibly rare glimpse of an initial gamma-ray burst at the beginning of a supernova,” said Peter Brown, a Penn State graduate student and a member of the Swift science team.

Astronomers are using Swift, whose science and flight operations are controlled by Penn State from the Mission Operations Center in State College, to continue to observe the event. Scores of satellites and ground-based telescopes also are now trained on the sight, watching and waiting. Amateur astronomers in the northern hemisphere with a good telescope in dark skies also can view the source.

The explosion has the trappings of a gamma-ray burst, the most distant and powerful type of explosion known. This event, however, was about 25 times closer and 100 times longer than the typical gamma-ray burst. “This burst is totally new and unexpected,” said Neil Gehrels, Swift principal investigator at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “This is the type of unscripted event in our nearby universe that we hoped Swift could catch.”

The explosion, called GRB 060218 after the date it was discovered, originated in a star-forming galaxy about 440 million light-years away toward the constellation Aries. This is the second-closest gamma-ray burst ever detected, if indeed it is a true burst.

Derek Fox, assistant professor of astronomy and astrophysics at Penn State, who is leading the monitoring effort of GRB 060218 on the Hobby-Eberly Telescope, commented, “This is the burst we’ve been waiting eight years for,” referring to the closest-ever gamma-ray burst, which was detected in 1998. “The special capabilities of Swift, which was not operating in 1998, combined with the intense campaign of ground-based telescopes, should help unravel this mystery,” said Fox.

“There are still many unknowns,” said Penn State Professor of Astronomy and Astrophysics John Nousek, the Swift mission operations director at Penn State University in University Park, Pennsylvania. The burst of gamma rays lasted for nearly 2,000 seconds; in contrast, most such bursts last a few milliseconds to tens of seconds. The explosion also was surprisingly dim. “This could be a new kind of burst, or we might be seeing a gamma-ray burst from an entirely different angle,” he said. The standard theory for gamma-ray bursts is that the high-energy light is beamed in our direction. “This off-angle glance–a profile view, perhaps–has given us an entirely new approach to studying star explosions. Had this burst been farther away, we would have missed it,” Nousek explained.

Because the burst was so long, Swift was able to observe the bulk of the explosion with all three of its instruments: the Burst Alert Telescope, which detected the burst; and the X-ray Telescope, and Ultraviolet/Optical Telescope, which provide high-resolution imagery and spectra across a broad range of wavelengths. Penn State lead the development of the X-ray and Ultraviolet/Optical Telescopes.

Scientists will attempt observations with the Hubble Space Telescope and the Chandra X-ray Observatory. Amateur astronomers in dark skies might be able to see the explosion with a 16-inch telescope as it hits 16th-magnitude brightness.

Swift is a NASA mission in partnership with the Italian Space Agency and the Particle Physics and Astronomy Research Council in the United Kingdom; it is managed by NASA Goddard, and Penn State controls its science and flight operations from the Mission Operations Center in University Park, Pennsylvania.

PSU News Release

First light of the VLT laser guide star. Image credit: ESO Click to enlarge
Scientists celebrate another major milestone at Cerro Paranal in Chile, home of ESO’s Very Large Telescope array. Thanks to their dedicated efforts, they were able to create the first artificial star in the Southern Hemisphere, allowing astronomers to study the Universe in the finest detail. This artificial laser guide star makes it possible to apply adaptive optics systems, that counteract the blurring effect of the atmosphere, almost anywhere in the sky.

On 28 January 2006, at 23:07 local time, a laser beam of several watts was launched from Yepun, the fourth 8.2m Unit Telescope of the Very Large Telescope, producing an artificial star, 90 km up in the atmosphere. Despite this star being about 20 times fainter than the faintest star that can be seen with the unaided eye, it is bright enough for the adaptive optics to measure and correct the atmosphere’s blurring effect. The event was greeted with much enthusiasm and happiness by the people in the control room of one of the most advanced astronomical facilities in the world.

It was the culmination of five years of collaborative work by a team of scientists and engineers from ESO and the Max Planck Institutes for Extraterrestrial Physics in Garching and for Astronomy in Heidelberg, Germany.

After more than one month of integration on site with the invaluable support of the Paranal Observatory staff, the VLT Laser Guide Star Facility saw First Light and propagated into the sky a 50cm wide, vivid, beautifully yellow beam.

“This event tonight marks the beginning of the Laser Guide Star Adaptive Optics era for ESO’s present and future telescopes”, said Domenico Bonaccini Calia, Head of the Laser Guide Star group at ESO and LGSF Project Manager.

Normally, the achievable image sharpness of a ground-based telescope is limited by the effect of atmospheric turbulence. This drawback can be surmounted with adaptive optics, allowing the telescope to produce images that are as sharp as if taken from space. This means that finer details in astronomical objects can be studied, and also that fainter objects can be observed.

In order to work, adaptive optics needs a nearby reference star that has to be relatively bright, thereby limiting the area of the sky that can be surveyed. To overcome this limitation, astronomers use a powerful laser that creates an artificial star, where and when they need it.

The laser beam, shining at a well-defined wavelength, makes the layer of sodium atoms that is present in Earth’s atmosphere at an altitude of 90 kilometres glow. The laser is hosted in a dedicated laboratory under the platform of Yepun. A custom-made fibre carries the high power laser to the launch telescope situated on top of the large Unit Telescope.

An intense and exhilarating twelve days of tests followed the First Light of the Laser Guide Star (LGS), during which the LGS was used to improve the resolution of astronomical images obtained with the two adaptive optics instruments in use on Yepun: the NAOS-CONICA imager and the SINFONI spectrograph.

In the early hours of 9 February, the LGS could be used together with the SINFONI instrument, while in the early morning of 10 February, it was with the NAOS-CONICA system.

“To have succeeded in such a short time is an outstanding feat and is a tribute to all those who have together worked so hard over the last few years,” said Richard Davies, project manager for the laser source development at the Max Planck Institute for Extraterrestrial Physics.

A second phase of commissioning will take place in the spring with the aim of optimizing the operations and refining the performances before the instrument is made available to the astronomers, later this year. The experience gained with this Laser Guide Star is also a key milestone in the design of the next generation of Extremely Large Telescope in the 30 to 60 metre range that is now being studied by ESO together with the European astronomical community.

Original Source: ESO News Release