Pinpointing the Distance to a Pulsar

Image credit: NSF

Astronomers have used the accuracy of the National Science Foundation’s Very Long Baseline Array (VLBA) to pinpoint the distance to a pulsar. The object, called PSR B0656+14, was previously thought to be up to 2,500 light-years away but it was at the same location in the sky as a supernova remnant which is only 1,000 light years away. This was thought to be a coincidence, but the new measurement from the VLBA pegs the pulsar at 950 light years away; the same distance as the remnant – they were both created by the same supernova blast.

Location, location, and location. The old real-estate adage about what’s really important proved applicable to astrophysics as astronomers used the sharp radio “vision” of the National Science Foundation’s Very Long Baseline Array (VLBA) to pinpoint the distance to a pulsar. Their accurate distance measurement then resolved a dispute over the pulsar’s birthplace, allowed the astronomers to determine the size of its neutron star and possibly solve a mystery about cosmic rays.

“Getting an accurate distance to this pulsar gave us a real bonanza,” said Walter Brisken, of the National Radio Astronomy Observatory (NRAO) in Socorro, NM.

The pulsar, called PSR B0656+14, is in the constellation Gemini, and appears to be near the center of a circular supernova remnant that straddles Gemini and its neighboring constellation, Monoceros, and is thus called the Monogem Ring. Since pulsars are superdense, spinning neutron stars left over when a massive star explodes as a supernova, it was logical to assume that the Monogem Ring, the shell of debris from a supernova explosion, was the remnant of the blast that created the pulsar.

However, astronomers using indirect methods of determining the distance to the pulsar had concluded that it was nearly 2500 light-years from Earth. On the other hand, the supernova remnant was determined to be only about 1000 light-years from Earth. It seemed unlikely that the two were related, but instead appeared nearby in the sky purely by a chance juxtaposition.

Brisken and his colleagues used the VLBA to make precise measurements of the sky position of PSR B0656+14 from 2000 to 2002. They were able to detect the slight offset in the object’s apparent position when viewed from opposite sides of Earth’s orbit around the Sun. This effect, called parallax, provides a direct measurement of distance.

“Our measurements showed that the pulsar is about 950 light-years from Earth, essentially the same distance as the supernova remnant,” said Steve Thorsett, of the University of California, Santa Cruz. “That means that the two almost certainly were created by the same supernova blast,” he added.

With that problem solved. the astronomers then turned to studying the pulsar’s neutron star itself. Using a variety of data from different telescopes and armed with the new distance measurement, they determined that the neutron star is between 16 and 25 miles in diameter. In such a small size, it packs a mass roughly equal to that of the Sun.

The next result of learning the pulsar’s actual distance was to provide a possible answer to a longstanding question about cosmic rays. Cosmic rays are subatomic particles or atomic nuclei accelerated to nearly the speed of light. Shock waves in supernova remnants are thought to be responsible for accelerating many of these particles.

Scientists can measure the energy of cosmic rays, and had noted an excess of such rays in a specific energy range. Some researchers had suggested that the excess could come from a single supernova remnant about 1000 light-years away whose supernova explosion was about 100,000 years ago. The principal difficulty with this suggestion was that there was no accepted candidate for such a source.

“Our measurement now puts PSR B0656+14 and the Monogem Ring at exactly the right place and at exactly the right age to be the source of this excess of cosmic rays,” Brisken said.

With the ability of the VLBA, one of the telescopes of the NRAO, to make extremely precise position measurements, the astronomers expect to improve the accuracy of their distance determination even more.

“This pulsar is becoming a fascinating laboratory for studying astrophysics and nuclear physics,” Thorsett said.

In addition to Brisken and Thorsett, the team of astronomers includes Aaron Golden of the National University of Ireland, Robert Benjamin of the University of Wisconsin, and Miller Goss of NRAO. The scientists are reporting their results in papers appearing in the Astrophysical Journal Letters in August.

The VLBA is a continent-wide system of ten radio- telescope antennas, ranging from Hawaii in the west to the U.S. Virgin Islands in the east, providing the greatest resolving power, or ability to see fine detail, in astronomy. Dedicated in 1993, the VLBA is operated from the NRAO’s Array Operations Center in Socorro, New Mexico.

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

Gamma Ray Bursts May Propel Fast Moving Particles

Image credit: NASA

Astronomers believe that gamma-ray bursts, the most powerful explosions in the Universe, may be generating ultrahigh-energy cosmic rays, the most energetic particles in the Universe. These cosmic rays have baffled astronomers because they’re moving faster than if they were thrown out of a supernova. Evidence gathered by NASA’s de-orbited Compton Gamma-Ray Observatory showed that in one instance of a gamma ray burst, these high-energy particles dominated the area giving a connection between them, but this is hardly enough evidence to say they’re conclusively linked.

The most powerful explosions in the universe, gamma-ray bursts, may generate the most energetic particles in the universe, known as the ultrahigh-energy cosmic rays (UHECRs), according to a new analysis of observations from NASA’s Compton Gamma-Ray Observatory.

Researchers report in the August 14 edition of Nature of a newly identified pattern in the light from these enigmatic bursts that could be explained by protons moving within a hair’s breadth of light speed.

These protons, like shrapnel from an explosion, could be UHECRs. Such cosmic rays are rare and constitute an enduring mystery in astrophysics, seemingly defying physical explanation, for they are simply far too energetic to have been generated by well-known mechanisms such as supernova explosions.

“Cosmic rays ‘forget’ where they come from because, unlike light, they are whipped about in space by magnetic fields,” said lead author Maria Magdalena Gonzalez of the Los Alamos National Laboratory in New Mexico and graduate student at the University of Wisconsin. “This result is an exciting chance to possibly see evidence of them being produced at their source.”

Gamma-ray bursts — a mystery scientists are finally beginning to unravel — can shine as brilliantly as a million trillion suns, and many may be from an unusually powerful type of exploding star. The bursts are common yet random and fleeting, lasting only seconds.

Cosmic rays are atomic particles (for example, electrons, protons or neutrinos) moving close to light speed. Lower-energy cosmic rays bombard the Earth constantly, propelled by solar flares and typical star explosions. UHECRs, with each atomic particle carrying the energy of a baseball thrown in the Major Leagues, are a hundred-million times more energetic than the particles produced in the largest human-made particle accelerators.

Scientists say the UHECRs must be generated relatively close to the Earth, for any particle traveling farther than 100 million light years would lose some of its energy by the time it reached us. Yet no local source of ordinary cosmic rays seems powerful enough to generate a UHECR.

The Gonzalez-led paper focuses not specifically on UHECR production but rather a new pattern of light seen in a gamma-ray burst. Digging deep into the Compton Observatory archives (the mission ended in 2000), the group found that a gamma-ray burst from 1994, named GRB941017, appears different from the other 2,700-some bursts recorded by this spacecraft. This burst was located in the direction of the constellation Sagitta, the Arrow, likely ten billion light years away.

What scientists call gamma rays are photons (light particles) covering a wide range of energies, in fact, over a million times wider than the energies our eyes register as the colors in a rainbow. Gonzalez’s group looked at the higher-energy gamma-ray photons. The scientists found that these types of photons dominated the burst: They were at least three times more powerful on average than the lower-energy component yet, surprisingly, thousands of times more powerful after about 100 seconds.

That is, while the flow of lower-energy photons hitting the satellite’s detectors began to ease, the flow of higher-energy photons remained steady. The finding is inconsistent with the popular “synchrotron shock model” describing most bursts. So what could explain this enrichment of higher-energy photons?

“One explanation is that ultrahigh-energy cosmic rays are responsible, but exactly how they create the gamma rays with the energy patterns we saw needs a lot of calculating,” said Dr. Brenda Dingus of LANL, a co-author on the paper. “We’ll be keeping some theorists busy trying to figure this out.”

A delayed injection of ultrahigh-energy electrons provides another way to explain the unexpectedly large high-energy gamma-ray flow observed in GRB 941017. But this explanation would require a revision of the standard burst model, said co-author Dr. Charles Dermer, a theoretical astrophysicist at the U.S. Naval Research Laboratory in Washington. “In either case, this result reveals a new process occurring in gamma-ray bursts,” he said.

Gamma-ray bursts have not been detected originating within 100 million light years from Earth, but through the eons these types of explosions may have occurred locally. If so, Dingus said, the mechanism her group saw in GRB 941017 could have been duplicated close to home, close enough to supply the UHECRs we see today.

Other bursts in the Compton Observatory archive may have exhibited a similar pattern, but the data are not conclusive. NASA’s Gamma-ray Large Area Space Telescope (GLAST), scheduled for launch in 2006, will have detectors powerful enough to resolve higher-energy gamma-ray photons and solve this mystery.

Co-authors on the Nature report also include Ph.D. graduate student Yuki Kaneko, Dr. Robert Preece, and Dr. Michael Briggs of the University of Alabama in Huntsville. This research was funded by NASA and the Office of Naval Research.

UHECRs are observed when they crash into our atmosphere, as is illustrated in the figure. The energy from the collision produces an air shower of billions of subatomic particles and flashes of ultraviolet light, which are detected by special instruments.

The National Science Foundation and international collaborators have sponsored instruments on the ground, such as the High Resolution Fly’s Eye in Utah (http://www.cosmic-ray.org/learn.html) and the Auger Observatory in Argentina (http://www.auger.org/). In addition, NASA is working with the European Space Agency to place the Extreme Universe Space Observatory (http://aquila.lbl.gov/EUSO/) on the International Space Station. The proposed OWL mission would, from orbit, look downward towards air showers, viewing a region as large as Texas.

These scientists record the flashes and take a census of the subatomic shrapnel, working backward to calculate how much energy a single particle needs to make the atmospheric cascade. They arrive at a shocking figure of 10^20 electron volts (eV) or more. (For comparison, the energy in a particle of yellow light is 2 eV, and the electrons in your television tube are in the thousand electron volt energy range.)

These ultrahigh-energy particles experience the bizarre effects predicted by Einstein’s theory of special relativity. If we could observe them coming from a remote corner of the cosmos, say a hundred million light years away, we’d have to be patient — it will take a hundred million years to complete the journey. However, if we could travel with the particles, the trip is over in less than a day due to the dilation of time of rapidly moving objects as measured by an observer.

The highest energy cosmic rays cannot even reach us if produced from distant sources, because they collide and lose energy with the cosmic microwave photons left over from the big bang. Sources of these cosmic rays must be found relatively close to us, at a distance of several hundred million light years. Stars that explode as gamma-ray bursts are found within this distance, so intensive observational efforts are underway to find gamma-ray burst remnants distinguished by radiation halos made by the cosmic rays.

Few kinds of celestial objects possess the extreme conditions required to blast particles to UHECR speeds. If gamma-ray bursts produce UHECRs, they probably do so by accelerating particles in jets of matter ejected from the explosion at close to the speed of light. Gamma-ray bursts have the power to accelerate UHECRs, but the gamma-ray bursts observed so far have been remote, billions of light years away. This doesn’t mean they can’t happen nearby, within the UHECR cutoff distance.

A leading contender for long-lived kinds of gamma-ray bursts like GRB941017 is the supernova/collapsar model. Supernovae happen when a star many times more massive than the Sun exhausts its fuel, causing its core to collapse under its own gravity while its outer layers are blown off in an immense thermonuclear explosion. Collapsars are a special type of supernova where the core is so massive it collapses into a black hole, an object so dense that nothing, not even light, can escape its gravity within the black hole’s event horizon. However, observations indicate black holes are sloppy eaters, ejecting material that passes near, but does not cross, their event horizons.

In a collapsar, the star’s core forms a disk of material around the newly formed black hole, like water swirling around a drain. The black hole consumes most of the disk, but some matter is blasted in jets from the poles of the black hole. The jets tear through the collapsing star at close to the speed of light, and then punch through gas surrounding the doomed star. As the jets crash into the interstellar medium, they create shock waves and slow down. Internal shocks also form in the jets as their leading edges slow and are slammed from behind by a stream of high-speed matter. The shocks accelerate particles that generate gamma rays; they could also accelerate particles to UHECR speeds, according to the team.

“It’s like bouncing a ping pong ball between a paddle and a table,” said Dingus. “As you move the paddle closer to the table, the ball bounces faster and faster. In a gamma-ray burst, the paddle and the table are shells ejected in the jet. Turbulent magnetic fields force the particles to ricochet between the shells, accelerating them to almost the speed of light before they break free as UHECRs.”

Detection of neutrinos from gamma-ray bursts would clinch the case for cosmic ray acceleration by gamma-ray bursts. Neutrinos are elusive particles made when high-energy protons collide with photons. Neutrinos have no electrical charge, so still point back to the direction of their source.

The National Science Foundation is currently building IceCube (http://icecube.wisc.edu/), a cubic kilometer detector located in the ice under the South Pole, to search for neutrino emission from gamma-ray bursts. However, the characteristics of nature’s highest-energy particle accelerators remain an enduring mystery, though acceleration by the exploding stars that make gamma-ray bursts has been in favor ever since Mario Vietri (Universita di Roma) and Eli Waxman (Weizmann Institute) proposed it in 1995.

The team believes that while other explanations are possible for this observation, the result is consistent with UHECR acceleration in gamma-ray bursts. They saw both low-energy and high-energy gamma rays in the GRB941017 explosion. The low-energy gamma rays are what scientists expect from high-speed electrons being deflected by intense magnetic fields, while the high-energy rays are what’s expected if some of the UHECRs produced in the burst crash into other photons, creating a shower of particles, some of which flash to produce the high-energy gamma rays when they decay.

The timing of the gamma-ray emission is also significant. The low-energy gamma rays faded away relatively quickly, while the high-energy gamma rays lingered. This makes sense if two different classes of particles – electrons and the protons of the UHECRs – are responsible for the different gamma rays. “It’s much easier for electrons than protons to radiate their energy. Therefore, the emission of low-energy gamma rays from electrons would be shorter than the high-energy gamma rays from the protons,” said Dingus.

The Compton Gamma Ray Observatory was the second of NASA’s Great Observatories and the gamma-ray equivalent to the Hubble Space Telescope and the Chandra X-ray Observatory. Compton was launched aboard the Space Shuttle Atlantis in April 1991, and at 17 tons, was the largest astrophysical payload ever flown at that time. At the end of its pioneering mission, Compton was deorbited and re-entered the Earth’s atmosphere on June 4, 2000.

Original Source: NASA News Release

Amateur Spots a Gamma Ray Burst Afterglow

Image credit: NASA

Berto Monard, an amateur astronomer from South Africa was lucky enough to spot the afterglow from a powerful gamma-ray burst – beating professional astronomers to the target. The 40-second-long burst was discovered by NASA’s HETE spacecraft, which provided Monard rough coordinates of where to look. He was able to provide the astronomy community with a precise location so they can follow up days or weeks later to try and determine what actually caused the explosion.

Armed with a 12-inch telescope, a computer, and a NASA email alert, Berto Monard of South Africa has become the first amateur astronomer to discover an afterglow of a gamma-ray burst, the most powerful explosion known in the Universe.

The discovery highlights the ease in tapping into NASA’s burst alert system, as well as the increasing importance that astronomy enthusiasts play in helping scientists understand fleeting and random events, such as star explosions and gamma-ray bursts.

This 40-second-long burst was detected by NASA’s High-Energy Transient Explorer (HETE) on July 25. Monard’s positioning of the lingering afterglow, and thus burst location, has given way to precision follow-up study, an opportunity that very well might have been missed: At the time of the burst, thousands of professional astronomers were attending the International Astronomical Union conference in Sydney, Australia, far away from their observatories.

“I have seen a multitude of stars and galaxies and even supernovae, but this gamma-ray burst afterglow is among the most ancient light that has ever graced my telescope,” Monard said. “The explosion that caused this likely occurred billions of years ago, before the Earth was formed.”

Gamma-ray bursts, many of which now appear to be massive star explosions billions of light years away, only last for a few milliseconds to upwards of a minute. Prompt identification of an afterglow, which can last for hours to days in lower-energy light such as X ray and optical, is crucial for piecing together the explosion that caused the burst.

Monard notified the pros of the burst location within seven hours of the HETE detection. The Interplanetary Network (IPN), comprising six orbiting gamma-ray detectors, confirmed the location shortly thereafter.

Because of the nature of gamma-ray light, which cannot be focused like optical light, HETE locates bursts to only within a few arcminutes. (An arcminute is about the size of an eye of a needle held at arm’s length.) Most gamma-ray bursts are exceedingly far, so myriad stars and galaxies fill that tiny circle. Without prompt localization of a bright and fading afterglow, scientists have great difficulty locating the gamma-ray burst
location days or weeks later.

The study of gamma-ray bursts (and increasing ease of amateur participation) comes through two innovations: faster burst detectors like HETE and a near-instant information relay system called the Gamma-ray Burst Coordinates Network, or GCN, which is located at NASA Goddard Space Flight Center in Greenbelt, Md.

The typical pattern follows: HETE detects a burst and, within a few seconds to about a minute, relays a location to the GCN. Instantly, the automated GCN notifies scientists and amateur astronomers worldwide about the burst event via email, pagers, and a Web site.

Monard is a member of the American Association of Variable Star Observers (AAVSO). This organization operates the AAVSO International High Energy Network, which acts as a liaison between the amateur and the professional communities. Monard essentially used GCN information passed through the AAVSO and other network groups and turned his telescope to the location determined by HETE.

“In the past two years, HETE has opened the door wide for rapid follow-up studies by professional astronomers,” said HETE Principal Investigator George Ricker of MIT. “Now, with GRB030725, the worldwide community of dedicated and expert amateur astronomers coordinated through the AAVSO is leaping through that door to join the fun.”

Monard, a Belgian national living in South Africa, has other discoveries under his belt, including ten supernovae and several outbursts from neutron star systems, as part of his participation with the worldwide Center for Backyard Astrophysics network and the Variable Star Network.

The AAVSO, founded in 1911, is a non-profit, scientific organization with members in 46 countries. It coordinates, compiles, digitizes and disseminates observations on stars that change in brightness (variable stars) to researchers and educators worldwide. Its International High Energy Network was created with cooperation from NASA.

HETE was built by the Massachusetts Institute of Technology under NASA’s Explorer Program. HETE is a collaboration among NASA, MIT, Los Alamos National Laboratory; France’s Centre National d’Etudes Spatiales, Centre d’Etude Spatiale des Rayonnements, and Ecole Nationale Superieure de l’Aeronautique et de l’Espace; and Japan’s Institute of Physical and Chemical Research (RIKEN). The science team includes members from the University of California (Berkeley and Santa Cruz) and the University of Chicago, as well as from Brazil, India and Italy.

Formation of Stars is On the Decline

Image credit: SDSS

The age of star formation in the Universe is drawing to a close, according to a new report from the Sloan Digital Sky Survey. A team of astronomers analyzed the colour of an enormous number of nearby galaxies and found that they contained less young stars than more distant galaxies. Since light takes so long to travel, the more distant galaxies are seen as they appeared many billion years ago. The number of new stars being formed has been on the decline since about 6 billon years ago, when our own Sun formed.

The universe is gently fading into darkness according to three astronomers who have looked at 40,000 galaxies in the neighbourhood of the Milky Way. Research student Ben Panter and Professor Alan Heavens from Edinburgh University’s Institute for Astronomy, and Professor Raul Jimenez of University of Pennsylvania, USA, decoded the “fossil record” concealed in the starlight from the galaxies to build up a detailed account of how many young, recently-formed stars there were at different periods in the 14-billion-year existence of the universe. Their history shows that, for billions of years, there have not been enough new stars turning on to replace all the old stars that die and switch off. The results will be published in the Monthly Notices of the Royal Astronomical Society on 21 August 2003.

“Our analysis confirms that the age of star formation is drawing to a close”, says Alan Heavens. “The number of new stars being formed in the huge sample of galaxies we studied has been in decline for around 6 billion years – roughly since the time our own Sun came into being.”

Astronomers already had evidence that this was the case, mainly from observing galaxies so far away that we see them as they were billions of years ago because of the great length of time their light has taken to reach us. Now the same story emerges strongly from the work of Panter, Heavens and Jimenez, who for the first time approached the problem differently and used the whole spectrum of light from an enormous number of nearby galaxies to get a more complete picture.

Galaxies shine with the combined light of all the stars in them. Most of the light from young stars is blue, coming from very hot massive stars. These blue stars live fast and die young, ending their lives in supernova explosions. When they have gone, they no longer outshine the smaller red stars that are more long-lived. Many galaxies look reddish overall rather than blue – a broad sign that most star formation happened long ago.

In their analysis, Panter, Heavens and Jimenez have used far more than the simple overall colours of the galaxies, though. The spectrum observations they used come from the Sloan Digital Sky Survey and the volume of data involved was so vast, that the researchers had to develop a special lossless data compression method, called MOPED, to allow them to analyse the sample in a reasonable length of time, without losing accuracy.

Original Source: RAS News Release

Astronomers Measure the Shape of a Supernova

Image credit: ESO

New data gathered by the European Southern Observatory’s Very Large Telescope (VLT) seems to indicate that supernovae might not be symmetrical when they explode – their brightness changes depending on how you look at them. This discovery is important, because astronomers use supernovae as an astronomical yardstick to measure distances to objects. If they’re brighter or dimmer depending on how you’re looking at them, it could cause errors in your distance calculations. But the new research indicates that they become more symmetrical over time, so astronomers just need to wait a little while before doing their calculations.

An international team of astronomers [2] has performed new and very detailed observations of a supernova in a distant galaxy with the ESO Very Large Telescope (VLT) at the Paranal Observatory (Chile). They show for the first time that a particular type of supernova, caused by the explosion of a “white dwarf”, a dense star with a mass around that of the Sun, is asymmetric during the initial phases of expansion.

The significance of this observation is much larger than may seem at a first glance. This particular kind of supernova, designated “Type Ia”, plays a very important role in the current attempts to map the Universe. It has for long been assumed that Type Ia supernovae all have the same intrinsic brightness, earning them a nickname as “standard candles”.

If so, differences in the observed brightness between individual supernovae of this type simply reflect their different distances. This, and the fact that the peak brightness of these supernovae rivals that of their parent galaxy, has allowed to measure distances of even very remote galaxies. Some apparent discrepancies that were recently found have led to the discovery of cosmic acceleration.

However, this first clearcut observation of explosion asymmetry in a Type Ia supernova means that the exact brightness of such an object will depend on the angle from which it is seen. Since this angle is unknown for any particular supernova, this obviously introduces an amount of uncertainty into this kind of basic distance measurements in the Universe which must be taken into account in the future.

Fortunately, the VLT data also show that if you wait a little – which in observational terms makes it possible to look deeper into the expanding fireball – then it becomes more spherical. Distance determinations of supernovae that are performed at this later stage will therefore be more accurate.

Supernova explosions and cosmic distances
During Type Ia supernova events, remnants of stars with an initial mass of up to a few times that of the Sun (so-called “white dwarf stars”) explode, leaving nothing behind but a rapidly expanding cloud of “stardust”.

Type Ia supernovae are apparently quite similar to one another. This provides them a very useful role as “standard candles” that can be used to measure cosmic distances. Their peak brightness rivals that of their parent galaxy, hence qualifying them as prime cosmic yardsticks.

Astronomers have exploited this fortunate circumstance to study the expansion history of our Universe. They recently arrived at the fundamental conclusion that the Universe is expanding at an accelerating rate, cf. ESO PR 21/98, December 1998 (see also the Supernova Acceleration Probe web page).

The explosion of a white dwarf star
In the most widely accepted models of Type Ia supernovae the pre-explosion white dwarf star orbits a solar-like companion star, completing a revolution every few hours. Due to the close interaction, the companion star continuously loses mass, part of which is picked up (in astronomical terminology: “accreted”) by the white dwarf.

A white dwarf represents the penultimate stage of a solar-type star. The nuclear reactor in its core has run out of fuel a long time ago and is now inactive. However, at some point the mounting weight of the accumulating material will have increased the pressure inside the white dwarf so much that the nuclear ashes in there will ignite and start burning into even heavier elements. This process very quickly becomes uncontrolled and the entire star is blown to pieces in a dramatic event. An extremely hot fireball is seen that often outshines the host galaxy.

The shape of the explosion
Although all supernovae of Type Ia have quite similar properties, it has never been clear until now how similar such an event would appear to observers who view it from different directions. All eggs look similar and indistinguishable from each other when viewed from the same angle, but the side view (oval) is obviously different from the end view (round).

And indeed, if Type Ia supernova explosions were asymmetric, they would shine with different brightness in different directions. Observations of different supernovae – seen under different angles – could therefore not be directly compared.

Not knowing these angles, however, the astronomers would then infer incorrect distances and the precision of this fundamental method for gauging the structure of the Universe would be in question.

Polarimetry to the rescue
A simple calculation shows that even to the eagle eyes of the VLT Interferometer (VLTI), all supernovae at cosmological distances will appear as unresolved points of light; they are simply too far. But there is another way to determine the angle at which a particular supernova is viewed: polarimetry is the name of the trick!

Polarimetry works as follows: light is composed of electromagnetic waves (or photons) which oscillate in certain directions (planes). Reflection or scattering of light favours certain orientations of the electric and magnetic fields over others. This is why polarising sunglasses can filter out the glint of sunlight reflecting off a pond.

When light scatters through the expanding debris of a supernova, it retains information about the orientation of the scattering layers. If the supernova is spherically symmetric, all orientations will be present equally and will average out, so there will be no net polarisation. If, however, the gas shell is not round, a slight net polarisation will be imprinted on the light.

“Even for quite noticable asymmetries, however, the polarisation is very small and barely exceeds the level of one percent”, says Dietrich Baade, ESO astronomer and a member of the team that performed the observations. “Measuring them requires an instrument that is very sensitive and very stable. ”

The measurement in faint and distant light sources of differences at a level of less than one percent is a considerable observational challenge. “However, the ESO Very Large Telescope (VLT) offers the precision, the light collecting power, as well as the specialized instrumentation required for such a demanding polarimetric observation”, explains Dietrich Baade. “But this project would not have been possible without the VLT being operated in service mode. It is indeed impossible to predict when a supernova will explode and we need to be ready all the time. Only service mode allows observations at short notice. Some years ago, it was a farsighted and courageous decision by ESO’s directorate to put so much emphasis on Service Mode. And it was the team of competent and devoted ESO astronomers on Paranal who made this concept a practical success”, he adds.

The astronomers [1] used the VLT multi-mode FORS1 instrument to observe SN 2001el, a Type Ia supernova that was discovered in September 2001 in the galaxy NGC 1448, cf. PR Photo 24a/03 at a distance of 60 million light-years.

Observations obtained about a week before this supernova reached maximum brightness around October 2 revealed polarisation at levels of 0.2-0.3% (PR Photo 24b/03). Near maximum light and up to two weeks thereafter, the polarisation was still measurable. Six weeks after maximum, the polarisation had dropped below detectability.

This is the first time ever that a normal Type Ia supernova has been found to exhibit such clear-cut evidence of asymmetry.
Looking deeper into the supernova

Immediately following the supernova explosion, most of the expelled matter moves at velocities around 10,000 km/sec. During this expansion, the outermost layers become progressively more transparent. With time one can thus look deeper and deeper into the supernova.

The polarisation measured in SN 2001el therefore provides evidence that the outermost parts of the supernova (which are first seen) are significantly asymmetric. Later, when the VLT observations “penetrate” deeper towards the heart of the supernova, the explosion geometry is increasingly more symmetric.

If modeled in terms of a flattened spheroidal shape, the measured polarisation in SN 2001el implies a minor-to-major axis ratio of around 0.9 before maximum brightness is reached and a spherically symmetric geometry from about one week after this maximum and onward.
Cosmological implications

One of the key parameters on which Type Ia distance estimates are based is the optical brightness at maximum. The measured asphericity at this moment would introduce an absolute brightness uncertainty (dispersion) of about 10% if no correction were made for the viewing angle (which is not known).

While Type Ia supernovae are by far the best standard candles for measuring cosmological distances, and hence for investigating the so-called dark energy, a small measurement uncertainty persists.

“The asymmetry we have measured in SN 2001el is large enough to explain a large part of this intrinsic uncertainty”, says Lifan Wang, the leader of the team. “If all Type Ia supernovae are like this, it would account for a lot of the dispersion in brightness measurements. They may be even more uniform than we thought.”

Reducing the dispersion in brightness measurements could of course also be attained by increasing significantly the number of supernovae we observe, but given that these measurements demand the largest and most expensive telescopes in the world, like the VLT, this is not the most efficient method.

Thus, if the brightness measured a week or two after maximum was used instead, the sphericity would then have been restored and there would be no systematic errors from the unknown viewing angle. By this slight change in observational procedure, Type Ia supernovae could become even more reliable cosmic yardsticks.
Theoretical implications

The present detection of polarised spectral features strongly suggests that, to understand the underlying physics, the theoretical modelling of Type Ia supernovae events will have to be done in all three dimensions with more accuracy than is presently done. In fact, the available, highly complex hydrodynamic calculations have so far not been able to reproduce the structures exposed by SN 2001el.
More information

The results presented in this press release have been been described in a research paper in “Astrophysical Journal” (“Spectropolarimetry of SN 2001el in NGC 1448: Asphericity of a Normal Type Ia Supernova” by Lifan Wang and co-authors, Volume 591, p. 1110).
Notes

[1]: This is a coordinated ESO/Lawrence Berkeley National Laboratory/Univ. of Texas Press Release. The LBNL press release is available here.

[2]: The team consists of Lifan Wang, Dietrich Baade, Peter H?flich, Alexei Khokhlov, J. Craig Wheeler, Daniel Kasen, Peter E. Nugent, Saul Perlmutter, Claes Fransson, and Peter Lundqvist.

Original Source: ESO News Release

Perseid Meteor Shower Next Week

Image credit: ESA

The annual Perseid meteor shower is due to make its appearance in mid-August this summer. The shower began on July 23 and will end on August 22, but the bulk of shooting stars will appear on August 13, when upwards of one meteor per minute is visible in the night sky. Unfortunately, the full Moon will brighten the sky and make some of the fainter meteors harder to see. To get the best view of the Perseids, get away from the city lights to a place which is as flat as possible to give you a wide view of the sky.

A fantastic, free light show occurred in the morning of Wednesday, 13 August 2003, in the form of the Perseid meteor shower!

This impressive set of shooting stars appears in the skies every year from around 23 July to 22 August, with its peak on 13 August. First recorded as long ago as 36 AD, the Perseids are also known as ‘the tears of St. Lawrence’ after the Roman martyr.

Typically, you can see this phenomenon with the naked eye, with a shooting star appearing every minute until about 03.00 CET on Wednesday morning. You may also see meteors a few days before or after this time.

However, this year the Moon will be full near the Perseid’s maximum, which will reduce observed rates by a factor of three or so. It will not be until around 2007 when the Moon’s phase is more favourable than that of last year.

Meteor showers occur when the Earth passes through the trail of debris often left behind by a comet. By studying meteor showers, scientists can learn more about cometary debris, but ESA is going a step further with its Rosetta comet-chasing mission which will examine a comet at close range.

Comets are considered to be the primitive building blocks of the Solar System, and the Rosetta mission could help us to understand if life on Earth began with the help of ‘comet seeding’.

The meteors we see are actually tiny bits of comet debris, most of which are only as big as a grain of sand, so they do not pose a threat to us. However, they do provide a spectacular light show as they vaporise on entering the Earth’s atmosphere. This particular shower is named after the Perseus constellation because the shooting stars can appear to start there, but the material was actually shed by the Comet Swift-Tuttle.

To get the best view of the light show, get as far away from city lights as you can since these affect your ability to see the meteor shower.

Make sure that you are comfortable – gazing at the sky for hours can cause neck strain. Find a reclining garden chair or lay out a blanket on the ground. The meteors can appear in any part of the sky, so make sure that you have as wide a view of it as possible.

However, if poor weather prevents you seeing this spectacular show, or you simply cannot stay awake that long, do not give up. You have a chance to view another set of shooting stars in November 2003 when the Leonid meteor shower comes our way. In the third week of November, the Leonids will appear – though 2002 was supposed to be their last big show for the next 30 years.

The Leonids are the leftovers from Comet 55P/Tempel-Tuttle, and ESA scientists regularly conduct intense observation campaigns of these to understand more about comets and cometary debris.

Original Source: ESA News Release

Canada’s Space Telescope Begins Operations

Image credit: CSA

After one month in space, Canada’s Microvariability & Oscillations of STars (MOST) space telescope began operations for the first time last week. A team of engineers and scientists from Dynacon, and the Universities of Toronto and BC issued the command that opens the door, allowing starlight into the sensitive observatory. MOST will measure the oscillation in the light intensity coming from various stars to help determine their composition and age.

After a perfect launch and orbit insertion one month ago, Canada’s first space telescope – called MOST (Microvariability & Oscillations of STars) – opened its eye to the cosmos for the first time last week. Astronomers traditionally call this milestone for a telescope “first light.”

A joint team of engineers and scientists from Dynacon Inc. and the Universities of Toronto and British Columbia issued the command to open the door on the MOST satellite to allow starlight to strike its sensitive electronic detectors. A star image obtained immediately after this operation confirmed that the optics and electronics are performing well.

MOST Mission Scientist and UBC astronomer Dr. Jaymie Matthews was elated at this successful operation: “One of my worst nightmares was having our superb instrument blind behind a stuck door. This is just another in a series of successful milestones which are a testament to the skills of all the Canadian hardware and software engineers on the MOST team.”

The Canadian Space Agency’s MOST space mission is designed to detect tiny vibrations in starlight and reflected light from planets outside the Solar System. These signals will enable Canadian astronomers to be the first to probe both the hidden deep interiors of stars and the outer atmospheres of mysterious extrasolar planets.

“With MOST, we will finally be able to determine the dynamic composition of stars,” said Steve Torchinsky, scientist with the CSA’s Space Astronomy Program. “Furthermore, since MOST is able to see the light reflected from planets and to record minuscule variations in luminosity, this will provide us with data we never had access to before, since no other telescope – not even Hubble – is capable of collecting this type of information.”

Despite such lofty goals, the MOST satellite has been dubbed the “Humble Space Telescope” because it’s just the mass and size of a suitcase. Its price tag is modest too: only about $10 million. To accomplish science that’s normally the domain of observatories 50 times larger and tens to hundreds of times more expensive, the MOST project has adopted a new approach to space science, as part of the Canadian Space Agency’s Small Payloads initiative.

Packed in the MOST space suitcase are new stabilising technologies from the Canadian aerospace firm Dynacon Inc., innovative microsatellite designs by the SpaceFlight Lab of the University of Toronto Institute for Aerospace Studies (UTIAS), and unique optics and electronics developed at the Department of Physics & Astronomy of the University of British Columbia (UBC).

MOST was launched from the Plesetsk Cosmodrome in northern Russia on 30 June 2003, entering orbit perfectly. It now circles the Earth every 100 minutes, pole over pole, at an altitude of 820 km. Since orbital insertion, the MOST team has been carefully activating and testing the satellite systems. The satellite was oriented so the telescope opening – still covered by a door to protect it from harmful direct sunlight – was pointed safely away from the Sun. Last Tuesday, MOST team leaders all agreed it was safe to open the door, built by Routes AstroEngineering Ltd. in Ottawa, Ontario.

At the moment, the MOST satellite is in its coarse pointing mode, looking towards the constellation Capricorn. The next step in the mission will be to activate the fine pointing system and redirect the telescope to a pre-selected target star for calibration. Routine scientific operations could begin within a few weeks, and the first public announcement of scientific results is anticipated in the fall.

Original Source: CSA News Release

Local Galactic Dust is on the Rise

Image credit: ESA

New observations from the European Space Agency’s Ulysses spacecraft show that galactic dust in the Milky Way is passing through our solar system more than normal. The Sun’s magnetic field normally forms a barrier around our solar system that forces dust to go around us, but the Sun has reached the high point of its 11-year cycle, and the magnetic field is highly disordered – so the interstellar dust is coming through the solar system more directly. Although it has no direct effects on the planets, the dust impacts asteroids and comets producing more fragments, and may increase the amount of material that rains down on the Earth.

Since early 1992 Ulysses has been monitoring the stream of stardust flowing through our Solar System. The stardust is embedded in the local galactic cloud through which the Sun is moving at a speed of 26 kilometres every second. As a result of this relative motion, a single dust grain takes twenty years to traverse the Solar System. Observations by the DUST experiment on board Ulysses have shown that the stream of stardust is highly affected by the Sun’s magnetic field.

In the 1990s, this field, which is drawn out deep into space by the out-flowing solar wind, kept most of the stardust out. The most recent data, collected up to the end of 2002, shows that this magnetic shield has lost its protective power during the recent solar maximum. In an upcoming publication in the Journal of Geophysical Research ESA scientist Markus Landgraf and his co-workers from the Max-Planck-Institute in Heidelberg report that about three times more stardust is now able to enter the Solar System.

The reason for the weakening of the Sun’s magnetic shield is the increased solar activity, which leads to a highly disordered field configuration. In the mid-1990s, during the last solar minimum, the Sun’s magnetic field resembled a dipole field with well-defined magnetic poles (North positive, South negative), very much like the Earth. Unlike Earth, however, the Sun reverses its magnetic polarity every 11 years. The reversal always occurs during solar maximum. That’s when the magnetic field is highly disordered, allowing more interstellar dust to enter the Solar System. It is interesting to note that in the reversed configuration after the recent solar maximum (North negative, South positive), the interstellar dust is even channelled more efficiently towards the inner Solar System. So we can expect even more interstellar dust from 2005 onwards, once the changes become fully effective.

While grains of stardust are very small, about one hundredth the diameter of a human hair, they do not directly influence the planets of the Solar System. However, the dust particles move very fast, and produce large numbers of fragments when they impact asteroids or comets. It is therefore conceivable that an increase in the amount of interstellar dust in the Solar System will create more cosmic dust by collisions with asteroids and comets. We know from the measurements by high-flying aircraft that 40 000 tonnes dust from asteroids and comets enters the Earth’s atmosphere each year. It is possible that the increase of stardust in the Solar System will influence the amount of extraterrestrial material that rains down to Earth.

Original Source: ESA News Release

Neutron Star Binaries are More Common in Clusters

Image credit: Chandra

Many of the stars that we see in globular star clusters are actually binary stars, formed when two stars get caught in each other’s gravity. But new research from the Chandra X-Ray Observatory shows that there are many more binary objects which are stars orbiting a neutron star or white dwarf. Chandra can detect the unique x-ray signature that a neutron star gives off, which is invisible in an optical telescope. The research seems to indicate that these neutron star binaries form much more commonly found in globular clusters than in other parts of a galaxy.

NASA’s Chandra X-ray Observatory has confirmed that close encounters between stars form X-ray emitting, double-star systems in dense globular star clusters. These X-ray binaries have a different birth process than their cousins outside globular clusters, and should have a profound influence on the cluster’s evolution.

A team of scientists led by David Pooley of the Massachusetts Institute of Technology in Cambridge took advantage of Chandra’s unique ability to precisely locate and resolve individual sources to determine the number of X-ray sources in 12 globular clusters in our Galaxy. Most of the sources are binary systems containing a collapsed star such as a neutron star or a white dwarf star that is pulling matter off a normal, Sun-like companion star.

“We found that the number of X-ray binaries is closely correlated with the rate of encounters between stars in the clusters,” said Pooley. “Our conclusion is that the binaries are formed as a consequence of these encounters. It is a case of nurture not nature.”

A similar study led by Craig Heinke of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. confirmed this conclusion, and showed that roughly 10 percent of these X-ray binary systems contain neutron stars. Most of these neutron stars are usually quiet, spending less than 10% of their time actively feeding from their companion.

A globular cluster is a spherical collection of hundreds of thousands or even millions of stars buzzing around each other in a gravitationally-bound stellar beehive that is about a hundred light years in diameter. The stars in a globular cluster are often only about a tenth of a light year apart. For comparison, the nearest star to the Sun, Proxima Centauri, is 4.2 light years away.

With so many stars moving so close together, interactions between stars occur frequently in globular clusters. The stars, while rarely colliding, do get close enough to form binary star systems or cause binary stars to exchange partners in intricate dances. The data suggest that X-ray binary systems are formed in dense clusters known as globular clusters about once a day somewhere in the universe.

Observations by NASA’s Uhuru X-ray satellite in the 1970’s showed that globular clusters seemed to contain a disproportionately large number of X-ray binary sources compared to the Galaxy as a whole. Normally only one in a billion stars is a member of an X-ray binary system containing a neutron star, whereas in globular clusters, the fraction is more like one in a million.

The present research confirms earlier suggestions that the chance of forming an X-ray binary system is dramatically increased by the congestion in a globular cluster. Under these conditions two processes, known as three-star exchange collisions, and tidal captures, can lead to a thousandfold increase in the number of X-ray sources in globular clusters.

In an exchange collision, a lone neutron star encounters a pair of ordinary stars. The intense gravity of the neutron star can induce the most massive ordinary star to “change partners,” and pair up with the neutron star while ejecting the lighter star.

A neutron star could also make a grazing collision with a single normal star, and the intense gravity of the neutron star could distort the gravity of the normal star in the process. The energy lost in the distortion, could prevent the normal star from escaping from the neutron star, leading to what is called tidal capture.

“In addition to solving a long-standing mystery, Chandra data offer an opportunity for a deeper understanding of globular cluster evolution,” said Heinke. “For example, the energy released in the formation of close binary systems could keep the central parts of the cluster from collapsing to form a massive black hole.”

NASA’s Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for the Office of Space Science, NASA Headquarters, Washington. Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime development contractor for the observatory. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass.

Original Source: Chandra News Release

Neutron Star Has Twin Tails

Image credit: ESA

Astronomers using the European Space Agency’s XMM-Newton space observatory have discovered a neutron star with two mysterious x-ray tails, stretching out almost a third of a light year. The neutron star is named Geminga, and it’s one of the closest known neutron stars, at a distance of only 500 light-years away. Unlike most neutron stars, Geminga is strangely quiet in the radio spectrum, but pulsates huge quantities of gamma radiation.

Astronomers using ESA?s X-ray observatory, XMM-Newton, have discovered a pair of X-ray tails, stretching 3 million million kilometres across the sky. They emanate from the mysterious neutron star known as Geminga. The discovery gives astronomers new insight into the extraordinary conditions around the neutron star.

A neutron star measures only 20-30 kilometres across and is the dense remnant of an exploded star. Geminga is one of the closest to Earth, at a distance of about 500 light-years. Most neutron stars emit radio emissions, appearing to pulsate like a lighthouse, but Geminga is ‘radio-quiet’. It does, however, emit huge quantities of pulsating gamma rays making it one of the brightest gamma-ray sources in the sky. Geminga is the only example of a successfully identified gamma-ray source from which astronomers have gained significant knowledge.

It is 350 000 years old and ploughs through space at 120 kilometres per second. Its route creates a shockwave that compresses the gas of the interstellar medium and its naturally embedded magnetic field by a factor of four.

Patrizia Caraveo, Instituto di Astrofisica Spaziale e Fisica Cosmica, Milano, Italy, and her colleagues (at CESR, France, ESO and MPE, Germany) have calculated that the tails are produced because highly energetic electrons become trapped in this enhanced magnetic field. As the electrons spiral inside the magnetic field, they emit the X-rays seen by XMM-Newton.

The electrons themselves are created close to the neutron star. Geminga?s breathless rotation rate ? once every quarter of a second ? creates an extraordinary environment in which electrons and positrons, their antimatter counterparts, can be accelerated to extraordinarily high energies. At such energies, they become powerful high-energy gamma-ray producers. Astronomers had assumed that all the electrons would be converted into gamma rays. However, the discovery of the tails proves that some do find escape routes from the maelstrom.

?It is astonishing that such energetic electrons succeed in escaping to create these tails,? says Caraveo, ?The tail electrons have an energy very near to the maximum energy achievable in the environment of Geminga.?

The tails themselves are the bright edges of the three-dimensional shockwave sculpted by Geminga. Such shockwaves are a bit like the wake of a ship travelling across the ocean. Using a computer model, the team has estimated that Geminga is travelling almost directly across our line of sight.

Studies of Geminga could not be more important. The majority of known gamma-ray sources in the Universe have yet to be identified with known classes of celestial objects. Some astronomers believe that a sizeable fraction of them may be Geminga-like radio-quiet neutron stars. Certainly, the family of radio-quiet neutron stars, discovered through their X-ray emission, is continuously growing. Currently, about a dozen objects are known but only Geminga has a pair of tails!

Original Source: ESA News Release