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Space telescope spots asteroid fly-by

AS ASTEROID 2005 YU55 swept past Earth in the early morning hours of Wednesday, November 9, NASA’s Swift satellite joined professional and amateur astronomers around the globe in monitoring the fast-moving space rock.

The video above shows the asteroid zooming through space near Earth. The other dots are not the Earth and Moon, but background stars.

Although Swift is better known for its study of high-energy outbursts and cosmic explosions, the versatile satellite has made valuable observations of passing comets and asteroids as well.

Swift’s unique ultraviolet observations will aid scientists in understanding the asteroid’s surface composition.

Classified as a potentially hazardous object, 2005 YU55 poses no threat of a collision with Earth for at least the next century. But understanding the details of how its surface reflects light and heat will allow improved assessments of future hazards.

A body in space absorbs sunlight and reradiates the energy as heat, and both of these processes produce a miniscule force that, over time, can alter the object’s trajectory.

For Swift, the challenge with 2005 YU55 was its rapid motion across the sky, which was much too fast for Swift to track. Instead, the team trained the spacecraft’s optics at two locations along the asteroid’s predicted path and let it streak through the field.

Adapted from information issued by NASA / Swift / Stefan Immler and DSS.

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Black hole eats a star

Artist's impression of a star being eaten by a black hole

The aftermath of a black hole's banquet of a star, was jets of energy blasted from the black hole, fortuitously pointed in our direction and detected by the Swift satellite. (Artist's impression)

A BRIGHT FLASH OF GAMMA RAYS observed on March 28 by the Swift satellite may have been the death rattle of a star falling into a massive black hole and being ripped apart.

When Swift detected the flash, astronomers initially thought it was a gamma-ray burst from a collapsing star.

However, research led by astronomers at the University of Warwick has confirmed that the flash—one of the biggest and brightest bangs yet recorded by astronomers—came from a massive black hole at the centre of a distant galaxy.

The black hole appears to have ripped apart a star that wandered too close, creating a powerful beam of energy that crossed the 3.8 billion light years to Earth.

Gamma-ray flare in a distant galaxy

A gamma-ray flare seen in a distant galaxy is thought to have been the death throes of a star being eaten by a black hole.

Careful analysis of the data and subsequent observations by the Hubble Space Telescope and the Chandra X-ray Observatory confirmed Bloom’s initial insight.

“Despite the power of this the cataclysmic event we still only happen to see this event because our Solar System happened to be looking right down the barrel of this jet of energy,” said Dr Andrew Levan, lead researcher from the University of Warwick.

What made this gamma-ray flare, called Sw 1644+57, stand out from a typical burst were its long duration and the fact that it appeared to come from the centre of a galaxy nearly 4 billion light-years away.

Since most, if not all, galaxies are thought to contain a massive black hole at the centre, a long-duration burst could conceivably come from the relatively slow disruption of an infalling star, the astronomers said.

“This burst produced a tremendous amount of energy over a fairly long period of time, and the event is still going on more than two and a half months later,” said Joshua Bloom, an associate professor of astronomy at the University of California Berkeley. “That’s because as the black hole rips the star apart, the mass swirls around like water going down a drain, and this swirling process releases a lot of energy.”

Adapted from information issued by the University of California, Berkeley, and University of Warwick. Images courtesy NASA / Swift / Stefan Immler / ESA / A. Fruchter, STScI / Mark A. Garlick.

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Rocky horror show in deep space

LATE LAST YEAR, ASTRONOMERS noticed an asteroid named Scheila had unexpectedly brightened, and it was sporting short-lived plumes. Data from NASA’s Swift satellite and Hubble Space Telescope showed these changes likely occurred after Scheila was struck by a much smaller asteroid.

“Collisions between asteroids create rock fragments, from fine dust to huge boulders, that impact planets and their moons,” said Dennis Bodewits, an astronomer at the University of Maryland in College Park and lead author of the Swift study.

“Yet this is the first time we’ve been able to catch one just weeks after the smash-up, long before the evidence fades away.”

Asteroids are rocky fragments thought to be debris from the formation and evolution of the solar system approximately 4.6 billion years ago. Millions of them orbit the Sun between Mars and Jupiter in the main asteroid belt. Scheila is approximately 110 kilometres wide and orbits the Sun every five years.

Astronomers have known for decades that comets contain icy material that erupts when warmed by the Sun. They regarded asteroids as inactive rocks whose destinies, surfaces, shapes and sizes were determined by mutual impacts.

However, this simple picture has grown more complex over the past few years.

During certain parts of their orbits, some objects, once categorised as asteroids, clearly develop comet-like clouds that can last for many months. Others display much shorter outbursts. Icy materials may be occasionally exposed, either by internal geological processes or by an external one, such as an impact.

HST image of asteroid Scheila

The Hubble Space Telescope imaged asteroid Scheila on December 27, 2010, when it was about 350 million kilometres away. The C-shaped cloud of particles and dust tail suggest the asteroid was struck by another object.

Asteroid or comet?

On December 11, 2010, images from the University of Arizona’s Catalina Sky Survey, a project of NASA’s Near Earth Object Observations Program, revealed Scheila to be twice as bright as expected and immersed in a faint comet-like glow. Looking through the survey’s archived images, astronomers inferred the outburst began between November 11 and December 3.

Three days after the outburst was announced, Swift’s Ultraviolet/Optical Telescope (UVOT) captured multiple images and a spectrum of the asteroid. Ultraviolet sunlight breaks up the gas molecules surrounding comets—water, for example, is transformed into hydroxyl (OH) and hydrogen.

But none of the emissions most commonly identified in comets, such as hydroxyl or cyanogen, show up in the UVOT spectrum. The absence of gas around Scheila led the Swift team to reject scenarios where exposed ice accounted for the activity.

Images show the asteroid was flanked by dual plumes formed as small dust particles excavated by the impact were pushed away from the asteroid by sunlight.

The teams found the observations were best explained by a small asteroid hitting Scheila’s surface at an angle of less than 30 degrees, leaving a crater 300 metres across. The researchers estimate the crash ejected more than 660,000 tons of dust.

Adapted from information issued by NASA’s Goddard Space Flight Centre. Images courtesy NASA / ESA / D. Jewitt (UCLA) / Goddard Space Flight Centre /Conceptual Image Lab.

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Crab’s candle starts to flicker

  • Crab Nebula is 6,500 light-years from Earth
  • It is the remains of an exploded star (a supernova)
  • Now shown to unexpectedly vary its energy output

DATA FROM SEVERAL NASA satellites has astonished astronomers by revealing unexpected changes in X-ray emission from the Crab Nebula, once thought to be the steadiest high-energy source in the sky.

“For 40 years, most astronomers regarded the Crab as a standard candle,” said Colleen Wilson-Hodge, an astrophysicist at NASA’s Marshall Space Flight Centre, who presented the findings recently at the American Astronomical Society meeting in Seattle.

“Now, for the first time, we’re clearly seeing how much our candle flickers.”

The Crab Nebula is the wreckage of an exploded star whose light reached Earth in 1054. Located 6,500 light-years away, it is one of the most studied objects in the sky.

At the heart of the expanding gas cloud lies what’s left of the original star’s core, a superdense neutron star that spins 30 times a second. All of the Crab’s high-energy emissions are thought to be the result of physical processes that tap into this rapid spin.

For decades, astronomers have regarded the Crab’s X-ray emissions as so stable that they’ve used it to calibrate space-borne instruments. They also customarily describe the emissions of other high-energy sources in “millicrabs,” a unit derived from the nebula’s output.

Crab Nebula

This view of the Crab Nebula comes from the Hubble Space Telescope and spans 12 light-years. The supernova remnant, located 6,500 light-years away, is among the best-studied objects in the sky. Image courtesy NASA / ESA / ASU / J. Hester.

“The Crab Nebula is a cornerstone of high-energy astrophysics,” said team member Mike Cherry at Louisiana State University (LSU), “and this study shows us that our foundation is slightly askew.”

Satellite tag teams

The story unfolded when Cherry and Gary Case, also at LSU, first noticed the Crab’s dimming in observations by the Gamma-ray Burst Monitor (GBM) aboard NASA’s Fermi Gamma-ray Space Telescope.

The team then analysed GBM observations of the object from August 2008 to July 2010 and found an unexpected but steady decline of several percent at four different “hard” X-ray energies.

With the Crab’s apparent constancy well established, the scientists needed to prove that the fadeout was real and was not an instrumental problem associated with the GBM.

“If only one satellite instrument had reported this, no one would have believed it,” Wilson-Hodge said.

Graph showing multi-wavelength observations of the Crab Nebula

Data from four satellites show that the Crab Nebula's energy output has varied. Powerful gamma-ray flares (pink vertical lines) have been detected as well. Graph courtesy NASA Goddard Space Flight Centre.

So the team amassed data from the fleet of sensitive X-ray observatories now in orbit—NASA’s Rossi X-Ray Timing Explorer (RXTE) and Swift satellites and the European Space Agency’s International Gamma-Ray Astrophysics Laboratory (INTEGRAL).

The results confirm a real intensity decline of about 7 percent at certain energy ranges. They also show that the Crab has brightened and faded by as much as 3.5 percent a year since 1999.

The scientists say that astronomers will need to find new ways to calibrate instruments in flight and to explore the possible effects of the inconstant Crab on past findings.

Showing some flare

Fermi’s other instrument, the Large Area Telescope, has detected unprecedented gamma-ray flares from the Crab, showing that it is also surprisingly variable at much higher energies.

The nebula’s power comes from the central neutron star, which is also a pulsar that emits fast, regular radio and X-ray pulses. This pulsed emission exhibits no changes associated with the decline, so it cannot be the source.

Instead, researchers suspect that the long-term changes probably occur in the nebula’s central light-year, but observations with future telescopes will be needed to know for sure.

Adapted from information issued by NASA MSFC.

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Blast blinds telescope

Gamma-ray burst GRB100621A

The brightest gamma-ray burst ever seen in X-rays temporarily blinded Swift's X-ray Telescope on 21 June 2010. This image merges the X-rays (red to yellow) with the same view from Swift's Ultraviolet/Optical Telescope, which showed nothing extraordinary.

  • Blast of X-rays from stellar explosion
  • Temporarily overwhelmed NASA’s Swift space observatory
  • Largest X-ray blast ever seen by Swift

A blast of the brightest X-rays ever detected from beyond our Milky Way galaxy’s neighbourhood temporarily blinded the X-ray eye on NASA’s Swift space observatory last month.

The X-rays had travelled through space for 5 billion years before slamming into and overwhelming Swift’s X-ray Telescope on 21 June.

The blindingly bright blast came from a gamma-ray burst (GRB), a violent eruption of energy from the explosion of a massive star morphing into a new black hole.

“This gamma-ray burst is by far the brightest light source ever seen in X-ray wavelengths at cosmological distances,” said David Burrows, senior scientist and professor of astronomy and astrophysics at Penn State University and the lead scientist for Swift’s X-ray Telescope.

Although the Swift satellite was designed specifically to study GRBs, the instrument was not designed to handle an X-ray blast this bright.

“The intensity of these X-rays was unexpected and unprecedented,” said Neil Gehrels, Swift’s principal investigator at NASA’s Goddard Space Flight Centre. He said the burst, named GRB 100621A, is the brightest X-ray source that Swift has detected since the observatory began work in early 2005.

Artist's impression of the Swift space observatory

The Swift space observatory (artist's impression) can swivel quickly in space to catch the fast flashes of energy from gamma-ray bursts.

“Just when we were beginning to think that we had seen everything that gamma-ray bursts could throw at us, this burst came along to challenge our assumptions about how powerful their X-ray emissions can be,” Gehrels said.

Blast so strong, Swift’s software shut down

“The burst was so bright when it first erupted that our data-analysis software shut down,” said Phil Evans, a postdoctoral research assistant at the University of Leicester in the United Kingdom who wrote parts of Swift’s X-ray-analysis software.

“So many photons were bombarding the detector each second that it just couldn’t count them quickly enough. It was like trying to use a rain gauge and a bucket to measure the flow rate of a tsunami.”

The software soon resumed capturing the evolution of the burst over time, and Evans recovered the data that Swift had detected during the software’s brief shutdown. The scientists then were able to measure the blast’s X-ray brightness at 143,000 X-ray photons per second during its fleeting period of greatest brightness.

That’s more than 140 times brighter than the brightest continuous X-ray source in the sky—a neutron star that is more than 500,000 times closer to Earth than the gamma-ray burst, and that sends a ‘mere’ 10,000 photons per second streaming toward Swift’s telescopes.

Artist's impression of a gamma-ray burst

Artist's impression of a gamma-ray burst, thought to occur when a large star explodes and implodes to become a black hole.

Rapid response to energy bursts

Gamma-ray bursts typically begin with a bright flash of high-energy gamma-rays and X-rays, then fade away like a fireworks display, sometimes leaving behind a disappearing afterglow in less-energetic wavelengths, including optical and ultraviolet.

Surprisingly, although the energy from this burst was the brightest ever in X-rays, it was merely ordinary in optical and ultraviolet wavelengths.

“We never thought we’d see anything this bright,” Burrows said.

The Swift observatory 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. The Burst Alert Telescope provides rapid initial location data.

The 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.

Adapted from information issued by Penn State / Barbara K. Kennedy / NASA / Swift / Stefan Immler.

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