"ROSETTA STONE" FOUND TO DECODE THE MYSTERY OF GAMMA RAY BURSTS

Scientists have pieced together the key elements of a gamma-ray burst, from star death to dramatic black hole birth, thanks to a March 29, 2003 explosion considered the "Rosetta stone" of such bursts.This telling March 29 burst in the constellation Leo, one of the brightest and closest on record, reveals for the first time that a gamma-ray burst and a supernova -- the two most energetic explosions known in the Universe -- occur essentially simultaneously, a quick and powerful one-two punch.

The results appear in the June 19 issue of Nature. The burst was detected by NASA's High-Energy Transient Explorer (HETE) and observed in detail with the European Southern Observatory's Very Large Telescope (VLT) at the Paranal Observatory in Chile.

"We've been waiting for this one for a long, long time," said Dr. Jens Hjorth, University of Copenhagen, lead author on one of three Nature letters. "The March 29 burst contains all the missing information. It was created through the core collapse of a massive star."

The team said that the Rosetta stone burst also provides a lower limit on how energetic gamma-ray bursts truly are and rules out most theories concerning the origin of "long bursts," lasting longer than two seconds.

Gamma-ray bursts temporarily outshine the entire Universe in gamma-ray light, packing the energy of over a million billion suns. Yet these explosions are fleeting -- lasting only seconds to minutes -- and occur randomly from all directions on the sky, making them difficult to study.

A supernova is associated with the death of a star about eight times as massive as the Sun or more. When such stars deplete their nuclear fuel, they no longer have the energy (in the form of radiation pressure outward) to support their mass. Their cores implode, forming either a neutron star or (if there is enough mass) a black hole. The surface layers of the star blast outward, forming the colorful patterns typical of supernova remnants.

Scientists have suspected gamma-ray bursts and supernovae were related, but they have had little observational evidence, until March 29.

 "The March 29 burst changes everything," said co-author Dr. Stan Woosley, University of California, Santa Cruz. Just as the Rosetta stone helped us understand a lost, ancient language, this burst will serve as a tool to decode gamma-ray bursts. It's now known for certain that at least some gamma-ray bursts are produced when black holes, or perhaps very unusual neutron stars, are born inside massive stars, according to the team GRB 030329, named after its detection date, occurred relatively close, approximately 2 billion light years away (at redshift 0.1685). The burst lasted over 30 seconds. ("Short bursts" are less than 2 seconds long.) GRB 030329 is among the 0.2% brightest bursts ever recorded. Its afterglow lingered for weeks in lower-energy X-ray and visible light.

With the VLT, Hjorth and his colleagues uncovered evidence in the afterglow of a massive, rapidly expanding supernova shell, called a hypernova, at the same position and created at the same time as the afterglow. The following scenario emerged: Thousands of years prior to this explosion, a very massive star, running out of fuel, let loose much of its outer envelope, transforming itself into a bluish Wolf-Rayet star. The Wolf-Rayet star -- containing about 10 solar masses worth of helium, oxygen and heavier elements -- rapidly depleted its fuel, triggering the Type Ic supernova / gamma-ray burst event. The core collapsed, without the star's outer part knowing. A black hole formed inside surrounded by a disk of accreting matter, and, within a few seconds, launched a jet of matter away from the black hole that ultimately made the gamma-ray burst.

The jet passed through the outer shell of the star and, in conjunction with vigorous winds of newly forged radioactive nickel-56 blowing off the disk inside, shattered the star. This shattering represents the supernova event. Meanwhile, collisions among pieces of the jet moving at different velocities, all very close to light speed, created the gamma-ray burst. This "collapsar" model, introduced by Woosley in 1993, best explains the observation of GRB 030329, as opposed to the "supranova" and "merging neutron star" models.

In previous gamma-ray bursts, scientists had found evidence of iron in the afterglow light, a signature of a star explosion. Also, the location of a supernova occurring in 1998, named SN1998bw, appeared to be in the same vicinity as a gamma-ray burst. The data was inconclusive, however, and many scientists remained skeptical of the association.

Supernova 1998bw whetted our appetite," said co-author Dr. Chryssa Kouveliotou of the NASA Marshall Space Flight Center in Huntsville, Ala. "But it took five more years before we could confidently say we found the smoking gun that nailed the association between gamma-ray bursts and supernovae, at least for some bursts."

"This does not mean that the gamma-ray burst mystery is solved," Woosley said. "We are confident that long bursts involve a core collapse, probably creating a black hole. We have convinced most skeptics. We cannot reach any conclusion yet, however, on what causes short gamma-ray bursts."

Short bursts might be caused by neutron star mergers. A NASA-led international satellite named Swift, to be launched in January 2004, will "swiftly" locate gamma-ray bursts and may capture short-burst afterglows, which have yet to be detected.

The VLT is the world's most advanced optical telescope, comprising four 8.2-meter reflecting Unit Telescopes and, in the future, four moving 1.8-meter Auxiliary Telescopes for interferometry. HETE was built by MIT as a mission of opportunity under the NASA Explorer Program, with collaboration among U.S. universities, Los Alamos National Laboratory, and scientists and organizations in Brazil, France, India, Italy and Japan.

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Comet Lovejoy Plunges into the Sun and Survives

COMET LOVEJOY

This morning, an armada of spacecraft witnessed something that many experts thought impossible. Comet Lovejoy flew through the hot atmosphere of the sun and emerged intact.

"It's absolutely astounding," says Karl Battams of the Naval Research Lab in Washington DC. "I did not think the comet's icy core was big enough to survive plunging through the several million degree solar corona for close to an hour, but Comet Lovejoy is still with us."

The comet's close encounter was recorded by at least five spacecraft: NASA's Solar Dynamics Observatory and twin STEREO probes, Europe's Proba2 microsatellite, and the ESA/NASA Solar and Heliospheric Observatory. The most dramatic footage so far comes from SDO, which saw the comet go in (below) and then come back out again (above).

In the SDO movies, the comet's tail wriggles wildly as the comet plunges through the sun's hot atmosphere only 120,000 km above the stellar surface. This could be a sign that the comet was buffeted by plasma waves coursing through the corona. Or perhaps the tail was bouncing back and forth off great magnetic loops known to permeate the sun's atmosphere. No one knows.

"This is all new," says Battams. "SDO is giving us our first look at comets traveling through the sun's atmosphere. How the two interact is cutting-edge research."

"The motions of the comet material in the sun's magnetic field are just fascinating," adds SDO project scientist Dean Pesnell of the Goddard Space Flight Center. "The abrupt changes in direction reminded me of how the solar wind affected the tail of Comet Encke in 2007 (view movie)."

Comet Lovejoy was discovered on Dec. 2, 2011, by amateur astronomer Terry Lovejoy of Australia. Researchers quickly realized that the new find was a member of the Kreutz family of sungrazing comets. Named after the German astronomer Heinrich Kreutz, who first studied them, Kreutz sungrazers are fragments of a single giant comet that broke apart back in the 12th century (probably the Great Comet of 1106). Kreutz sungrazers are typically small (~10 meters wide) and numerous. The Solar and Heliospheric Observatory sees one falling into the sun every few days.

At the time of discovery, Comet Lovejoy appeared to be at least ten times larger than the usual Kreutz sungrazer, somewhere in the in the 100 to 200 meter range. In light of today's events, researchers are re-thinking those numbers.

"I'd guess the comet's core must have been at least 500 meters in diameter; otherwise it couldn't have survived so much solar heating," says Matthew Knight. "A significant fraction of that mass would have been lost during the encounter. The remains are probably much smaller."

SOHO and NASA's twin STEREO probes are monitoring the comet as it recedes from the sun. It is still very bright and should remain in range of the spacecrafts' cameras for several days to come.

What happens next is anyone's guess.

"There is still a possibility that Comet Lovejoy will start to fragment," continues Battams. "It' been through a tremendously traumatic event; structurally, it could be extremely weak. On the other hand, it could hold itself together and disappear back into the recesses of the solar system."

"It's hard to say," agrees Knight. "There has been so little work on what happens to sungrazing comets after perihelion (closest approach). This continues to be fascinating."

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Comet Lovejoy

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NASA's Fermi Shows That Tycho's Star Shines in Gamma Rays

Gamma Ray

In early November 1572, observers on Earth witnessed the appearance of a "new star" in the constellation Cassiopeia, an event now recognized as the brightest naked-eye supernova in more than 400 years. It's often called "Tycho's supernova" after the great Danish astronomer Tycho Brahe, who gained renown for his extensive study of the object. Now, years of data collected by NASA's Fermi Gamma-Ray Space Telescope reveal that the shattered star's remains shine in high-energy gamma rays.

The detection gives astronomers another clue in understanding the origin of cosmic rays, subatomic particles -- mainly protons -- that move through space at nearly the speed of light. Exactly where and how these particles attain such incredible energies has been a long-standing mystery because charged particles speeding through the galaxy are easily deflected by interstellar magnetic fields. This makes it impossible to track cosmic rays back to their sources.

"Fortunately, high-energy gamma rays are produced when cosmic rays strike interstellar gas and starlight. These gamma rays come to Fermi straight from their sources," said Francesco Giordano at the University of Bari and the National Institute of Nuclear Physics in Italy. He is the lead author of a paper describing the findings in the Dec. 7 edition of The Astrophysical Journal Letters.

Better understanding the origins of cosmic rays is one of Fermi's key goals. Its Large Area Telescope (LAT) scans the entire sky every three hours, gradually building up an ever-deeper view of the gamma-ray sky. Because gamma rays are the most energetic and penetrating form of light, they serve as signposts for the particle acceleration that gives rise to cosmic rays.

"This detection gives us another piece of evidence supporting the notion that supernova remnants can accelerate cosmic rays," said co-author Stefan Funk, an astrophysicist at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), jointly located at SLAC National Accelerator Laboratory and Stanford University, Calif.

In 1949, physicist Enrico Fermi -- the satellite's namesake -- suggested that the highest-energy cosmic rays were accelerated in the magnetic fields of interstellar gas clouds. In the decades that followed, astronomers showed that supernova remnants may be the galaxy's best candidate sites for this process.

When a star explodes, it is transformed into a supernova remnant, a rapidly expanding shell of hot gas bounded by the blast's shockwave. Scientists expect that magnetic fields on either side of the shock front can trap particles between them in what amounts to a subatomic pingpong game.

"A supernova remnant's magnetic fields are very weak relative to Earth's, but they extend across a vast region, ultimately spanning thousands of light-years. They have a major influence on the course of charged particles," said co-author Melitta Naumann-Godo at Paris Diderot University and the Atomic Energy Commission in Saclay, France, who led the study with Giordano.

As they shuttle back and forth across the supernova shock, the charged particles gain energy with each traverse. Eventually they break out of their magnetic confinement, escaping the supernova remnant and freely roaming the galaxy.

The LAT's ongoing sky survey provides additional evidence favoring this scenario. Many younger remnants, like Tycho's, tend to produce more high-energy gamma rays than older remnants. "The gamma-ray energies reflect the energies of the accelerated particles that produce them, and we expect more cosmic rays to be accelerated to higher energies in younger objects because the shockwaves and their tangled magnetic fields are stronger," Funk added. By contrast, older remnants with weaker shockwaves cannot retain the highest-energy particles, and the LAT does not detect gamma rays with corresponding energies.

The supernova of 1572 was one of the great watersheds in the history of astronomy. The star blazed forth at a time when the starry sky was regarded as a fixed and unchanging part of the universe. Tycho's candid account of his own discovery of the strange star gives a sense of how radical an event it was.

The supernova first appeared around Nov. 6, but poor weather kept it from Tycho until Nov. 11, when he noticed it during a walk before dinner. "When I had satisfied myself that no star of that kind had ever shone forth before, I was led into such perplexity by the unbelievability of the thing that I began to doubt the faith of my own eyes, and so, turning to the servants who were accompanying me, I asked them whether they too could see a certain extremely bright star…. They immediately replied with one voice that they saw it completely and that it was extremely bright," he recalled.

The supernova remained visible for 15 months and exhibited no movement in the heavens, indicating that it was located far beyond the sun, moon and planets. Modern astronomers estimate that the remnant lies between 9,000 and 11,000 light-years away.

After more than two and a half years of scanning the sky, LAT data clearly show that an unresolved region of GeV (billion electron volt) gamma-ray emission is associated with the remnant of Tycho's supernova. (For comparison, the energy of visible light is between about 2 and 3 electron volts.)

Keith Bechtol, a KIPAC graduate student who is also based at SLAC, was one of the first researchers to notice the potential link. "We knew that Tycho's supernova remnant could be an important find for Fermi because this object has been so extensively studied in other parts of the electromagnetic spectrum. We thought it might be one of our best opportunities to identify a spectral signature indicating the presence of cosmic-ray protons," he said.

The science team's model of the emission is based on LAT observations, along with higher-energy TeV (trillion electron volt) gamma rays mapped by ground-based facilities and radio and X-ray data. The researchers conclude that a process called pion production best explains the emission. First, a proton traveling close to the speed of light strikes a slower-moving proton. This interaction creates an unstable particle -- a pion -- with only 14 percent of the proton's mass. In just 10 millionths of a billionth of a second, the pion decays into a pair of gamma rays.

If this interpretation is correct, then somewhere within the remnant, protons are being accelerated to near the speed of light, and then interacting with slower particles to produce gamma rays, the most extreme form of light. With such unbelievable goings-on in what's left of his "unbelievable" star, it's easy to imagine that Tycho Brahe himself might be pleased.

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Cassini to Make a Double Play

A quartet of Saturn's moons, from tiny to huge, surround and are embedded within the planet's rings in this Cassini composition.

In an action-packed day and a half, NASA's Cassini spacecraft will be making its closest swoop over the surface of Saturn's moon Dione and scrutinizing the atmosphere of Titan, Saturn's largest moon.

The closest approach to Dione, about 61 miles (99 kilometers) above the surface, will take place at about 1:39 a.m. PST (4:39 a.m. EST) on Dec. 12. One of the questions Cassini scientists will be asking during this flyby is whether Dione's surface shows any signs of activity. Understanding Dione's internal structure will help address that question, so Cassini's radio science instrument will learn how highly structured the moon's interior is by measuring variations in the moon's gravitational tug on the spacecraft. The composite infrared spectrometer instrument will also look for heat emissions along fractures on the moon's surface.

Cassini will also be probing whether Dione, like another Saturnian moon, Rhea, has a tenuous atmosphere. Scientists expect a Dionean atmosphere – if there is one – to be much more ethereal than even Rhea's. Research published in journal Geophysical Research Letters and led by Sven Simon, a Cassini magnetometer team member at the University of Cologne, Germany, found magnetic field disturbances around Dione, hinting at a tenuous atmosphere. But scientists hope to get stronger confirmation by "tasting" the space around the moon with Cassini's ion and neutral mass spectrometer.

On Cassini's journey out from Dione toward Titan, the imaging science subsystem will turn back to look at Dione's distinctive, wispy fractures and a ridge called Janiculum Dorsa.

Cassini will approach within about 2,200 milles (3,600 kilometers) of the Titan surface, at about 12:11 p.m. PST (3:11 PM EST) on Dec. 13. At Titan, the composite infrared spectrometer will be making measurements to understand how the seasonal transition from spring to summer affects wind patterns in the atmosphere near Titan's north pole. It will also search for mist.

The visual and infrared mapping spectrometer and imaging science subsystem will be observing the same equatorial deserts where the imaging science subsystem saw sudden and dramatic surface changes last year, when Titan was experiencing early northern spring. One possibly theory is that rainstorms caused these changes. As Cassini recedes from Titan, the imaging cameras will also continue to observe the moon for another day to monitor any new weather systems. The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington.

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