Science Article

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ASTROPHYSICS:
A Very Good Year For Explosions
Robert Irion

Abundant cataclysms studied in 2005 kept astrophysicists tuned to extreme neutron stars in our galaxy and beyond, as well as the most distant blasts yet seen.

If you catch them at happy hour, an alarming number of high-energy astrophysicists will admit that they liked to blow things up as children. Nowadays they have graduated to bigger and better things--and blasts. In 2005, in fact, their field enjoyed its most explosive year in decades.
Telescopes caught one startling blast after another, with convulsions on an ultramagnetic neutron star beyond the center of our Milky Way ending 2004 with a bang. Rapid bursts in remote galaxies appeared to come from long-sought collisions between two neutron stars or a neutron star and a black hole. And the most distant explosions ever seen, hailing from the first billion years of cosmic history, marked the deaths of giant stars.
The discoveries marked a stunning inaugural year for NASA's Swift satellite, launched in November 2004 to detect the fleeting explosions called gamma ray bursts (GRBs) (Science, 8 October 2004, p. 214). Other satellites and a growing roster of telescopes on the ground--including many new robotic systems--partnered with Swift to observe GRBs and their home galaxies in gamma rays, x-rays, optical and infrared light, and radio waves.
The results, especially the outbursts from neutron stars, yielded vivid insights into the violent universe. "We have hoped for these observations for years," says theorist Stephan Rosswog of the International University Bremen in Germany. Observers were thrilled as well, after years of doubt that they would ever catch up to the transient sky. "If you get onto the telescope quickly enough, you can learn amazing new things about why these objects explode," says radio astronomer Bryan Gaensler of the Harvard- Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts.
The long and short of GRBs
Of all the explosions observed by Swift and its telescopic partners, "short" GRBs garnered the most headlines last year. These pulses of gamma rays, lasting fractions of a second, had eluded explanation for 35 years. "It was an open playing field for theorists," says Edo Berger, a Hubble postdoctoral fellow at the Carnegie Observatories in Pasadena, California. "Then, in just two or three months, we answered the basic questions about them. It was really amazing."
Astrophysicists had been confident that short GRBs erupt from different sources than do "long" ones, which linger many seconds to minutes. Several years ago, research showed that long GRBs arise when the spinning cores of massive stars collapse into black holes. Tight beams of gamma rays tunnel outward through the stars, which then detonate in powerful supernovas visible in optical light. But this messy process is too drawn-out to explain short GRBs.
To account for those bursts, astrophysicists favored quick and deadly mergers of neutron stars: the dense remnants of large stars with cores that fall just short of making black holes. Models of two crashing neutron stars seemed consistent with the sketchy data about short GRBs. Further, astrophysicists had identified neutron-star binaries in the Milky Way and had confirmed that some of them slowly spiral together. Fiery collisions in other galaxies seemed inevitable.
Telescopes caught several such flares in 2005. There were telltale signs of compact mergers: brief gamma ray flashes, no accompanying supernovas, and energies just 0.1% to 1% as prodigious as those of long GRBs. And in three out of four well-studied cases, the short GRBs appeared to blow up in the outskirts of old burned-out galaxies, where stars haven't formed for at least two billion years. Supernovas have long stopped exploding there, but their shrunken neutron-star remnants could still be slowly converging. The fourth short GRB, spotted on 9 July by NASA's High-Energy Transient Explorer-2 satellite, appeared in a completely different setting: a dwarf galaxy that was still creating new stars. But it could still have come from the same sort of collision, theorists say, because some neutron stars--including a tight binary in our own active galaxy--merge much more quickly if they start out close together.
University press releases and NASA's publicity juggernaut declared that compact binary mergers "solved" the short GRB mystery. But many astrophysicists urged restraint. "Everyone jumped on the neutron star merger bandwagon, but there may be other physical causes," says Neil Gehrels of NASA's Goddard Space Flight Center in Greenbelt, Maryland, Swift's principal investigator.
For one, an unknown fraction of short GRBs may come from neutron stars plunging into black holes. Models suggest that such bursts would display a distinct pattern: flares of x-rays minutes later, as the black hole finishes off debris torn from the neutron star by intense tidal forces. A short GRB that Swift spotted on 24 July emitted such delayed flares, leading NASA to proclaim discovery of a neutron star-eating black hole.
But that's not the only explanation for the July event. Theorist Andrew MacFadyen of the Institute for Advanced Study in Princeton, New Jersey, and colleagues proposed that a single neutron star could suck enough gas from a nearby companion star, creating an object massive and dense enough to form its own black hole. The collapse would spark a short GRB, followed minutes later by x-ray flares as the blast wave struck the parasitized star.
The small number of short GRBs studied in detail so far makes any claims of black holes or other sources tenuous at best, Gehrels agrees. "We think neutron star--neutron star mergers are the most common," he says. "But once we've seen 10 to 100 of these, we'll know a lot better whether any of them stick out as unusual."
LIGO lies in wait
One potential observation at the time of a short GRB would settle all debate: gravitational waves. Einstein's general theory of relativity predicts that inward spiraling binary neutron stars or black holes should distort and ripple the fabric of space-time, producing such waves. The shapes of the resulting waves would depend on the masses of the two objects, the eccentricities of their orbits, and our viewing angle, which affects the patterns of waves we observe. As a result, detecting gravitational waves along with a GRB "would really nail the nature of the compact binary," Rosswog says.
And astrophysicists may finally have the tool to see Einstein's waves. The two facilities of the Laser Interferometer Gravitational-wave Observatory (LIGO) in Hanford, Washington, and Livingston, Louisiana, have reached their promised sensitivities for the project's first phase and will gather scientific data throughout 2006.
At today's sensitivity, LIGO could firmly detect a typical neutron star merger 30 million light-years away, says physicist David Shoemaker of the Massachusetts Institute of Technology in Cambridge. That range extends to 70 million light-years if the viewing angle is good, and even farther if a black hole is involved. "We are certainly optimistic," Shoemaker says. "There is no doubt we are in completely new territory in terms of the probability of observing something."
A perfect magnetic storm
Although technically not a 2005 event, an extraordinary outburst on the far side of the Milky Way on 27 December 2004 dominated much of the discussion of short GRBS in the past year. The unusual blast raised the odds that many gamma ray flashes pop off in relatively nearby galaxies--and from radically different sources.
The explosion came from an object about 50,000 light-years away called SGR 1806-20, an exotic neutron star ensnared by the strongest magnetic fields known (Science, 23 April 2004, p. 534). Other "magnetars" had erupted with violent flares in 1979 and 1998, but the December event astonished observers. It was brighter than any solar flare, even from its great distance. The x-rays and gamma rays swamped nearly every orbiting detector. Fingernail-sized particle counters on a few satellites kept up with the onslaught, revealing that the explosion released as much energy in a 0.2-second spike as the sun churns out in 250,000 years.
The flare's features jibed with a magnetar model developed in the 1990s by theorists Robert Duncan of the University of Texas at Austin and Christopher Thompson of the Canadian Institute for Theoretical Astrophysics in Toronto. In their scenario, the neutron star's interior is shot through with fantastically tangled magnetic fields, a remnant of the star's youthful spin. Judging by the immense punch from SGR 1806-20, the magnetic field may reach 1016 gauss--three times as high as Duncan had previously believed, and 10,000 to 100,000 times stronger than fields on most neutron stars. Over time, the field lines untwist and diffuse toward the surface, forcing the star's magnetized crust to shift. When these shifts become extreme, the entire surface fails and yields. The external field lines, suddenly displaced, whip into new configurations. The implosive release of magnetic tension triggers a blast of gamma rays and other radiation.
Researchers are debating the contents of this blast wave. One clue comes from a nebula expanding into space around the magnetar at 30% the speed of light. High-resolution radio images revealed a surprisingly stretched glowing cloud, created by accelerated particles. "Contrary to expectations, the explosion may not have spread over the entire star," says Bryan Gaensler of CfA. "Material may have been thrown off one side or focused into a jet." Gaensler and his colleagues will use the Very Large Array of 27 radio telescopes in Socorro, New Mexico, on 4 February to scrutinize the nebula's evolving shape.
But evidence suggests that most of the flare's energy didn't emerge in this lopsided particle flow. The blast's initial energy spectrum was nearly that of a perfectly radiating blackbody with a temperature of 2 billion degrees kelvin, Duncan says. "To make that happen you need a clean source of energy from magnetic reconnection, with little matter involved."
Theorist Roger Blandford has a picture of how the 27 December flare proceeded. The magnetar's external fields initially assumed a "smoke ring" geometry used in a spheromak, a prototype of a magnetically controlled nuclear fusion device, says Blandford, director of the Kavli Institute for Particle Astrophysics and Cosmology in Stanford, California. "If you suddenly release this confined field, it's like an electromagnetic bomb that expands relativistically. There is still some plasma to create the gamma rays, but it's mostly magnetic field." The doughnut-shaped geometry of the magnetic stresses neatly explains the squashed nebula that resulted, he adds.
The magnetar flare also renewed interest in whether similar events in other galaxies produced many of the short GRBs that Swift and previous gamma ray satellites have observed. Even though the SGR 1806-20 outburst came from a single neutron star, it bears an eerie resemblance to explosions from merging neutron stars, says astronomer Joshua Bloom of the University of California (UC), Berkeley. "If you squint your eyes, they almost look the same." The only difference is that astrophysicists can resolve more details for the Milky Way blast, such as x-ray oscillations possibly due to vibrations of the neutron star's crust.
Astrophysicists now think a short GRB detected on 3 November 2005 was a magnetar flare in a nearby group of well-known galaxies. Astronomer Kevin Hurley of UC Berkeley, who coordinates a network of solar-system probes capable of detecting such flares, believes that extragalactic magnetars produce 1/5 to 1/6 of all short GRBs.
A team in the United Kingdom reached a similar conclusion by examining archival records of short GRBs recorded by NASA's Compton Gamma Ray Observatory, which flew from 1991 to 2000. Astronomer Nial Tanvir of the University of Hertfordshire, U.K., found a modest correlation between the locations of about 500 short bursts seen by the satellite and the positions of galaxies in our neighborhood of the universe, within about 300 million light-years. Although those "local" galaxies are just a tiny fraction of all galaxies in the cosmos, they may have produced 10% to 25% of Compton's short GRBs, the team reported in the 15 December Nature. This suggests that magnetar flares--rather than much rarer neutron-star collisions--do indeed account for most of the short GRBs in nearby galaxies.
For Blandford, SGR 1806-20 was the highlight of a rich period in astrophysics. "This was a rather magical thing to happen," he says. "We were lucky to see it with so many telescopes."
The great bright hopes
No luck was involved in the other explosive advance of 2005: GRBs from the era of galaxy formation. Swift has seen two of them so far, most notably a burst on 4 September from a star that died when the universe was just 900 million years old. A 14 August GRB was less well studied but appeared to date to a cosmic age of 1.1 billion years.
Both astrophysicists and cosmologists covet GRBs from even earlier epochs. Astrophysicists hope such primeval bursts will give clues to the types of stars that existed within a few hundred million years of the big bang. The first generation of stars, called "Population III," consisted only of primordial hydrogen and helium. These stars made carbon, oxygen, and heavier elements such as iron, starting the chemical evolution of the universe that continues today. Models suggest that Population III stars were at least 100 times as massive as our sun--huge enough to explode as supernovae (Science, 4 January 2002, p. 66). However, physical conditions may have stifled GRBs from the dying stars.
One barrier is the massive envelope of hydrogen in a Population III star. That gas could have acted like a wet blanket, damping the jets of a GRB and preventing their escape when the star's core collapses. New research suggests one way out: If a binary companion strips much of this material, then the GRB blast might break out into space, according to calculations by astrophysicists Volker Bromm of the University of Texas at Austin and Abraham Loeb of CfA.
Bromm and Loeb think Swift's detector might not be quite sensitive enough to spot faint radiation from the earliest Population III GRBs, those that happened within the first 200 million to 500 million years of cosmic time. But if pristine pockets of Population III star formation persisted a few hundred million years later than that, Swift might catch some of their deaths. "Whatever Swift does see, it will help us construct better models of the history of star formation at these times," Bromm says.
Cosmologists are equal fans of Swift, for a different reason: GRBs are ideal probes of the early universe. "For a short time, they are so much brighter than quasars at those distances," says astrophysicist Donald Lamb of the University of Chicago. "They are the great bright hopes of cosmology." Like needle-sharp searchlights, GRBs would illuminate all material along the way to Earth. In particular, cosmologists are eager to learn about how radiation from the earliest stars and galaxies sculpted and ionized the ingredients of the young cosmos. Each distant GRB will expose a bit more of that growth history, Lamb says.
Lamb is optimistic that about 10% of Swift's GRBs will date back to the first billion years of the universe. He thinks a few may even unveil the environment of embryonic galaxies just 500 million years after the big bang. But to take full advantage of the potential science, the largest telescopes on the ground must be ready to gather light before the bursts fade. That hasn't happened yet; for the 4 September GRB, it took 3.5 days for Japan's 8.2-meter Subaru telescope at Mauna Kea, Hawaii, to take marginal data. "We have to get our house in order," Lamb comments.
Still, there's no denying Swift's landmark find. By responding to a faint cry of gamma rays that had journeyed across space for 12.77 billion years, the satellite and its partner telescopes exposed light from the most distant single star yet seen--the type of object that set the stage for a mature universe brimming with violence.

----- If you don't tell them, I won't either. :-) -----

(edit) I love the phrase 'perfectly radiating blackbody with a temperature of 2 billion degrees kelvin' :-)

Oh yeah...!! The researchers generally use logarithmic graph plots to catch all those orders of magnitude! It's nice to be in a 'quiet' corner of the universe.
I still try to get my head around the 'rigidity' of spacetime, meaning that it takes quite an energy density to produce even a small disturbance. If we are discovering effects of the order of fractions of a nuclear width far out here in the backwaters, imagine the conditions near the sources - unsafe for sure.
As for 'high-energy astrophysicists will admit that they liked to blow things up as children' ...... well, duh ... didn't we all? :-)