The Pulsar Menagerie
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P ULSARS NEWS the Harvard-Smithsonian Center for Astro- physics in Cambridge, Massachusetts. ECTION S The Pulsar Menagerie Einstein’s test bed Another graduate student was central to the The lighthouses of the universe take on countless guises, as astronomers next pulsar triumph. This time, the Nobel have learned to their ongoing delight during the last 4 decades committee rewarded both the student and PECIAL his adviser with the physics prize, in 1993. S When pulsars spun their way into astro- and observers alike. “Pulsars have blessed us The setting was a search for new pulsars nomical lore in 1967, their debut was with stunning and unexpected discoveries in 1974 at Arecibo Observatory. Russell hardly glamorous. The radio telescope that every few years,” says astrophysicist David Hulse, working under astronomer Joseph found them was not a photogenic dish, but Nice of Princeton University in New Jersey. Taylor of the University of Massachusetts, a 4.5-acre British field wired with 2048 Amherst, devised a computer algorithm that gangly aerials. The first signal, spotted in The iconic Crab picked out pulsars 10 times more sensitively August, was an odd “piece of scruff ” em- Among the 1500 pulsars now known, just than did previous efforts. One object, called bedded within kilometers of chart record- one has an ironclad link to a fiery birth dis- PSR B1913+16 for its sky coordinates, spun ings. Puzzled for months by that flickering play witnessed by scientists: the famous pul- 17 times per second—but its timing pattern light, the University of Cambridge re- sar inside the Crab Nebula. Chinese kept changing. “My reaction was not search team called it LGM-1, in case it astronomers first recorded the Crab super- ‘Eureka, it’s a discovery,’ but ‘Nuts, what’s came from little green men. nova as a “guest star” on 4 July 1054. Today, wrong now?’ ” recalls Hulse, now a fusion By December, it was clear that the the pulsar’s fierce output of energy lights up scientist at the Princeton Plasma Physics pulses—precisely one every 1.33731109 the blast’s expanding tangle of debris. Laboratory. “I was frustrated and annoyed.” seconds—streamed from an exotic, com- Giant radio dishes in Green Bank, West Vexation turned to elation when Hulse pact body. The team’s first report, led by Virginia, and near Arecibo, Puerto Rico, divined the truth: The pulsar darted in an 8- radio astronomer Antony Hewish, pro- spotted the Crab pulsar in 1968. As- hour orbit around another body. Taylor posed “stable oscillations of white dwarf tronomers were startled by its rapid-fire rushed to Arecibo, and the pair confirmed or neutron stars.” But a blaze of research blips, 30 each second. Even more thrilling the system that became known as the Hulse- in 1968 left no doubt that the group had was a perceptible change in its spin rate: Taylor binary. The pulsar’s orbit revealed The Crab’s clock was that its partner was a nonpulsing neutron losing 36.5 billionths of star. Taylor worked with astrophysicist Joel a second per day. Weisberg, now at Carleton College in The results vindicat- Northfield, Minnesota, and others to show ed a bold model put that Einstein’s general theory of relativity forward by theorist was needed to describe the strong gravita- Thomas Gold of Cor- tional embrace of the two objects—the first nell University in Itha- such test outside our solar system. ca, New York. In Gold’s Most critically, the team showed that the view, pulsars were spin- two bodies inexorably spiral together, at ex- ning neutron stars that actly the rate predicted by Einstein 60 years gradually wound down earlier. Gravitational waves carry away the under the influence of lost orbital energy. “It’s indirect, like show- intense magnetic fields. ing that radio waves exist because you know Energy lost by a pulsar’s the radio transmitter uses power,” Hulse deceleration, he rea- says. “But it was the first evidence for the soned, would illuminate existence of gravitational waves.” the nebula. Part of the energy would stream Outrageous spin High-energy fire hose. The Vela pulsar (bottom left of each image) into space within two By the late 1970s, research on pulsars had expels a rapidly fluctuating jet of particles half a light-year long, cylinders of light, dwindled, says astronomer Donald Backer revealed by the Chandra X-ray Observatory. sweeping through the of the University of California (UC), Berke- cosmos like lighthouse ley. It was a perfect time for a whopper, and unveiled an object prophesied in the 1930s beams as the pulsar spun. The broad details Backer’s team found it at Arecibo in 1982: by astrophysicists Walter Baade, Fritz of Gold’s model still reign today. PSR B1937+21, rotating a jaw-dropping Zwicky, and Lev Landau: an ultradense The Crab quickly rose to iconic status. 642 times per second. Yet another graduate neutron star, perhaps 20 kilometers across, But as it turns out, the pulsar is far from typ- student, Shrinivas Kulkarni (now at the left by a massive star’s death in a superno- ical. It’s unusually bright, spewing a huge California Institute of Technology in va explosion long ago. portion of its energy into radio waves and Pasadena), made the key initial find at the ET AL. Hewish went on to share the 1974 Nobel x-rays. Moreover, it raised hopes that hordes telescope. Prize in physics—an honor that also should of pulsars would turn up in other supernova This “millisecond pulsar” sparked a theo- have recognized his graduate student Jocelyn remnants, but most searches have struck ry frenzy. It was nowhere near a supernova Bell Burnell for her central role in the discov- out. “There is another branch of compact remnant, but its rotational speed—exceeding ery, many astronomers feel. Since then, a objects created, and they’re definitely differ- 1/10th the speed of light at its equator— stream of findings has made pulsars the dar- ent from the canonical pulsars everyone ex- looked like that of a freshly minted neutron lings of high-energy astrophysics for theorists pected,” says astrophysicist Patrick Slane of star. Astrophysicists Malvin Ruderman, the CREDIT: NASA/CXC/PSU/G. PAVLOV 532 23 APRIL 2004 VOL 304 SCIENCE www.sciencemag.org P ULSARS S PECIAL late Jacob Shaham, and colleagues at Colum- dio astronomer Dale Frail of the National Ra- tivity than the Hulse-Taylor binary bia University in New York City suggested dio Astronomy Observatory in Socorro, New (Science, 9 January, p. 153). that the pulsar was old but got a new lease on Mexico, saw minuscule fluctuations in the ar- Already, astrophysicists are mystified life by yanking material from a companion rival times of pulses from PSR B1257+12. by the energetic interplay between the neu- S star. Gas plunging onto the greedy neutron Their model suggested that two planets, a few tron stars. Intense winds from the faster ro- ECTION star spun it up to outrageous rates, like a mer- times larger than Earth, tugged the pulsar to tating pulsar create a tear-shaped shock and fro ever so slightly as they orbited. wave around the slower pulsar. Teams are The claim was controversial, but probing this process as one pulsar dips be- Wolszczan thinks the impact was worth hind the other, every 2.4 hours. the early catcalls. “After a relatively In one interpretation of the data, the fast short period of disbelief, the average pulsar is churning out 100,000 to 1 million reaction was that the existence of plan- times more charged gas than expected ets around a neutron star must mean from the seething region above its surface, that the planet-production process in says theorist Jonathan Arons of UC Berke- general was a robust one,” he says. In- ley. “The physics is not quite incredible, deed, more than 100 other planets are but it’s close,” he says. now known, although PSR B1257+12 As searches for new pulsars continue, is still the only burned-out corpse of a astronomers await the next varieties on Pulsars Surf the Cosmic Waves Imagine floating in the middle of a large lake, where you can’t quite see the shore. If peo- ple throw big rocks into the water, you’ll feel a riot of ripples. With the right tools, you might be able to tell how many rocks splashed in—and how big they were. In space, the “rocks” are binary pairs of giant black holes at the cores of galaxies, and the Tilt-a-whirl. Misalignment between a pulsar’s “ripples” are gravitational waves. Long before they crash, the spiraling black holes churn the spin axis and magnetic axis sends an inclined waters of spacetime. The waves, spanning light-years, traverse the entire universe. As each one radio beam whirling into space. passes by, it distorts the apparent distances between objects in a subtle but distinct pattern. Millisecond pulsars surf those waves. As they bob to and fro, their signals arrive in ry-go-round getting constant shoves. telescopes on Earth at slightly different times. By timing an array of such pulsars for Since then, astronomers have found near- years, astronomers think they might spot the imprints of gravitational waves in this ly 100 such “recycled” pulsars. Some are iso- decade—perhaps beating out several major physics experiments on the ground. lated, but others have a partner in the process Astronomer Donald Backer of the Univer- of being eaten or literally blown away by the sity of California, Berkeley, thinks his team is pulsar’s radiation. For instance, the “black “within a factor of 5” of the necessary preci- widow” pulsar, found in 1988 by astronomer sion to detect the waves, using about a dozen Andrew Fruchter of the Space Telescope Sci- pulsars in the Northern Hemisphere.