A New Type of Star

A pulsar shoots out powerful bursts of x-rays.

By Kathy A. Svitil|Monday, July 01, 1996

Last December 2 a strange signal reached Earth from a source thousands of light-years away in the center of our galaxy. The signal, composed of energetic outbursts of X-rays--up to 18 an hour, and each more powerful than the radiation from 100,000 suns--was immediately noticed by nasa’s Burst and Transient Source Experiment. batse, which flies onboard the orbiting Compton Gamma Ray Observatory, is designed to detect any gamma ray emissions (and energetic X-rays) in the sky. But while batse was busy gathering data on the unusual beacon, no scientists were yet aware of its presence. December 2 was a Saturday, and even astrophysicists occasionally take the weekend off.

On Monday morning, however, when batse researchers examined the weekend’s data, they knew right away that they had something exciting. In the idiom of astrophysicists, the beacon was a burster--a neutron star sending out periodic bursts of X-rays (which by Monday had dropped to a frequency of about one an hour). But that discovery would not be the end of the story. About a week and a half after batse detected the bursting X-ray emissions, it picked up another X-ray signal. This one was much less energetic, but it was coming from the same general area of the sky as the X-ray outbursts.

Within a few days, batse researchers had determined that the second signal was from a pulsar--a neutron star emitting a continuous beam of X-rays from its magnetic poles. The X-ray signals appear to pulse because the beam sweeps across the sky as the star rotates, and only periodically falls on Earth. These pulses, the researchers determined, were coming steadily at a rate of two per second. Then, in early January, the intensity of the original bursting signal increased. When the bursts became stronger, we could see modulations in that signal much more easily than before, and we could detect pulsations in it too, says astrophysicist Chryssa Kouveliotou, a batse senior research scientist. The timing of the two signals, researchers found, was identical. We knew that the burster and the pulsar were the same source, and we knew that we had a unique object.

This bizarre bursting pulsar, as it came to be known (officially, it’s groj1744-28, or 1744 for short), shouldn’t exist, according to conventional theory. Both pulsars and bursters, astrophysicists have long thought, involve a binary system: two stars orbiting each other. One of those stars is an incredibly dense neutron star, ten or so miles in diameter, but with the mass of our sun. The other star is an expanding red giant, with a bloated outer shell of hydrogen that gets whisked away by the strong gravitational pull of its partner. The matter forms a disk around the neutron star and gradually spirals to the star’s surface. In the case of 1744, the two stars are some 12 million miles apart--just one-third the distance separating Mercury and our sun-- and orbit each other once every 12 days.

Pulsars, according to conventional theory, are neutron stars with immense magnetic fields--about a trillion times the strength of Earth’s-- that funnel hydrogen pulled from their red-giant neighbor continuously down onto their magnetic poles. (The electrically conductive ionized hydrogen creates its own magnetic field, which allows the pulsar’s magnetic field to push and pull it around.) The pulsar’s gravity accelerates the hydrogen to about half the speed of light, and on impact the ions release tremendous amounts of energy in the form of X-rays, which stream out along the pulsar’s magnetic poles.

The generation of X-rays on bursters, astronomers have thought, is different. Bursters, says Coleman Miller, an astrophysicist at the University of Chicago, are neutron stars that have weaker magnetic fields than pulsars--although still a hundred million or a billion times stronger than Earth’s. A burster’s magnetic field isn’t quite powerful enough to corral and funnel hydrogen toward the burster’s poles. In that case, Miller says, the matter falls all over the surface of the star. As with a pulsar, the hydrogen releases energy as it slams into the surface, but unlike a pulsar, the hydrogen is more dispersed--so the X-rays released upon impact radiate outward much more diffusely.

Eventually, enough hydrogen accumulates on the star’s surface that the individual atoms are forced so close together that they fuse. Once the fusion gets going, Miller says, you end up with all the hydrogen that had accumulated going into helium in a flash--in other words, in a giant X-ray-producing thermonuclear explosion. When more hydrogen settles onto the star and heaps up to a critical density, another explosion occurs, releasing another burst of X-rays, and so on.

Bursters and pulsars, then, are both thought to be manifestations of neutron stars. The neutron stars that give rise to pulsars just happen to have a more powerful magnetic field than does a burster’s progenitor. (Astronomers have no generally accepted explanation for what causes any neutron star’s magnetic field, no matter the strength.) Somehow, though, 1744 combines bursting and pulsing behavior in a single neutron star.

Miller and his University of Chicago colleague Don Lamb, along with Ron Taam of Northwestern University, may have an explanation. They think a very strong magnetic field, tens of trillions of times the size of Earth’s and much stronger even than a normal pulsar’s field, generates the bursting pulsar’s signals. That field, they say, would funnel hydrogen from a red giant toward a small spot at the neutron star’s magnetic poles, just as in a normal pulsar. And as the matter dropped onto the star, it would release a steady stream of X-rays--again, like a normal pulsar.

The difference, says Miller, is that the prodigious magnetic field would allow large amounts of hydrogen to accumulate on 1744’s poles. On a normal pulsar, only a small amount of matter accumulates on the poles before it fuses and self-destructs. Why? Radiation--in the form of photons- -can’t easily escape from hydrogen on the pulsar’s surface. The gravity is so strong, and the hydrogen atoms packed so densely at the poles of the pulsar, that photons from the hydrogen run into and get absorbed by clouds of electrons around the packed hydrogen. Very little radiation escapes, and the hydrogen rapidly heats up and fuses, releasing some X-rays. Since relatively little matter piles up before it fuses--and fusion is a relatively weak source of X-rays--the X-rays produced in this way on a normal pulsar are dwarfed by the X-rays produced by hydrogen as it falls onto the pulsar.

On a bursting pulsar, though, the incredibly strong magnetic field would help the hydrogen to cool and pile up without fusing. The electrons in the hydrogen would be trapped and move up and down the bursting pulsar’s intense magnetic field lines, but not across them. Photons in that case would be less likely to hit electrons and could carry away more energy, cooling the hydrogen. So hydrogen could build up to the point at which when it fused, it would release an enormous burst of X-rays in addition to the regular stream of X-rays produced by the pulsar action.

Further observations may reveal whether this theory about the origin of the signals from 1744 is right. In any event, 1744 has forced astrophysicists to rethink basic ideas about the nature of bursters and pulsars. It is a unique source, says Miller. We were lucky to have caught it.

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