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Crusty young star
makes its presence felt

Gamma ray flash zaps satellites, illuminates Earth, and sheds light on several mysterious stellar events

Sept. 29, 1998: A powerful flash of gamma rays, strong enough to be detected through a satellite's own shielding and to turn night into day in the Earth's outer atmosphere, has led to confirmation of the existence of super-magnetized stars.

Right: An artist's concept depicts the magnetic field lines rising from the surface of a magnetar, and the plasma clouds around the star. Links to 720x486-pixel, 38KB 72 dpi JPG. Credit: Dr. Robert Mallozzi, University of Alabama in Huntsville.

"The discovery of magnetars is a major breakthrough in astrophysics", said Dr. Chryssa Kouveliotou of the Universities Space Research Association, working at NASA's Marshall Space Flight Center.

"If the theory holds up, it will mean that there are probably a million old magnetars drifting around our Galaxy, and perhaps as many as 10 to 100 million, because magnetars must have been forming throughout the history of our Galaxy. Most of these stars have now gone inactive and are difficult to detect."

The flash of gamma rays was detected on Aug. 27 by at least seven spacecraft in Earth orbit and in deep space. It capped several months of observations of an object known as SGR 1900+14, a Soft Gamma Repeater located in the constellation Aquila (the eagle) near Sagittarius (the archer). Note to editors: High-resolution and PDF copies of these images are available on a separate Magnetar Image Page.
Left: SGR 1900+14 - observed by the Italian-Dutch Beppo SAX satellite - during a dormant phase in 1997 (left side) and in September 1998 (right side) after it erupted with a series of energetic flares. Links to 720x486-pixel, 34KB JPG. Credit: Chryssa Kouveliotou, USRA, and Peter Woods, UAH.

Connecting mysteries

Astronomers think the Aug. 27 boomer was caused by an out-of-control magnetic field realigning itself in a manner similar to what happens inside solar flares. While in magnetars their huge magnetic field is capable of cracking a neutron star's rigid surface to bits, it also connects three different mysteries. Each mystery involves neutron stars. A neutron star is created when a massive star explodes in an event called a supernova. The remaining core of the supernova, which has slightly more mass than our own sun but can no longer burn fuel, compresses under gravity into a neutron star about 20 km (12 mi) across.

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The mysteries are:

  • What are SGRs? Since 1979, astrophysicists have discovered four (possibly five) neutron stars that repeatedly emit powerful bursts in the gamma ray spectrum. Their positions are known, but their activity is unpredictable.
  • What are Anomalous X-ray Pulsars (AXPs), neutron stars whose slow periods and odd X-ray spectra indicate they are old, yet are associated with young supernova remnants?
  • Why do so many supernova remnants appear to have no pulsars at all?

The answer is magnetars, neutron stars whose magnetic fields are the strongest in the universe. They spend several millennia spinning and creating a great, erratic fuss, bursting in gamma radiation as SGRs. Then they slow down as their "juice" is depleted and glow as AXPs for several tens of thousands of years. Finally, they fade to near invisibility.

The discovery of a pulsar that slowed down dramatically in SGR 1900+14 has allowed scientists to confirm this theory, first advanced in 1992 by Dr. Robert Duncan of the University of Texas at Austin and Dr. Christopher Thompson of University of North Carolina at Chapel Hill.

Duncan and Thompson's calculations indicate that ultra-strong magnetic fields are generated only in neutron stars that are born spinning very rapidly: these become magnetars. Those rotating more slowly at birth become radio pulsars, a kind of neutron star - discovered in 1967 - that was already familiar to astronomers.

A colossal crackup

According to the magnetar theory, the common flashes that are the hallmark of SGRs are caused by "starquakes" in the outer rigid crust of the magnetar. As a magnetar's colossal magnetic field shifts, it strains the crust with monstrous magnetic forces, often breaking it. (An ordinary pulsar's magnetic field is not strong enough to do this.) When the crust snaps, it vibrates with seismic waves like in an earthquake and emits a flash of soft gamma-rays.

Just such a series of flashes was seen this summer by a number of spacecraft, notably the Rossi X-ray Timing Explorer (RXTE), Beppo-SAX, the Advanced Satellite for Cosmology and Astrophysics (ASCA), and four U.S. defense and civilian weather satellites, all in Earth orbit; Wind, about a million miles from Earth; the Ulysses solar polar probe near the orbit of Jupiter, and the Near-Earth Asteroid Rendezvous (NEAR) spacecraft near the orbit of Mars.

... like a diamond in the sky ...

Of all the atoms in the periodic table, why is the surface of a neutron star made of iron? The answer is that iron is at the bottom of the energy valley for fission and fusion.

Fission, the process used in A-bombs, is the easier of the two processes. Just one fast neutron can split a nucleus of plutonium 239 or uranium 235 which already is on the edge of instability.



Twinkle, twinkle, little star.
Now we know just what you are.
Nuclear furnace in the sky,
You'll burn to ashes, by and by.


Fusion, the process that goes on in a star's core and in H-bombs, is tougher. It requires incredible pressure to overcome the natural repulsion between hydrogen and helium nuclei and join them into a heavier nucleus. This is why fusion reactors remain elusive, and H-bombs require A-bomb triggers.

It takes ever more energy to split atoms as you move from uranium down the list of elements, or to fuse atoms as you move from hydrogen upward. At iron - with 26 protons plus 30 neutrons - it takes as much energy to fission as to fuse.

Left: A cross-section of a neutron star. Beneath the iron surface, nuclei in the crust quickly go to higher atomic numbers (e.g., lead) bloated with neutrons. Deeper, the crust has free neutrons floating between the nuclei, along with relativistic electrons. Finally, at the base of the crust the nuclei get truly enormous until they literally touch - and then melt to become the liquid interior. Links to 720x486-pixel, 46KB 72 dpi JPG. Credit: NASA/Marshall Space Flight Center.


As it ages, a star goes through a series of burn cycles, converting hydrogen into helium "ash," and that ash into heavier elements, with each cycle burning out faster and yielding less energy than the one before it. In the last cycle, silicon - No. 14 - "burns" to form iron: 14 + 14 = 28. After that, further fusion absorbs rather than yields energy. The fire goes out, and the star collapses.



But tick, tick, tick, pulsating star,
Now we wonder what you are.
Magneto-nuclear-plasma ball,
You're making monkeys of us all.
(anon.)


So where do we get the heavy metals we have now? The implosion of the star's outer layers powers one last burst of fusion that also absorbs energy and slightly cools the blast. Many of these elements are thrown into deep space to become the dust that eventually condenses into new solar systems.

Left behind is the neutron star, a spinning, liquid blob of jam-packed neutrons convered with a crust of heavy nuclei compressed by intense gravity into a solid, crystalline crust, far harder than the finest diamond anywhere.

For the past several years, Kouveliotou has been using the Burst and Transient Source Experiment aboard the Compton Gamma Ray Observatory, the RXTE, and other satellites to unravel the mysteries of SGRs.

   
Two different views of SGR 1900+14 as seen by the Beppo-SAX satellite (left; links to 720x486-pixel, 28KB 72 dpi JPG) and Japan's ASCA (right; 720x486-pixel, 101KB 72 dpi GIF).

Kouveliotou discovered the first magnetar candidate, SGR 1806-20, this May. Her collaborators comprise an international team of scientists, among whom is Dr. Tod Strohmayer, a member of the RXTE science team at NASA's Goddard Space Flight Center, who first identified the rapid spindown of SGR 1900+14.

They observed SGR 1900+14 with the RXTE between May 31 and June 9 for a total of 41,700 seconds (11.6 hours) and found it had a period of 5.159142 seconds. Comparing the various RXTE observations with each other and with earlier data from ASCA, Kouveliotou's team calculated that SGR 1900+14 was slowing down ever so slightly. Each rotation is about 0.00000000057 second slower than the one before. That's about one second every 290 years. It doesn't seem like much, but for the fact that since SGR 1900+14 is a neutron star, it weighs at least 1-1/2 times as much as our Sun. Something incredibly powerful is putting the brakes on it.

Magnetic attraction

"It implies that the magnetic field is almost a quadrillion (1015) gauss," Kouveliotou said. While the science team was analyzing the data from 1900+14, the star unleashed another surprise.

The most spectacular event came on Aug. 27 when RXTE was not even pointed at SGR 1900+14. The burst was so strong and clear that RXTE's Proportional Counting Array detected it through the shielding intended to keep stray radiation out. Then the array locked up because of the overload. Because Ulysses and NEAR were in deep space, scientists could triangulate the position by slight differences in arrival times.

Right: The relative positions of spacecraft at the time of the Aug. 27, 1998, flare. (Links to 720x486-pixel, 28KB 72 dpi JPG.)

At the same time, Stanford University scientists saw the ionization of the Earth's nightside outer atmosphere increased to near-dayside levels on the side facing SGR 1900+14.

"What is most interesting here," said Dr. Umran Inan of Stanford, "is the sheer energy of the event, indicating that the ionosphere within view of the flash was ionized as much as it is during day time!"

"In California, where I work, we're always waiting for the 'big one.' When I saw this flash, I knew the wait was over," said Kevin Hurley of the University of California, Berkeley.

Fading away

Yet by the time it reached Earth, it posed no health risk - the dose was far less than a dental X-ray and the outer atmosphere absorbed all the energy.

The Ulysses and RXTE data clearly show the radiation counts rocketing from background (near zero) to several thousand per second - the energy measured here was two times greater than any other recorded burst - then trailing off like a dying lighthouse. It keeps rotating, but the lamp steadily fades away.

Left: The radiation from SGR 1900+14, as seen by a detector on Ulysses, spiked quickly and soon settled into a series of ever-smaller spikes that clearly revealed the neutron star's rotational period. (Links to 720x486-pixel, 42KB 72 dpi JPG.)

"The clarity of these data is unusual," Kouveliotou said. "Normally, we have to sift through the data to extract a signal. This pulsar stood up to say, 'Here I am!'"

Three of the four confirmed SGRs (1900+14, 1806-20 and 0526-66) have localized, point like X-ray counterparts; 1806-20 and 1900+14 have regular pulsations and 0526-66 had an 8-second period in its 1979 event. It is by comparing the change in the rotational period of an SGR across several observations that scientists can measure the magnetic field.

A flat existence


Warning: Do not expose to magnetic
fields greater than 1014 Gauss.

 So just what would it be like to visit a magnetar? Well, you don't want to bring your ATM card.

A pulsar-strength magnetic field - at the star's surface - would kill you instantly by rearranging all the atoms and molecules in your body. Anything over about 1 billion gauss - a billion times the strength of Earth's magnetic field - would kill you.


Even at a distance of about 200,000 km (160,000 mi), a magnetar's field would be as strong as a refrigerator magnet is up close. The field could certainly erase your credit cards and suck pens out of your pocket as far away as half the distance to the Moon, perhaps farther.

Of course if you were that close, losing your pens and credit cards would be the least of your worries. The steady X-rays - not to mention the flashes and flares - along with a deadly wind of charged particles, driven by vibrations of the magnetic field from many small fractures going on all the time, deep in the crust, would kill you in short order. So, even if one day we have ships that can take us to the stars, ordering "shields up" for a closer look will be a tough job.

But don't worry about missing the sights. A neutron star's gravity is so intense at the surface that Earth's tallest mountain ranges would be flattened to about the height of an ant.

Robert Duncan and Dave Dooling

"SGRs, like pulsars, slow down because of magnetic dipole radiation, nibbling away at their rotation," Kouveliotou explained. "What's significant with SGRs is that their slowdown is so much faster."

According to the magnetar theory, the common flashes that are the hallmark of SGRs are due to starquakes. As a magnetar's huge magnetic field drifts though the neutron star, the magnetic strain on the iron crust causes it to deform, often breaking it. Like an earthquake, this vibrates the star with seismic waves, and drives shaking magnetic waves outward to energize particles outside the star and emit flashes of soft gamma-rays. To cause starquakes, the magnetic field must be enormous - at least 1014 gauss, at least 100 times stronger than a "normal" pulsar!

Interview with Dr. Chryssa Kouvelioutou

Interview with Dr. Kouveliotou
Credit NASA / Marshall Space Flight Center
Requires Realvideo player (version 5). Click to download the RealVideo Player (from RealNetworks, Inc.
RealPlayer icon, click to download

Right: This chart depicts the complete history of the study of Soft Gamma Repeaters, including major outbursts by the four known repeaters, and the periods when spacecraft instruments were available to detect SGR flashes. Credit: NASA/Marshall Space Flight Center. (Links to an 853x731-pixel, 36KB GIF.)

The big event

Nonetheless, the common SGR flashes are mild compared to SGR flares like the Aug. 27 event. The magnetar theory holds that during a flare, the magnetic field rearranges itself to a state of lower energy. When it does, this probably cracks the crust profoundly, at many places all over the star. (Similar magnetic rearrangements - at much lower energy levels - often occur in X-ray flares from the Sun which, of course, has no solid surface.)

In the first moments of a magnetar flare, the release of pure magnetic energy drives out an tremendous explosion of superheated particles and gamma rays. This was observed as the intense gamma-ray pulse in the first second of the Aug. 27 event.

The explosion leaves behind a residue of hot particles which are held close to the star by the magnetic field, because charged particles cannot freely flow across a strong magnetic field. This magnetically trapped cloud cools and shrinks by emitting soft gamma rays and X-rays, as observed in the fading tail of the Aug. 27 event. As the star rotates, the trapped cloud of particles is seen from different angles, causing the intensity to rise and dip regularly over each 5.16-second rotation cycle.

Left: SGR 1900+14 is located along the plane of our Milky Way galaxy. In the night sky, it's in the constellation Aquila (the Eagle) almost into Sagittarius (the archer). Most of the SGRs and AXPs are along the galactic plane because that's where most of the stars in our galaxy are located. The image here was produced by the Diffuse Infrared Background Experiment on NASA's Cosmic Background Explorer (CoBE). Links to 720x486-pixel, 80KB 72 dpi JPG.

Hurley calculates that a thousand stars like the Sun shining for a whole year would be needed to give as much energy as in the "soft tail" of the signal he saw with Ulysses. He estimates that the magnetic field must be unusually strong in order to hold all this energy close to the star. Indeed, simple calculations indicate that field must be in the magnetar range.

Under the current theory, a magnetar spends the first 10,000 years of its life as an SGR. Its iron crust shifts and wrinkles (although the height of these wrinkles is less than a few millimeters!), just like Earth's own crust is dragged about by the convection of the mantle. Then, the SGR weakens and becomes an AXP for another several tens of thousands of years, and finally fades to black, drifting unnoticed through the heavens. Magnetars could well be the rule rather than the exception, and the galaxy may be littered with a few million stellar corpses like this.

As always, more studies

Although many lines of evidence seem to favor the magnetar theory, Duncan urges caution while scientists are still analyzing the data.

"The August 27th flare is exciting because it will provide us with the kind of detailed information we need to thoroughly test the theory," he said.

"This is a new kind of star with bizarre properties that weren't even imagined until a few years ago," said Dr. Paul Hertz of NASA's Structure and Evolution of the Universe Program. "If confirmed, it is an amazing discovery. It makes you wonder what else is out there."

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Author: Dave Dooling
Curator: Linda Porter
NASA Official: Gregory S. Wilson