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Soft Gamma ray Repeaters (SGRs), a type of neutron star with an intense magnetic field, are the brightest of the known bursters which exhibit cyclic characteristics. Supernovae and GRBs are much brighter still, but the remnant is completely disrupted in this process, and they occur in our Galaxy only once in few hundred years (supernovae) or once in perhaps a million years (GRBs). SGRs, on the other hand, occur relatively frequently (~10 to 20 per year) and have a consistent emission source point. However, it is true that for years, astronomers didn't distinguish between SGR bursts and the much more frequently-observed "ordinary" or "classic" gamma-ray bursts (GRBs). It wasn't until 1987 that SGRs were clearly recognized as a distinct set of objects. The name "soft gamma repeaters" focuses on properties which distinguish SGR bursts from GRBs.

At the surface of this remnant a chunk of magnetizable metal like iron would feel a force equal to 150 million times the Earth's gravitational pull on it. At this intensity, the magnetic field's movements wrinkle the crust of the neutron star and cause starquakes that are believed to be the source of the soft gamma-ray bursts. That energy is actually released in two forms -- a burst of gamma-rays and X-rays and an ejection of subatomic particles at nearly the speed of light. The gamma-ray and X-ray burst lasts no more than a few minutes, while the ejected subatomic particles, interacting with the star's magnetic field, can produce detectable amounts of radio emission for several days.

The "Index" Event and subsequent SGR occurences

The Index event, called the Sagittarius burster, occurred on January 7, 1979, and during the first 3 months of 1979, three of the four known SGRs were discovered. Although the Index event was notable in a historical sense, the real show was the GRB 790305 event.

On March 5, 1979, gamma ray detectors on nine spacecraft recorded an intense radiation spike of energy in the Gamma-ray frequencies. Lasting just 2/10th of a second, it released as much energy as the sun releases in 1,000 years. It was then followed by a 200-second emission that showed a clear 8-second pulsation period.

The Right Ascension (RA) and Declination (DEC) tied the burst to a supernova remnant known as N49 in the Large Magellanic Cloud. Immediately, scientists recognized something odd. N49's youth - it's only a few thousand years old - contrasted with its 8-second spin, typical of a much older neutron star. The question of the moment was "what is slowing the rotation (hence pulsation) of this supermassive object ?"

This was the brightest SGR burst ever detected from outside our solar system, and had exceptionally high energy emissions. Indeed, it was exceptional in almost every way. Only nine days after that a third SGR became active in a new part of the Galaxy, giving three bursts in a three day period - thus three of the four known SGRs were discovered in this extremely short period of time.

The gamma rays in these SGR bursts are "spectrally soft" compared to those in "true" GRBs. This means that the average energy per gamma-ray photon is less. In fact, most SGR photons are really high-energy or "hard" X-rays. A more descriptive name would thus be "hard X-ray flashers," but we are stuck with "soft gamma repeaters" because of the way these objects were historically distinguished from GRB sources.

The "soft" in "soft gamma repeaters" does NOT mean "faint." Luminosity or brightness, the energy radiated away per second, is related to the NUMBER of photons being emitted, times the energy per photon---and the number of photons coming from SGR bursts is enormous. The term "soft" simply means that the energy PER PHOTON is less than in GRBs. Note that SGRs are spectrally soft ONLY in comparison to GRBs---they are harder than all other known astronomical phenomena.

Normal SGR bursts can radiate away as much energy in a single second as the Sun does in a whole year. (By "normal" I mean to exclude the March 5th 1979 event, which was more than 1,000 times brighter.) SGR bursts commonly last for a few tenths of a second, although some last for several seconds. All identified SGRs lie within our galaxy (3 of them) or in a clump of stars just outside our galaxy (1 of them).

Although only four SGRs have been detected so far, many millions almost certainly exist in our galaxy, and a similar number probably exist in every other galaxy. But SGRs cease emitting bright bursts after only about 10,000 years so only the youngest few have been detected.

It is interesting to compare SGRs with other repeating burst sources in the Galaxy. Astronomers have identified many such objects, and given them names like: "Type I and Type II X-ray Bursters, Black Hole X-ray Transients, Cataclysmic Variables, and Novae." These bursters are compact stars (white dwarfs, neutron stars, or black holes) into which material is falling from an orbiting companion star, in a double-star system. All of these other kinds of repeating bursts are FAINTER than normal SGR bursts by a factor of ~10,000 or more, except for the Black Hole X-ray Transients, which are fainter by only 1,000. However, the bursts from these other sources sometimes last much longer than SGR bursts, so the total energy in a burst can be comparable. (Because of their brevity, "flashers" would have been a better name for the SGRs.)

For a brief time, the March 5th 1979 event was brighter than a supernova. At 10:51 A.M. Eastern Standard Time two Soviet interplanetary space probes, Venera 11 and Venera 12, were drifting out through the solar system, when they detected an unprecedented flux of gamma rays. Gamma ray detectors built by a team of French and Russian scientists jumped from 0 to 40,000 counts and then off-scale in a fraction of a millisecond---first on Venera 11, then 5 seconds later on Venera 12. The detectors, not been designed for such energtic emissions, became saturated, and went off-line. This resulted in losing count of the gamma rays emitted during this event. Eleven seconds later, gamma rays were detected by an American space probe, Helios 2, which also caused its detectors read off-scale.

A plane wavefront of gamma rays was evidently sweeping through the Solar system at the speed of light. It soon reached Venus, where the Pioneer Venus Orbiter's gamma ray detector also became saturated and went off-line, and 7 seconds later it reached Earth. In Earth orbit, three 10 year old Vela satellites and a Soviet satellite named Prognoz 7 were overwhelmed with a burst of gamma rays. The Einstein X-ray Observatory, an orbiting X-ray telescope, also registered a strong signal.

As the wavefront passed out of the solar system, it hit two more space probes: the International Sun-Earth Explorer (ISEE) in orbit around the gravitational null or Lagrangian point of the Sun-Earth system, and the International Cometary Explorer (ICE). The ICE gamma-ray detector was pointed away from the oncoming gamma rays, but they passed through the solid body of the spacecraft, partially scattered and absorbed, but still were able to kick the detector up to maximum. Sixteen years later a team of Los Alamos scientists would make elaborate computer simulations of gamma rays passing through the ICE spacecraft in an attempt to extract more information about this intense burst.

All the detectors agreed that the burst began with a "hard pulse" of gamma rays lasting 0.2 seconds. This pulse was about 100 times more intense than any burst of cosmic gamma rays that had been detected up to that time. Nineteen years later, it still holds the record, by about a factor of something like 10. The hard pulse saturated the detectors. It was followed by a much fainter "soft tail" of soft gamma rays (or hard X-rays), lasting over 3 minutes, steadily fading. As it faded, the soft tail also varied in intensity in something like a sine wave, but with two peaks per cycle, and with a cycle period of 8.0 s. The 8-second modulations were clearly observed by many different detectors for more than 20 cycles. Nothing like this soft tail has ever been seen again. (Click here for an image of the light curve of the March 5th event) (graph courtesy R. Duncan, May, 1998) .

Fourteen and a half hours later, at 1:17 A.M. E.S.T. on March 6th, another, a fainter burst came from the same spot in the sky, lasting only 1.5 seconds. In retrospect we can see that it was a normal SGR burst in all its properties. Then, a month later on April 4th and again on April 24, more SGR-type bursts came, each lasting about 0.2 second. Over the next four years, 16 SGR-type bursts were seen from this source. Then in May 1983, the bursting ceased. No bursts have been detected from this source since.

Many people suggested that the SGR-like bursts were a residual effect of the huge March 5th event, perhaps a sign that the burster was "settling" into its post-burst state. Russian astrophysicists noted that spectrum of the soft tail of the March 5th event---that is, the distribution of energies of the detected hard X-ray photons---was almost identical to the spectrum of the SGR-like bursts which followed. Thus the soft tail could be considered a "super long-duration" SGR-type event, although the hard initial pulse was unique to March 5, 1979.

In the weeks and years after March 5, 1979, scientists analyzed data from the different spacecraft. Each detector had a clock that tagged the time on when the gamma rays first hit, to the nearest millisecond. By comparing these times from spacecraft at different places in the solar system, astronomers were able to tell at what angle the plane wavefront of gamma rays had passed through the solar system. This in turn told them WHERE in the sky the burst came from. It took more than a year to do this accurately. The result was a huge surprise.

The initial gamma ray wavefront of the March 5th event was so intense that it is thought that it must have been emitted by an almost mass-free liberation of energy blown out from the star at near-relativistic speeds. In a system like this it is reasonable to believe that this could have been powered by a magnetic flare event. Although it is expected that the fireball will disperse, it left behind its constituent products. In this specific case, the expected detrius are particles that are not free to escape because of the strength of its magnetic field.

In a general way, this star's magnetic field resembles that of an ordinary magnet: field lines are emitted from the north polar region and gently curve around, ending at the south polar region, and are anchored on the neutron star's surface. A matrix of hot gas comprised of charged particles (electrons and anti-electrons, or positrons) cannot freely pass across these field lines and escape because of the magnetic forces present there. After the March 5th event, it has been surmised that arch-shaped zones of hot particle gas must have been left behind. As a result of thses particles inability to cross this boundary layer, the particles can't escape, and the object gradually radiates away all their energy in the form of hard X-rays. The residual "soft tail" of the March 5th event thought to be the X-ray emission of this magnetically-trapped hot particle gas.

The result of being anchored at both poles was that the zones of glowing gas rotated every 8 seconds. X-rays carried off more and more energy, so the zones got smaller in size. Similar zones are made by the strong magnetic waves driven by a tectonic mechanism called a starquake. This may explain why the X-ray spectrum (the distribution of energies of X-ray photons) was essentially the same in the soft tail of the March 5th event as it was in subsequent, normal SGR bursts. However, normal SGR bursts never last long enough to show a stable 8-second periodicity.

The star's magnetic field had to be strong enough to trap the hot particle gas which was emitted during the soft tail of the March 5th event. X-ray measurements of the soft tail, summed over its whole 3-minute duration, tell us (roughly) what the total energy of the trapped gas was. A prodidgious amount of energy escaped at the beginning (as evinced by the hard initial spike) and the magnetic trapping forces were pushed to their limits. This implies that 4 X 10¹⁴ Gauss is an ESTIMATE of the field strength, not just a lower limit on it. The fact that this estimate agrees with other independent arguments is encouraging.

The Source of the SGR

The source turned out to lie INSIDE the tiny area of the sky which is covered by a "supernova remnant": the glowing cloud of debris left over from a massive stellar explosion. However, this particular supernova remnant (SNR), with the catalog name N49, is not in our own Milky Way galaxy. Instead, SNR N49 is in a "dwarf satellite galaxy" of the Milky Way called the Large Magellanic Cloud (LMC). The LMC is an irregular knot of stars that is prominent in the sky in the Southern Hemisphere. It is one of the nearest clumps of stars outside our galaxy, 180,000 light years from Earth. The LMC is called a "dwarf satellite galaxy" because it is smaller than most galaxies and it orbits the Milky Way. For an X-ray map of SNR N49 click here.

Supernovae are common in the LMC; in fact, one was observed to go off there in February 1987---"Supernova 1987A." Could the March 5th burster actually be much closer to us than the LMC? Almost certainly not. This would require that it just happens to have a position that overlaps with the tiny SNR in the LMC, which would be a tremendously improbable coincidence. Thus there is little doubt that the source was actually in the LMC, 180,000 light-years, or 10¹⁸ miles, away.

This was a shock. Everyone had expected that the source would be in the near galactic neighborhood, at most a few hundred light years away. This meant that the burst actually occured 180,000 years ago, long before the dawn of history, but it took this long for the gamma rays to reach us. The "plane wavefront" passing through the solar system was actually part of an expanding sphere of radiation, 180,000 light years in radius; it only SEEMED flat locally because the sphere's radius was huge compared to the size of the solar system.

The fact that the source is so far away means that the burst was enormously bright, intrinsically. At its peak the burster was shining about 10 times brighter than all the stars in our galaxy put together, or about 10 times brighter than a supernova explosion at its peak photon brightness. (Note that galactic stars and supernovae both radiate mostly optical & UV photons, whereas the March 5th burster radiated mostly gamma rays; but the energy loss rates can be compared.) In the first two-tenths of a second, the burster radiated away as much energy as the Sun radiates in 1000 years.

There was one more tantalizing clue... The position of burster, as precisely determined using data from 8 different spacecraft, did NOT lie at the center of the spherical SNR, but significantly displaced toward the edge. This displacement was verified in 1991 when a faint, steady "point source" of X-rays was found at the position of the burster, allowing its position to be accurately measured. (These X-rays are evidently emitted by the burster. Astronomers call it a "point source" because its size and shape are not measured: it is so small that it is indistinguishable from a point with present X-ray telescopes.)

SGR 1900+14 - a Magnetar or something else ?

The flash of gamma rays was detected on Aug. 27, 1998 by at least seven spacecraft in Earth orbit and in deep space. This was the result of several months of observations of an object known as SGR 1900+14 (The numbers 1900+14 refer to its coordinates in the sky: 19 hours, 00 minutes right ascention, +14 degrees declination.), a Soft Gamma Repeater located in the constellation Aquila (the eagle) near Sagittarius (the archer).

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. It is believed that within a magnetar, the huge magnetic field is capable of cracking a neutron star's rigid surface to bits.

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 appears to be magnetars, where the magnetic field strength is thought to be the strongest in the universe. They spend several millennia spinning and creating great, erratic emissions - bursting in gamma radiation as SGRs. These magnetars/SGRs slow down as their field strength is depleted and glow as AXPs for several tens of thousands of years, then they fade to near invisibility.

The discovery of a pulsar that slowed down dramatically in SGR 1900+14 has allowed scientists to confirm the magnetar theory that was 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 the incredibly strong magnetic fields are generated only in neutron stars that are born spinning very rapidly - these become magnetars, while those rotating more slowly at birth become radio pulsars.

Magnetars and SGRs - how they are related

Fission, the process used in A-bombs, is the easier of the two processes involved in the magnetar theory to understand. Just one fast neutron can split a nucleus of plutonium 239 or uranium 235 which already is on the edge of instability. Fusion, on the other hand, is a little more complex. As it ages, a star goes through a series of burn cycles, converting hydrogen into helium "ash," and that ash into heavier elements. Each cycle burns out faster and yields less energy than the one before it. In the last cycle, silicon "burns" to form iron - after that, further fusion absorbs rather than yields energy. The fire goes out, and the star collapses.

The implosion of the star's outer layers powers one last burst of fusion that also absorbs energy and slightly cools the blast. What is left behind is a neutron star - a spinning, liquid conglomeration of densely 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.

Under the current theory, a magnetar spends the first 10,000 years of its life as an SGR. The common flashes that are indicative of SGRs are caused by "starquakes" in the outer rigid crust of the magnetar. It is believed that as a magnetar's colossal magnetic field shifts, it strains the crust with extremely powerful magnetic forces, often breaking it. An ordinary pulsar's magnetic field is not strong enough to do this. When the crust of a magnetar snaps, it vibrates with seismic waves like in an earthquake and emits a flash of soft gamma-rays.

SGR 1806-20

Just such a series of flashes was seen during the summer of 1998 by a number of spacecraft - 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 also recorded this event. Dr. Chrissa Kouveliotou discovered the first magnetar candidate, SGR 1806-20, in May of 1998, while Dr. Tod Strohmayer, a member of the RXTE science team at NASA's Goddard Space Flight Center, was the first to identify the rapid spindown of SGR 1900+14.

SGR 1900+14 was observed with the RXTE between May 31 and June 9 for a total of 41,700 seconds (11.6 hours) and found it had a rotational period of 5.159142 seconds. Comparing the various RXTE observations with each other and with earlier data from ASCA, Dr. Kouveliotou's team calculated that SGR 1900+14 was slowing down ever so slightly - each rotation was about 0.00000000057 second slower than the one before, which amounts to about one second every 290 years. Ssince SGR 1900+14 is a neutron star, it weighs at at least 1-1/2 times as much as our Sun. Something incredibly powerful must be slowing it down.

"It implies that the magnetic field is almost a quadrillion (10¹⁵) gauss," Dr. Kouveliotou said. While the science team was analyzing the data from 1900+14, something else was discovered. 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!" Additionally, three of the four confirmed SGRs (1900+14, 1806-20 and 0526-66) have localized, point like X-ray counterparts.

Magnetar Flares

Typical 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 as intense as the gamma-ray pulse in the first second of the Aug. 27 event. This explosion left behind a residue of hot particles which are held close to the star by the magnetic field, because charged particles could not freely flow across the strong magnetic field. This magnetically trapped cloud cools and shrinks by emitting soft gamma rays and X-rays - called a "soft tail" - which was observed in the fading tail of the Aug. 27 event.

As the magnetar 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. Dr. Kevin 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. The magnetic field must be unusually strong in order to hold all this energy close to the star, and calculations indicate that field must be in the magnetar range.

Known SGRs