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Robert C. Duncan, University of Texas at Austin
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| Observations of SGRs give evidence that these stars have extrordinarily strong magnetic fields. One especially important and reliable measurement of SGR magnetism—indeed a crucial test of the theory—was first done by Chryssa Kouveliotou and her collaborators in 1998. This gave evidence for a field of 8 X 1014 Gauss, where "Gauss" is a unit of magnetic field strength. (1 Gauss = 10-4 Tesla) |
| Here I use scientific notation, so 1014 means 10 raised to the 14th power, which equals 1 with 14 zeros after it. Thus 8 X 1014 is 8 with 14 zeros after it, or 800,000,000,000,000. |
| To put this in perspective, here is a table of magnetic field strengths |
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| The Earths magnetic field, which deflects compass needles | measured at the N magnetic pole | |
| A common, hand-held magnet | like those used to stick papers on a refrigerator | 100 Gauss |
| The magnetic field in strong sunspots | (within dark, magnetized areas on the solar surface) | 4000 Gauss |
| The strongest, sustained (i.e., steady) magnetic fields achieved so far in the laboratory | generated by huge electromagnets | 4.5 X 105 Gauss |
| The strongest man-made fields ever achieved, if only briefly | made using focussed explosive charges; lasted only 4 - 8 microseconds. | 107 Gauss |
| The strongest fields ever detected on non-neutron stars | found on a handful of strongly-magnetized, compact white dwarf stars. (Such stars are rare. Only 3% of white dwarfs have Mega-gauss or stronger fields.) | 108 Gauss |
| Typical surface, polar magnetic fields of radio pulsars | the most familiar kind of neutron star; more than a thousand are known to astronomers | |
| Magnetars | soft gamma repeaters and anomalous X-ray pulsars (These are surface, polar fields. Magnetar interior fields may range up to 1016 Gauss, with some field lines probably wrapped in a toroidal, or donut geometry inside the star.) |
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Physicists have not made steady fields stronger than 4.5 x 105 Gauss in the lab because the magnetic stresses of such fields exceed the tensile strength of terrestrial materials. If you try to make stronger fields, magnetic forces will blow apart your electromagnet. |
| Using chemical high explosives to drive implosions, it is possible to compress a magnetic field and reach higher field strengths, at least for a tiny fraction of a second. This has been done at Los Alamos Laboratory in the U.S., and at a nuclear weapons lab in Sarov, Russia, attaining fields of about 107 Gauss before the equipment was destroyed. |
Atoms in very strong magnetic fieldsThe strongest magnetic field that you are ever likely to encounter personally is about 104 Gauss if you have Magnetic Resonance Imaging (MRI) scan for medical diagnosis. Such fields pose no threat to your health, hardly affecting the atoms in your body. Fields in excess of 109 Gauss, however, would be instantly lethal. Such fields strongly distort atoms, compressing atomic electron clouds into cigar shapes, with the long axis aligned with the field, thus rendering the chemistry of life impossible. A magnetar within 1000 kilometers would thus kill you via pure static magnetism -- if it didn't already get you with X-rays, gamma rays, high energy particles, extreme gravity, bursts and flares... |
| In fields much stronger than 109 Gauss, atoms are compressed into thin needles. At 1014 Gauss, atomic needles have widths of about 1% of their length, hundreds of times thinner than unmagnetized atoms. Such atoms can form polymer-like molecular chains or fibers. A carpet of such magnetized fibers probably exist at the surface of a magnetar, at least in places where the surface is cool enough to form atoms. |
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Grey boxes like this indicate material for advanced readers.
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Ultra-strong magnetic fieldsMany fascinating physical effects occur in magnetic fields with strength exceeding the "quantum electrodynamic field strength" of BQ =Although magnetar fields are strong by most measures, they are weak compared to the strongest possible field that could theoretically exist in nature, which is 1049 - 1053 Gauss. A field stronger than this would literally break down the vacuum and decay, via the quantum mechanical process of magnetic monopole creation. However, there is no known way that such strong fields could be generated. Magnetars are the most strongly-magnetized objects yet known in the Universe. |
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The U.S. Dept. of Defense launched the "Vela" satellites in the late 1960s
to search for gamma rays in space, in order to verify a treaty banning nuclear
tests in outer space. To everyone's surprise, many brief bursts of gamma
rays were found. They seemed to come randomly from all directions in
the sky. Thus a new natural phenomenon, "gamma-ray bursts" (GRBs), was
discovered and announced to astronomers in 1973.
Gamma rays are made up of very high-energy photons, more energetic than X-ray photons, which in turn have higher energy than ultraviolet, optical, infrared, microwave, and radio-frequency photons (listed in order of decreasing energy, or increasing "spectral softness"). Gamma rays can only be studied from outer space, because they are absorbed by the Earth's atmosphere as they ionize atoms. (That is, as they knock electrons out of atoms in the air, turning them into positively-charged ions.) Fortunately, gamma ray detectors are relatively cheap, compact and easy to put on spacecraft, so they were "piggybacked" on various space missions during the 1970's to give data on GRBs. By 1979, there were almost a dozen gamma ray detectors on Earth satellites and on space probes scattered around the solar system. 1979 was a fantastic year for the study of soft gamma repeaters (SGRs), although nobody realized it at the time. The first SGR burst ever detected, from a source in the constellation Sagittarius, occurred on January 7, 1979. Then a truly powerful SGR outburst---indeed, by far the most intense blast of gamma rays that had ever been detected from outside our solar system (until another SGR outburst broke the record in 1998)---came just two months later, on March 5, 1979. This tremendous flare eventually allowed the SGR mystery to be unraveled. 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. So during the first 3 months of 1979, three of the five known SGRs were discovered. For years, astronomers didn't distinguish between SGR bursts and the much more frequently-observed "ordinary" or "classic" gamma-ray bursts (GRBs). (During the 1990's, with modern detectors, about one GRB per day was detected from some place in the sky. Fewer SGR bursts were seen, perhaps 10 to 50 per year, although they came in groups.) 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. Unlike GRBs, which have never been verified to come more than once from the same spot in the sky, SGR bursts repeat sporadically from the same source. The gamma rays in SGR bursts are also "spectrally soft" compared to those in 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, not true gamma 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. |
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| 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 giant flares of March 5, 1979 and August 27, 1998 which were more than 1,000 times brighter.) SGR bursts commonly last for only a small fraction of a second, although some last for several seconds. All identified SGRs lie within our galaxy (4 of them) or in a clump of stars just outside our galaxy (1 of them). |
| [Our galaxy, the Milky Way, is a huge, flattened agglomerate of stars, shaped roughly like a disk about 100,000 light-years in diameter and about 1000 light-years thick, where a light-year is the distance that light travels in one year: about 6 trillion miles. If seen from the outside, the Milky Way would look like a luminous pinwheel, made of about 100 billion (1011) stars. The Sun and Earth lie about 3/4ths of the way out from the center.] |
| Although only five 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---a brief instant of cosmic time---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 the brightest 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 good name for the SGRs.) |
| In summary, SGRs are, by far, the brightest known bursters which repeat. Supernovae and GRBs are much brighter still, but they are one-shot events, destroying the bursting star. Supernovae and GRBs are rare, occuring in our Galaxy only once in few hundred years (supernovae) or once in perhaps a million years (GRBs). This means that almost all detected supernovae and GRBs come from other, distant galaxies. |
| This brings us to the pivotal event in the history of SGRs: an outburst that was, for a brief time, brighter than a supernova. |
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March 5th, 1979 .... |
| At 10:51 A.M. Eastern Standard Time, far out in space, two Soviet interplanetary space probes, Venera 11 and Venera 12, were drifting through the inner solar system when they were walloped by an unprecedented flux of gamma rays. Onboard gamma ray detectors jumped from 100 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 had not been designed for such a flood of energy and they "saturated," losing count of the gamma rays pouring through them. Eleven seconds later, more gamma rays blasted an American space probe, Helios 2, in orbit around the Sun, also knocking its detectors 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 went over the top. Then only 7 seconds later it reached Earth. Nobody noticed as it passed: life went on calmly beneath the protective atmosphere. It was a rainy, dreary, cold Monday morning on the U.S. East coast; chilly and clear elsewhere in the country. The lead story in U.S. newspapers concerned President Carter's attempts to advance an Israeli-Egyptian peace treaty. (I was 4000 miles across the Atlantic Ocean then, a young student at Cambridge University.) |
| Meanwhile up in Earth orbit, three Vela satellites and a Soviet satellite named Prognoz 7 were swamped with a sudden flood of gamma rays. The Einstein X-ray Observatory, an orbiting X-ray telescope, also showed a strong signal. Gamma rays were diffusing copiously through the metal radiation shields surrounding its detectors. |
| As the wavefront passed out of the solar system, it hit one more space probe: the International Sun-Earth Explorer (ISEE) in orbit around the gravitational null or Lagrangian point of the Sun-Earth system. (A few years later this probe left the Lagrangian point, and was sent drifing through the wider solar system in an effort to study comets, at which time it was renamed the International Cometary Explorer or ICE.) The gamma-ray detector on ISEE was pointed away from the oncoming gamma rays, but they passed through the solid body of the spacecraft, partially scattered and absorbed and were still 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 ISEE spacecraft in an attempt to extract more information about this intense burst. |
 Light curve of the March 5th event, as recorded by gamma-ray detectors aboard the Venera 12 space probe. (From E.P. Mazets et al., 1979, Nature 282, p. 587.) |
| All the detectors showed 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 held the record, by about a factor of 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 had ever been seen by astronomers, or was seen again for 19 years. |
| 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. |
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| In the months 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.
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. |
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| 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 3000 years. | ||
| There was one more tantalizing clue... The position of burster, as precisely determined using data from 7 different spacecraft, did not lie at the center of the spherical SNR, but significantly displaced toward the edge. (See the figure above.) 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.) | ||
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| What caused the March 5th event? Assuming that the March 5 burster formed in the supernova which gave rise to SNR N49, as seems likely, then we can infer that.... | ||||||||||||
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| Nobody understood why a neutron star would have this strange set of properties, or what would cause it to burst so spectacularly. | ||||||||||||
| Many theories were proposed in the 1980s, suggesting, for example, that the March 5th event was due to a small planet or a large asteroid slamming into a neutron star, or a "phase transition" in the core of a neutron star (i.e., the neutron star's core somehow abruptly changed its state as it cooled, like water does when it freezes, releasing energy in the process), or even more speculative suggestions involving hypothetical new objects, such as "a quark nugget falling onto a strange quark star." Most of these ideas accounted for only a limited subset of the known facts. Almost none of them attracted many believers, or were the subject of more than one research paper. | ||||||||||||
| It was particularly difficult for theorists to account for the enormous gamma-ray brightness of the hard initial pulse of the March 5th event. If you try to power this from some material falling onto a neutron star (e.g., from a planet or asteroid) then the pressure associated with the outflowing gamma rays itself halts the inflow, and cuts off the energy supply. But if you try to power it from a source deep inside the neutron star, like a phase transition, then it is hard to get all the energy out quickly and completely enough in the form of gamma rays. | ||||||||||||
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| In the 1990's the SGR mystery stimulated many astronomers to point all kinds of telescopes at these objects. The X-ray point source at the location of the March 5th burster was discovered in 1991. Then in 1993 and 1994 the locations of two more SGRs were pinned down. | ||||||||||
While scores of astronomers were working to achieve these observational
advances in the 1990's, the "magnetar" theory of SGRs was being developed.
This theory is still being tested and debated. As more observations come in,
it might be disproven. However, a wide variety of evidence now seems to favor
it.
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| The "magnetar" theory of SGRs came about from an attempt to understand a completely different issue, namely: the origin of magnetic fields in radio pulsars. |
| Radio pulsars are "garden variety" neutron stars: over 1000 have been detected since they were discovered in 1968. They emit beams of radio waves which sweep through space as the stars rotate, like lighthouse beams, thus from afar pulsars seem to flicker or pulsate at their rotation periods. Careful measurements have shown that pulsar periods increase over time, implying that the stars are gradually spinning down. This is attributed to their magnetic fields. The field lines are anchored to the neutron star surface, because they are generated by circulating electric currents inside the star. Thus as the star turns the field also must turn. This drives magnetic waves outward, along with diffuse winds of charged particles (which emit the radio beams from just above the magnetic poles), carrying off energy and causing the star to gradually spin down. The measured rate of spin-down allows the magnetic field to be estimated. For almost all young pulsars it is a few times 1012 Gauss at the magnetic poles. |
| In 1986, Christopher Thompson (originally from Winnipeg, Canada; now at the Canadian Institute for Theoretical Astrophysics in Toronto) and I began to study a question that many astrophysicists had wondered about, namely: why are pulsar magnetic fields about 1012 Gauss? At the time, we were both at Princeton University: Chris was a graduate student and I was a postdoctoral fellow. We were intrigued by computer simulations which had shown that neutron stars get all mixed up just after they form. We wondered how this would affect their magnetic fields. |
| Neutron stars are very hot when they first form. The computer models, by Adam Burrows of the University of Arizona and James Lattimer of the State University of New York at Stony Brook, showed that the dense fluid of neutrons inside a nascent neutron star roils and churns to help carry out heat, a little like water boiling in a pot. Such circulation is called "convection." It happens because hot parcels of fluid rise, while cool ones sink. (In a hot nuclear fluid, the density of electrons also affects fluid buoyancy and helps drive mixing.) |
| We found that the hot, ultra-dense neutron star fluid also can conduct electricity very well, because it contains some free electrons and protons along with the more abundant neutrons. These charged particles readily carry currents. This means that any magnetic field lines caught in the fluid initially are swept along by the convective motions; they cannot just "ignore" the moving fluid because the fluid is an electrical conductor. |
| If the star is born rotating fast enough, the combined effects of rotation and convection, which both drag field lines through the star, can build up the star's overall magnetic field, via a complicated process known as "dynamo action." Dynamos operate in the interior of the Earth and the Sun, giving them their magnetic fields. |
| We were amazed to find that, if a dynamo worked with ideal efficiency in a hot, newborn neutron star, it would generate a field of about 1016 Gauss: 10,000 times stronger than was actually found in pulsars! As the star cools, convection and dynamo action cease. This happens after only about 10 or 20 seconds in a neutron star, but 10 seconds is enough time for a very strong field to build up. After that, the field can remain trapped by the heavy, stratified liquid of neutrons and protons inside the neutron star. |
| This led us to conclude that the familiar radio pulsars were neutron stars in which large-scale dynamos had essentially failed to operate, probably because they were not born rotating fast enough. The spin period of the Crab pulsar at birth was about 20 milliseconds; we found that it needed to be considerably less than that for a dynamo to work. |
| The question of why a pulsar field was 1012 Gauss thus turned out to involve some subtle details of the residues of magnetism left over after a large-scale dynamo fails. We made some progress in estimating this, but we also couldn't help wondering: what happens if the dynamo succeeds? |
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Although it is born spinning somewhat faster than a pulsar, a magnetar spins down much more quickly, because the magnetic waves (and the related, diffuse winds of charged particles) which carry off the star's rotational energy are very efficient when the field is strong. This means that magnetars rarely send out widely-sweeping radio "lighthouse" beams as do radio pulsars. Except in a fleeting interval just after it is formed, a magnetar spins so slowly that its spindown-powered beams are exceedingly narrow or completely turned-off. (Recall that the radio beams in an ordinary pulsar come from a rotation-driven outflow of charged particles above the magnetic poles. When the rotation rate drops this ceases.) To put it a different way: all of the observed emissions of ordinary radio pulsars (except for a faint X-ray glow from its tiny, hot surface) are powered by a slow loss of the rotational energy that the star is born with. A radio pulsar's magnetic field is essentially stable; its main role is to passively facilitate the loss of rotational energy. In a magnetar, on the other hand, the rotational energy quickly becomes negligible. However, we realized that the magnetic field itself can provide an energy source for potentially observable emissions. A magnetar's field is strong enough to push material around in the star's interior and crust, leading to the dissipation of a significant amount of magnetic energy during the first ten thousand years. |
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This has several consequences: |
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Why does the star have a crust?A neutron star is mostly made of a dense liquid of neutrons, with a trace of protons and electrons. This is "nuclear fluid": the pure stuff of the atomic nucleus. It is denser than liquid water on Earth by a factor exceeding 1014. This great disparity is understandable since ordinary atoms, like those of water, are mostly made of empty space with a smattering of lightweight electrons that flitter around a tiny, heavy nucleus. The nucleus is what holds almost all the mass. In contrast, neutron star matter dispenses with the nearly-empty space: it is "wall-to-wall nucleus." A single tablespoon of nuclear fluid from deep inside a neutron star contains about 10 billion tons of material: as much matter as in a large mountain on Earth.The ultra-dense nuclear fluid would explode like a nuclear bomb if you brought it to Earth, but inside a neutron star it is stable because it is held under tremendous pressure. However, in the outer layers of a neutron star (as in the outer layers of all stars) the pressure and the temperature both drop, although the force of gravity is still enormous. Here the fluid solidifies into a heavy crust, about a mile in depth. It is made of heavy atomic nuclei arranged in a quasi-cubic lattice, with electrons flowing between, somewhat like a terrestrial metallic alloy but much denser. The properties of a neutron star's crust were worked out by Malvin Ruderman of Columbia University and other astrophysicists. The upper layers are made of iron, but the nuclei in the solid lattice get increasingly heavy and bloated with neutrons as you go deeper. Free neutrons are also present, along with free electrons, in the inner crust, at depths below about 1/2 mile. That is, a liquid of single neutrons and (negatively-charged) electrons permeates the (positively-charged) lattice. These free neutrons are continually getting stuck and unstuck to the neutron-bloated nuclei. At the base of the crust the bloated nuclei touch and merge; and the whole sub-crust star resembles one giant nucleus. In a pulsar this outer, solid shell is essentially stable, but in a magnetar, it is stressed by unbearable magnetic forces as the field diffuses through it, and as the magnetic field in the liquid core drags on it from below. This deforms the crust and causes the magnetic field penetrating the crust to shift and move. Occasionally the crust and the field above it become catastrophically unstable. | ||||
Outburst
Magnetar outbursts release a tremendous amount of magnetic energy very rapidly.
They tend to come in bunches, at times when the crust is yielding to strong
magnetic stresses. As the instability grows,
the changing, shearing,
twisting field drives strong dissipative currents above the star,
energizing particles trapped in the exterior magnetic field. Simultaneously,
the magnetic field rearranges itself to a state of lower energy. This
produces
a burst of hard X-rays (soft gamma rays) observed as ordinary, powerful
SGR bursts.
Note that magnetic forces can also deform a radio pulsar's crust, but a typical pulsar's magnetic field is not strong enough to rapidly deform the crust and drive episodes of bright outbursts. The field must be roughly 1014 Gauss or more to cause the deep-crust solid to fail. | ||||
Giant FlaresOccasionally, the magnetic field becomes unstable on much larger scales, and rapidly rearranges itself to a state of lower energy. Giant flares inevitably involve significant shifts in the crust structure as well.In a magnetar, the energy available for these magnetic flares is tremendous because the field is so strong. A magnetar's magnetic energy is easily sufficient to power the 1979 March 5th or 1998 August 27th giant flares; as well as all the common, short-duration bursts from SGRs. Somewhat analogous outbursts often happen on the Sun, in explosive events called "solar flares." During a solar flare, magnetic field lines near the Sun's surface change the pattern by which they connect to each other, a process called "magnetic reconnection" which releases pure magnetic energy. This happens in magnetars too. Because it occurs quickly and cleanly (without much matter involved) it is capable of producing intense, short bursts of gamma rays. Indeed, solar flares often emit part of their energy in gamma rays. |
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| In 1992 we published a paper in The Astrophysical Journal proposing the magnetar theory of SGRs. We outlined most of the ideas described above, including the idea of reconnection-powered bursts (and magnetically-powered steady X-rays, buried in footnote 6 of the paper). We also noted that: | ||
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| The theory was widely met with skepticism. This is a healthy part of the scientific process, causing us to work harder. In 1995 we published a long paper in the Monthly Notices of the Royal Astronomical Society (a British journal) with many more details. We outlined seven different ways to estimate the magnetic field of the March 5th burster, all of which seemed to indicate a field greater than 1014 Gauss. | ||
| In particular, we argued that if, and only if, the field exceeds 1014 Gauss, it could: | ||
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| This final point needs a bit of explanation. The initial pulse
of the March 5th event was so hard and bright that it must have been
emitted by a
pure (nearly mass-free) explosion of energy, or a "fireball," blown out from
the star at nearly the speed of light. This could have been powered by a
magnetic flare. The fireball evidently contained little matter, except for lightweight electron-positron pairs which are ubiquitous in tremendously hot gas. (Positrons are anti-electrons, a kind of antimatter. Particle-antiparticle pairs are spontaneously created from photons whenever sufficient energy is present.) If the fireball had been polluted by more than a trace of heavy particles (neutrons and protons) then it would have lost energy in blowing out the heavy matter, and it wouldn't have emitted such intense, hard gamma rays. It is natural to expect that after the fireball dispersed, it left behind a residue. In this case, the expected residue is a hot cloud of electron-positron pairs, trapped near the star by the strong magnetic field. |
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Near a neutron star, magnetic field lines are anchored at both ends on
the stellar surface, describing arches outside the star. (In this way,
they resemble the field lines of an iron bar magnet, which arch from the north
to the south poles.)
Electrons and positrons are electrically charged, so they gyrate around
field lines, but they cannot drift perpendicular to them.
They can drift freely parallel to
field lines, but in this direction they quickly run up against the
stellar surface at the footpoints
of a magnetic arch. Thus the magnetic field acts like a
"bottle," holding charged particles.
Now, there are also plentiful X-rays and gamma-rays inside the
magnetic bottle,
which can cross the field lines, but these photons bounce
around between
electrons and positrons and don't get far. (Besides bouncing,
gamma-ray photons also continually
make electron-positron pairs and get regenerated when pairs annihillate.
This further impedes their motion.)
Only at the surface of the bottle can the photons stream freely away. Thompson and I realized that zones of such hot, magnetized photon-pair gas must have been left behind following the March 5th hard pulse, anchored onto the neutron star. We called this phenomenon the "trapped fireball." The hot gas loses energy as photons stream away from its outermost, exposed layers. Electrons and positrons in this outer shell steadily annihillate, so their energy is radiated away too. The trapped fireball inevitably shrinks over time. Nested sheaths of field-lines empty out, in succession. This could explain the March 5th light curve. As the trapped fireball shrunk, its glowing surface area diminished and it got dimmer. Since the magnetic field lines were anchored to the rotating neutron star, the zones of glowing gas also turned every 8 seconds. We must have viewed the fireball from ever-changing angles, repeating on an 8-second cycle. Peaks in the brightness occured when the most luminous part faced toward us; and dips when the fireball was mostly occulted by the star. Meanwhile, it steadily shrunk and dimmed. After about 3 minutes, the trapped fireball evidently evaporated away entirely. |
| Similar zones of hot, trapped particles are probably made when magnetic energy is released in common SGR bursts. This can 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 the subsequent short bursts. But typical SGR bursts don't last long enough to show 8-second rotational dips. They simply don't have enough energy to persist that long. A low-energy trapped fireball evaporates quickly. |
| This brings us to point (7) above. The star's magnetic field had to be strong enough to confine the hot electron-positron-photon gas which emitted 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 very simple calculation then shows that, in order to hold a particle gas with that much energy close to a neutron star (close enough to show such dramatic dips as it turns), using purely magnetic forces, the field must have been greater than about 4 X 1014 Gauss. |
| Now, because a huge amount of energy escaped at the beginning (as evinced by the hard initial spike) the magnetic trapping forces evidently were pushed to their limits. This suggests 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 (e.g., the spindown argument, etc.) is encouraging. Most of our estimates were based on the 1979 March 5th event, so that event might be called a "smoking gun" for extremely strong magnetic fields. |
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All of the above arguments for magnetars were proposed before the end of 1995. However, at that time few astronomers were interested in studying abstruse issues of neutron star magnetism, or considering reinterpretations of data which were 16 years old. This situation changed dramatically in 1998 when many new observational results came flooding in.
It began in May 1998, when Chryssa Kouveliotou of NASA Marshall
Space Flight Center and an international team of 10 collaborators
showed that the X-ray emissions from
SGR 1806-20 pulsate on a regular 7.5 second cycle.
These pulsations are almost certainly due to the rotation of a
neutron star.
As the star turns, bright and dim zones on its surface
and in its surrounding magnetosphere ("hot and cold spots") evidently
rotate in and out of our view.
This means that SGR 1806-20 has a rotation period of 7.5 s, similar to that
of the March 5th burster (8.0 s).
Moreover, Kouveliotou and her collaborators
measured the rate at which the pulsations (or rotations) were
slowing down : namely 0.26 seconds per century.
This might not sound like much, but it demonstrated that the
braking on the star's spin is profound. It allowed
Kouveliotou et al. to make a fairly direct
estimate of the magnetic field, using the same method used in radio pulsar
studies. If the braking was due to magnetic waves and associated particle flows
carrying away
energy and angular momentum, as seemed plausible, then the field strength was
This dramatic result, published in the the 21 May 1998 issue of Nature raised much interest among astrophysicists. Moreover, it seemed as if the SGRs themselves decided to respond. During the last week of May 1998 SGR 1900+14 emitted over 50 detected bursts, some with unprecedented energy. It continued bursting into early June, when a completely new SGR in our galaxy, SGR 1627-41, showed itself for the first time. This new star emitted about 100 bursts over the next two months, as described above. Then, in August, Kevin Hurley and his co-workers announced that they had detected 5.16 second pulsations in the continuous X-rays from SGR 1900+14 ( IAU Circular 7001 ). Kouveliotou, Tod Strohmayer (NASA Goddard Space Flight Center) and Hurley, working with six other scientists, soon found that the X-ray pulsations of this star, like SGR 1806-20, were gradually slowing down. For the magnetic field to cause the star to spin down at the observed rate, it would need to be about 5 X 1014 Gauss. But before the researchers had a chance to write a paper about this, they were scrambling to point all available X-ray telescopes toward SGR 1900+14 again... |
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On August 27, 1998 a giant flare from SGR 1900+14 set new records for
the most intense flux of gamma-rays ever detected from a source outside
our solar system. It blitzed gamma-ray
and X-ray detectors on seven different spacecraft at locations throughout the
solar system. Especially useful
data were recorded by three experiments:
the Russian
Konus detector on the geo-space science
Wind space probe
which was
orbiting near the Sun-Earth equilibrium point ("L1"), upstream of the
Earth in the solar wind; the Italian-Dutch Beppo-SAX
gamma-ray/X-ray observatory, in low Earth orbit;
and a gamma-ray detector aboard the
Ulysses spacecraft, a joint effort of the European Space
Agency and NASA
that was orbiting the Sun in a polar orbit at roughly the
distance of Jupiter.
NASA's Rossi X-ray Timing Explorer (RXTE), another Earth-orbiting X-ray observatory, was pointed away from SGR 1900+14 when the burst occured, but it nevertheless recorded a strong signal. High-energy photons were diffusing through the metal shields surrounding its X-ray detectors. However, one proven workhorse for SGR studies, the Burst and Transient Source Experiment (BATSE) aboard NASA's orbiting Compton Gamma-ray Observatory, detected nothing. The BATSE team, led by mild-mannered Charles Meegan (who is BATSE-MAN) ran out of luck that day: the Compton Observatory was on the far side of the Earth at the time of the flare. The flare hit the Earth on it's night side, in the zenith over the western Pacific Ocean, at 1:22 A.M. Hawaii time. It was intense enough to strongly ionize the Earth's outer atmosphere, affecting radio communications. This requires some explanation. Radio waves, especially long-wavelength ones like those on the AM dial, bounce between the "ionosphere" and the Earth's surface as they propagate around our planet. The ionosphere is a layer of diffuse, ionized gas -- atoms of air which have lost electrons and become positively-charged ions -- in the upper reaches of the atmosphere. High-energy photons from the Sun keep the thin air up there well-ionized during the day, so the daytime ionosphere lies about 60 kilometers above the Earth's surface. At night, electrons recombine with ions, causing the inner edge of the ionosphere to recede upward, to 80 - 90 kilometers. This is why you can pick up very distant AM stations on your radio at night: radio signals generally travel farther if they must make fewer (power-sapping) bounces. In the early morning of August 27th 1998, Stanford University engineers monitoring very-long-wavelength U.S. Navy radio transmissions (which carry coded messages for nuclear submarines) found that the altitude of the ionosphere plummeted for a five-minute period beginning at 3:22 A.M. PDT. Some mysterious source of ionization was apparently driving the ionized layer down to daytime altitudes (about 60 km). Curiously, the height of the ionosphere was also observed to vary cyclically over a period of 5.16 seconds... Of course, they had detected the rotation period of SGR 1900+14 in a remarkable new way, proving that you don't need a sensitive X-ray telescope to measure the spin period of a SGR. If you can wait for a giant flare, you can use a "Whole Earth Telescope"-- the bulk ionosphere of the whole planet -- to see the rotation period of a tiny neutron star, twenty thousand light years away. As the wavefront of gamma-rays swept out of the solar system, the last spacecraft it reached was NASA's Near-Earth Asteroid Rendevous (NEAR) space probe, enroute to a rendevous with the asteroid Eros. The flare was bright enough to force NEAR's gamma-ray detectors into a protective shut-down mode. Here is a graph of the intensity of the August 27 flare, as recorded by the Ulysses gamma-ray detectors (sensitive to gamma rays in the range of 25-150 "kilo-electron volts" or "keV," a unit of photon energy). |
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The two giant flares ( 1979 March 5. and 1998 August 27) were similar in many ways. Each began with a brief, hard spike of very intense gamma-rays, followed by a soft oscillating tail. In the August 27th event, the oscillations follow a regular 5.16-second cycle. (The cycles appear progressively shorter as you go to the right on the above figure, but that is only because the time-scale is not linear -- it gets more "squashed" as you move to the right, in a "logarithmic" way.)
Although the August 27 event was intrinsically less powerful than March 5th (roughly by a factor ~10), it came from a source much closer to Earth, so it appeared brighter. Many improvements in detectors and data recording equipment had occured between 1979 and 1998, so we have much better data about the 1998 flare. For example, in 1979, almost no information about the terminal stages of the flare was recorded, due to the limited data storage capabilities of 1970's-era spacecraft computers. In 1998, the end of the flare was recorded by three different experiments ( Konus, Ulysses and Beppo-SAX). Remarkably, the bright flare emissions dropped quite abruptly, essentially to zero, at a time 380 seconds after the onset of the flare (see figure at right). This is as expected in the magnetar model, because a trapped fireball on the surface of a magnetar must "evaporate away" completely in a finite time, as electron-positron pairs in the fireball annihillate and their energy is carried away by the flare's hard X-rays. In contrast, emissions from a cooling "hot spot" on the surface of a star would gradually fade away as the hard X-rays "soften" -- i.e., the X-rays photon energies would gradually shift to lower values. |
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Strange Quark Stars and Afterglows
(some parenthetical remarks, for advanced readers) |
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Fading "hot spot" behavior is predicted by an alternative model for SGRs based upon the idea of "strange quark stars." This model posits that SGRs are made of "strange quark matter," a hypothetical alternative state for very dense material (different from ordinary nuclear matter, as found in neutron stars). Theorists have noted that a glob or "nugget" of strange quark matter falling onto a strange quark star (if such an event somehow happened) and slamming onto its surface at high speed, would create a hot spot on the surface which could emit something resembling a SGR giant flare. This is because strong quantum "color" forces between quarks would hold down matter rather than gravity, allowing extremely bright gamma-ray and hard X-ray emissions to come from the star, without losing too much energy to the process of blowing out matter. However, the abrupt termination of the August 27th flare seems inconsistent with this scenario. Note that even in the magnetar model, there should be a residual "hot spot" on the neutron star surface after the trapped fireball evaporates, which emits an X-ray "afterglow." (This was first noted by Thompson and I in our 1995 paper in Monthly Notices of the Royal Astronoical Society.) But this afterglow is orders of magnitude fainter that the bright, trapped fireball emissions in the flare, so it cannot be detected by all-sky gamma-ray detectors like the one on Ulysses . To detect SGR afterglow requires a true X-ray telescope, pointed at the source. Indeed a fading afterglow was found in the days following the 1998 August 27th event when RXTE and other X-ray telescopes were turned toward the SGR. Afterglows following three subsequent, bright bursts from SGR 1900+14 have been measured as well (on 1998 August 29; 2001 April 18 and 2001 April 28). These afterglows are consistent with cooling hot-spots on magnetars, but not on strange quark stars. |
Perhaps the most striking feature of the August 27th event was the emergence
of a strong four-peaked pattern in the light curve after about 40 seconds,
as shown in these data from the Ulysses and Beppo-SAX
gamma-ray detectors.
The four-peaked pattern persisted through much of the rest of the flare, as shown in the following graph. (Note: each box below represents one 5.16-second rotation cycle of the star. The Beppo-SAX spacecraft recorded the detailed pattern of flare emission for 7 1/2 more cycles than did Ulysses .) 
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These remarkable data indicate that the geometry of the trapped fireball became more complicated in regions close to the star, once the far-reaching, smoother emission zones cleared. This will be interesting to fit with detailed models, in order to learn more about the flare mechanism and the neutron star’s magnetic field.
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