original of this page
|1 Strong magnetic fields *||2 Discovery of SGRs|
|3 SGR Bursts||4 The March 5th Event|
|5 The burster is located||6 March 5th Flare Theories|
|7 Other SGRs *||8 Neutron star magnetic fields|
|9 A new kind of star? *||10 A brief history of magnetars|
|11 Trapped fireballs *||12 A year of magnetar breakthoughs *|
|13 The August 27th event *||14 Anomalous X-ray Pulsars *|
|15 Magnetic twists and X-rays *||16 Magnetar links *|
|17 Epilog: new horizons in magnetar science *||.|
* New or expanded sections, March 2003
|Observations of SGRs give evidence that these stars have extrordinarily strong magnetic fields. One especially simple and reliable estimate of SGR magnetism, first done by C. Kouveliotou and her collaborators in 1998, 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.|
|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 hulking 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 field lines probably wrapped in a toroidal, or donut geometry inside the star.)
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 fields (new subsection, Jan. 2003)The 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.|
|Grey boxes like this indicate material for advanced readers.|
Ultra-strong magnetic fieldsMany
fascinating physical effects occur in magnetic fields
with strength exceeding the "quantum electrodynamic
field strength" of BQ
|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.
|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.|
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.|
|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.
The source turned out to lie inside the tiny area of the sky which is covered by a "supernova remnant": the glowing cloud of hot gas 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 a small galaxy that orbits the Milky Way.
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 10,000 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.)|
|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....|
|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.|
|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.|
|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?|
We estimated that, at the pole of a dynamo-active young neutron star, the magnetic field could realistically reach 1014 - 1015 Gauss--- 100 to 1000 times stronger than in ordinary pulsars. What would such a strongly-magnetized neutron star, or `magnetar' look like?
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.
This has several consequences:
Why does the star have a crust?
(This subsection expanded in Jan. 2003). 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.
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.
Occasionally, 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.
|(For more information about magnetar instabilities, see my 2004 paper, Triggers of magnetar outbursts. This review article was written for non-specialist scientists. I tried to avoid complex equations, so it might be accessible to adventurous non-scientists who are willing to skip over occasional technical parts.)|
|In 1992 we published a paper in The Astrophysical Journal proposing the magnetar theory of SGRs (Duncan & Thompson, Ap.J. 392, L9). 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:|
|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 very long paper in the Monthly Notices of the Royal Astronomical Society (a British journal) with many more details (Thompson & Duncan, M.N.R.A.S. 275, 255). 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:|
|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
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.
|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
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.|
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
carrying away energy and angular momentum, as seemed
plausible, then the field strength was
This 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...
| 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).
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.
|Strange Quark Stars and Afterglows
(some parenthetical remarks, for advanced readers)
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, 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
|These remarkable data indicate that the geometry of
the trapped fireball was quite complicated in regions
close to the star, once the far-reaching, smoother
emitting zones "cleared." This gives the
first direct evidence that the magnetic field of a
neutron star is complex near the star. That is, it is not
a simple field with just a north and south pole, like a
|This is the only section which is not
yet written. I will be posting some information here in
early 2004. I am also writing a thorough, non-technical
discussion of AXPs, which will appear in an upcoming
issue of Sky
and Telescope magazine.
This section is not yet written. Check back later if you want to read it. This section is not yet written. Check back later if you want to read it. This section is not yet written. Check back later if you want to read it. This section is not yet written. Check back later if you want to read it. This section is not yet written. Check back later if you want to read it. This section is not yet written. Check back later if you want to read it. This section is not yet written. Check back later if you want to read it.
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| The crust of a neutron
star has odd properties. It is very difficult to compress
crust material very much, or to move elements of crust up
or down, because strong gravity and pressure forces
maintain a firm balance, holding the crust on the level
and at nearly constant volume. It is much easier to move
parts of the crust horizontally, in ways which apply only
"shear strains" to it.
(To understand shear strains, imagine grasping a block of rubber between your hands. Now push your left hand away from you, and pull with the right toward you. The block deforms into a parallelpiped. You have applied a shear strain to the block: the elastic restoring force is perpendicular to the line between your hands.)
Pressure forces which resist compression in a neutron star crust are much stronger than elastic forces which resist shear strains, because they originate from different microscopic components. Pressure comes mostly from the quantum-mechanical incompressibility of the electron and neutron fluids (so-called "degeneracy" forces), while the shear elasticity comes solely from the weaker electrostatic (Coulomb) restoring forces of the positively-charged cubic lattice of nuclei.
In a young magnetar, the magnetic field evolves over time, seeking a lower energy state and in the process, subjecting the crust to strong magnetic forces. For example, magnetic field lines continually drift through the star's liquid interior, stressing the crust from below. The field isn't strong enough to drive much compression or vertical motion in the crust, but it can drive significant twisting motion of patches of crust along horizontal directions, since this involves pure shear strains. (Because work = force x distance, these kind of deformations also engender the most energy transfer between the field and the crust.) Whenever this happens, magnetic field lines outside the star also get twisted because they are anchored to the crust. This can have significant, observable consequences, as shown in the following figure.
Now imagine that a circular "cap" of crust gets twisted due to stresses applied on it from below, as shown by the brown arrows on the surface of the left-hand star. When this happens, the field lines outside the star get twisted, as shown on the right. This inevitably leads to a current I along the arch, as will be clear to readers who recall Ampere's law from freshman college physics.
Ampere's law - the equation in the box, within the figure - says says that the "circulation of B," which is the line-integral of the magnetic field B around a closed loop (in this case, the green circle around the arch of twisted field lines) is proportional to the electrical current I passing through the loop. Because the field is twisted, there is a component of B pointed around the green loop, yielding a non-zero circulation. Thus there must be a current: charged particles must stream along the twisted bundle of magnetic field lines, which form an arch above the star. Electrons flow one way, while protons and ions flow the other way.
Currents flowing outside a magnetar give rise to detectable X-ray emissions (represented by orange squiggley arrows in the figure) in two ways. Protons and heavier ions slam onto the star's surface at one footpoint, heating a patch of crust there and causing it to glow in (relatively soft) X-rays. Electrons flowing in the other direction achieve velocities close to the speed of light, because such lightweight particles are easy to accelerate. They impact and heat the other footpoint; but more importantly, these fast-moving electrons often collide with X-ray photons outside the star, giving up much of their kinetic energy. Many soft, thermal X-ray from the star's glowing surface thus get kicked up to high energies. The resultant emissions can plausibly match those of SGRs and AXPs, which include soft and hard components. In this way, the energy of a magnetic twist outside a magnetar is gradually dissipated into X-rays.
Patches of a magnetar's crust can twist elastically, with spiral patterns of shear strain. When the magnetic forces get strong enough, they can also yield to the magnetic stress and move "plastically," truly changing the crust's structure. Such twisting of "caps" of crust probably trigger SGR outburst episodes and flares.
These crustal motions generally drive currents outside the star, and generate X-rays. In particular, when localized, lateral plastic slippage of the crust creates strongly sheared regions in the magnetic field above the star's surface, strong currents will flow. Magnetic reconnection during outbursts tends to smooth such discontinuities, but it cannot eliminate all exterior currents. When any closed loop around any part of the shifting crust's surface develops a non-zero circulation of B, a current will flow.
This probably explains why the X-rays from SGRs and AXPs sometimes vary in brightness, often concurrently with bursts and flares. Moreover, if magnetic twists (and currents) extend out far from the star, to regions where outgoing magnetic waves are generated due to the stellar rotation, then they will affect the rate at which the star spins down.
Rather dramatic variations in spindown rates have been observed in some SGRs. These variations might be driven by internal changes within the stars. But some of the spindown variability might also be due to changing twists in a "globally-twisted magnetosphere." In this scenario, whole hemispheres of the magnetar tend to twist relative to each other, driven by interior magnetic stresses. The whole, global, exterior magnetic field thus becomes twisted, and carries currents. This includes many field lines extending out very far from the star, so outgoing magnetic waves are significantly affected, and the spindown rate changes when the large-scale twist geometry does.
A globally-twisted magnetic field implies a very strong, "wound-up" magnetic field deep inside the star to drive the twist. Such a field, with many field lines in a toroidal (donut-like) geometry, would drive hemispheres of the crust to twist relative to each other as it seeks to unwind, like the spring of a watch. If magnetars form from very rapidly-rotating newborn neutron stars, then the interior fields is expect to be "wound up" by the rapid initial rotation.
References Magnetic twists and X-ray emissions were first discussed by C. Thompson, R.C. Duncan, P. Woods et al. 2000, Astrophysical Journal, 543, 340. The globally-twisted magnetosphere was discussed by C. Thompson, M. Lyutikov and S.R. Kulkarni 2002, Astrophysical Journal, 574, 332.
|Here are links to some informative, non-technical webpages about SGRs and AXPs. Those with yellow backgrounds are especially recommended because they discuss developments that I did not (adequately) cover above.|
Note: A recent Google
search of "magnetars" turned up over 10,000
webpages. I have seen only a small fraction of these, so
I probably have omitted some good ones. Please let me
know via e-mail if you know about links that I should
Many astronomers are now studying SGRs and AXPs. These stars are most luminous in X-rays, but some have been detected in visible and infrared wavebands as well, where they show pulsating (and possibly semi-polarized) emissions. Astronomers have also detected changes in X-ray and infrared emissions, including changes in pulse shapes and in the X-ray photon energy distributions; and variations in the rates of spindown. These observed changes sometimes occur in concert and/or together with burst and flare activity.
Studies of these phenomena promise to tell us much about the physics of neutron stars and their surroundings. Magnetars seem especially well suited for such investigations. Radio pulsars are much more stable stars, so their relatively-unchanging emissions provide less interesting probes of the stars' structure.
This ongoing study of SGRs and AXPs is one growing area in magnetar astrophysics. Another possible growth area involves unexplained astronomical phenomena for which magnetars might prove culpable. Here are some intriguing possibilities.
|This grey box is more speculative than the rest of this website, and can be skipped by readers who are not interested in unresolved questions on the frontiers of astrophysics.|
Cosmic rays, discovered and named in 1912, are actually not rays at all. They are very fast-moving protons or atomic nuclei (i.e., self-bound knots of protons and neutrons). A diffuse flux of such particles rains down on the Earth all the time, from the depths of space.
Because cosmic rays travel at nearly the speed of light, they have very high energies. Most cosmic rays are believed to be accelerated in the shockwaves of supernova explosions, but the most energetic cosmic rays cannot be accounted for in this way. These rare "Ultra High-Energy Cosmic Rays" (UHECRs) have truly amazing energies. They probably are single protons or light nuclei (e.g., helium nuclei) each carrying as much kinetic energy as a bowling ball rolling toward the pins, or a speeding ball in baseball or cricket (choose your sport).
UHECRs are detected when they strike the upper layers of our atmosphere and dissipate their energy, creating "showers" of high-energy particles which can be seen as flashes of light. The study of these mysterious particles is a hot topic in astronomy right now. (For a non-technical discussion, see the cover story in the March 2003 issue of Sky & Telescope.)
UHECRs cannot travel truly vast distances (i.e., billions of light-years) across the Universe because they tend to lose energy by scattering against the ubiquitous low-energy photons that fill space: the "cosmic microwave background" left over from the Big Bang. This means that very distant quasars or active galactic nuclei (giant black holes surrounded by accretion disks and with outflowing jets) cannot be the sources of UHECRs. Neither can those cosmic gamma-ray burst (GRB) sources which are known to be billions of light years away. One must look instead for sources within about 100 million light- years of Earth, where there are no quasars.
Newborn magnetars may the sources, as suggested in a recent paper by Jonathan Arons of Berkeley. With their very rapid spins and high magnetic fields, young magnetars generate very strong electric fields that could accelerate particles to ultra-high energies. UHECR energies would then come ultimately from the rapid spin of a nascent neutron star. There are many galaxies within 100 million light-years of Earth in which magnetars presumably form, easily enough to account for the observed UHECRs.
For UHECRs to emerge from a fast-spinning magnetar within a supernova, the high-energy outflow from the magnetar must quickly "punch" out of the (relatively slow-moving) supernova gas, leaving open channels or jets. Otherwise the UHECRs would lose too much energy by interacting with the hot gas of the exploding star. Arons argues that the powerful wind of particles and radiation blown out from a young magnetar at nearly the speed of light is fully capable of "punching out."
How might this scenario be verified? Galaxies within 100 million lightyears of Earth tend to be concentrated in a vast structure called the "supergalactic plane" which manifests itself as a band across the sky, nearly perpendicular to the plane of our Galaxy (i.e., to the Milky Way). For example, the Virgo cluster of galaxies, about 60 Million light-years away, lies in the supergalactic plane, as do most nearby galaxies and galaxy clusters. Recent observations (now being checked) suggest that UHECRs tend to come from the supergalactic plane. Magnetars are arguably the most plausible UHECR sources known in this local zone of the Universe.
Gamma-ray bursts (GRBs) are occasional flashes of very high-energy photons which are observed come from spots all over the sky. They tend to be much harder than SGR bursts, and show no evidence for repeating. Two distinct physical classes of GRBs are known: long-duration and short-duration GRBs. Long-duration GRBs tend to have softer (lower-energy) photons. The dividing line between the two classes is about 2 seconds; i.e., most GRBs which last for less than 2 seconds are in the short duration physical class (although a few intermediate bursts are difficult to classify). About two-thirds of the GRBs cataloged by the landmark BATSE experiment of the 1990's were long-duration ones. It is possible (although far from certain) that magnetars play some role in explaining both kinds of GRBs.
Many long-duration GRBs have been proven to come from very great, "cosmological" distances (i.e., their sources are billions of light years away, which means that the gamma rays have traveled across an appreciable fraction of the 14-billion-year-old observable Universe before reaching us). In some cases, X-ray, optical, and radio-wave afterglows from long-duration GRBs have been detected, allowing their positions to be precisely pinned-down and matched with a very distant galaxy which presumably contains the GRB source. For a detailed, non-technical account of this important breakthrough in our understanding of long-duration GRBs, see the book Flash! by Govert Schilling (Cambridge University Press, 2002).
At present, the most popular model for long-duration GRBs invokes the core-collapse of a very massive, rotating star, as in familiar models for supernova, but under circumstances in which the core becomes too heavy to form a stable neutron star. The result is a rapidly-spinning black hole with an ultra-magnetized disk of very dense matter orbiting around it. This probably drives jets of fast-moving matter outward along its rotation axis, which could "punch" out of the surrounding gas and perhaps be observable as GRBs when the jet happens to point toward Earth. Because a black hole's gravity is so strong, such a scenario potentially produces more power than any model involving neutron stars, which is advantageous when trying to explain the tremendous power of cosmic GRBs.
Alternative models, in which the "central engine" powering the GRB is a rapidly-spinning, nascent magnetar, have also been suggested by many astrophysicists (beginning with our 1992 magnetar-proposal paper, section 3.3). With less available power, this is not widely favored. However, a subset of long-duration GRBs with low power was recently identified by Jay Norris of NASA's Goddard Space Flight Center. These GRBs are thought to be intrinsically faint because they show long "lag times" between the arrival of hard and soft gamma-ray photons. (This lag time is empirically anti-correlated with brightness in GRBs.) About 100 of these events were found in the BATSE catalog of GRBs. Their sky positions tend to be concentrated near the Supergalactic Plane, as expected if they come from stars in galaxies within a few hundred million lightyears of Earth. (Here is a popular account of Norris' work,and his scientific paper.)
It is tempting to speculate that these low-brightness GRBs are the magnetar-powered subset of long-duration GRBs. If so, these could be the same objects which produce UHECRs, as in Aron's model. These might be due to magnetar-forming supernovae in which the outer (hydrogen) envelope of the star has been lost prior to the explosion, allowing easier emergence of high-energy emissions flowing from the central engine.
About 1/3 of the events in the BATSE catalog are short-duration GRBs. These brief events have distinct properties (including harder gamma-ray photons) so they seem to come from a different kind of source than the long-duration GRBs. There is circumstantial evidence that short GRB sources are also distributed widely through the observable Universe (i.e., detectable out to many billions of light-years) which would imply that they are intrinsically very bright. However, as of March 2003, no X-ray, optical or radio afterglow has yet been detected following a (verified) short GRB, so their locations have not been accurately pinned-down and studied. As a consequence, the distances of short GRB sources from Earth remain uncertain.
Some short GRBs are probably magnetar flares like the March 5, 1979 event, coming from SGRs in galaxies beyond the Milky Way and Magellanic Clouds. BATSE could have detected the hard spike of the March 5th event out to a distance of about 40 million light-years, almost reaching the Virgo cluster of galaxies. However, BATSE was not sensitive enough to detect the ensuing soft, oscillating tail of the March 5th event if its source was more than 2 million light-years away (roughly the distance to our nearest-neighbor Andromeda Galaxy). Thus, for flaring SGRs between 2 and 40 million lightyears distant, the hard spike would have been recorded as a typical short-duration GRB by BATSE. If all magnetar flares are as bright as the March 5th event, there should be about a dozen such events listed in the BATSE catalog of GRBs, with considerable uncertainty. (I estimate a 5% chance that BATSE detected more than 30.)
In order to determine how many --and which -- short-duration GRBs are magnetar flares, we need to obtain precise positions of these events on the sky. (BATSE localized short-duration GRBs only to within a few degrees.) NASA's Swift satellite, scheduled for launch in December 2003, is designed to do just that. Upon detecting a GRB, Swift will swiftly (within 20-70 seconds) turn and point both an X-ray telescope and an optical-UV telescope toward it. If the event shows an afterglow, then the X-ray and UV-optical telescopes will pinpoint its location.
Magnetar flares will be obvious to Swift, since they will be found to come from regions of active star formation in relatively nearby galaxies. Moreover, Swift's X-ray telescope might catch the oscillating, soft tail of the event (lasting 3-5 minutes) allowing the flaring magnetar's rotation period to be measured.
It is interesting to speculate that some magnetar flares might be much more powerful than the March 5th event, or appear much brighter when viewed from certain directions ("beamed emissions"). If this is true, then it is possible that all short-duration GRBs are extragalactic magnetar flares from sources within about 1 billion lightyears of Earth, although this seems unlikely. (Another possibility that cannot yet be ruled out is that some short GRBs come from old magnetars in the halo of our Galaxy.)
Even if only a few percent of BATSE bursts prove to be extragalactic magnetar flares, as seems likely, the fraction of magnetar flares will increase in the future, as GRB detectors get more sensitive. Indeed, magnetar flares and Norris' GRBs might eventually dominate the counts of short and long GRBs, respectively. The trends in GRB data which suggest this are clear. After we become capable of detecting most of the very bright, truly cosmological GRBs in the observable Universe, improvements to detectors won't add many more of these. Instead, we will find more and more intrinsically-dim bursts, since detectors will find these out to greater and greater `sampling depths' within the local space around us.
The study of magnetars will be invigorated by these new data. Even for events with no data on soft tails (i.e., no Swift-like rapid follow-up with a sensitive X-ray telescope), a large catalog of hard spikes from magnetar flares would hold a wealth of fascinating information about magnetic instabilities in neutron stars.
Reference: see my 2001 paper, Gamma-Ray Bursts from Extragalactic Magnetar Flares.
Magnetars in the Milky Way
We now turn from observed, mysterious astronomical phenomena which might be caused by magnetars, to a reliably-predicted manifestation of magnetars which has yet to be observed.
How many magnetars are there in our Milky Way galaxy? A dozen SGRs and AXPs are now known in the Milky Way and in the neighboring Magellanic Clouds, all with rotation periods between 5 and 12 seconds. Because these stars are spinning down rapidly, they cannot be very old; otherwise they would have already reached much longer periods. Their spins imply that they are all roughly 10,000 years old or younger, in agreement with the ages of the supernova remnants found surrounding some of them (and within which the stars evidently formed).
Now, if there exist 10 magnetars younger than 10,000 years in our Galaxy, then magnetars must be forming at a rate of about 1 per 1000 years. In reality, the birthrate is probably higher because we haven't yet found all the young magnetars in the Galaxy. (SGRs and AXPs are still being found.)
Another way to estimate the birthrate is to take a census of neutron stars associated with known, young supernova remnants (SNRs). Since the number of magnetar candidates associated with SNRs is comparable to the number of radio pulsars so associated, an appreciable fraction (perhaps half?) of all neutron stars may be magnetars. Now, the total neutron star formation rate is almost as large as the rate of core-collapse supernova in the Galaxy. (Only a minor fraction of core collapse events are thought to make black holes). Based on counts of supernovae within many other galaxies like the Milky Way, this rate is estimated in the range of 1 per 100 years. This might be closer to the true Galactic magnetar birthrate than 1 per 1000.
Let's suppose that the birthrate is 1 per 300 years. Then in the 10-billion- year age of our Galaxy, about 30 million magnetars would have formed. If the supernova rate was higher in the past, due to vigorous star formation during the early history of the Milky Way (as seems likely) then the count would be higher.
Where are all these stars? The theory of neutron star
magnetic evolution gives clues.
|(The next two paragraphs are intended for advanced readers. Please skip them if they seem confusing.)|
| In the sub-crust fluid interior of a
neutron star, magnetic field lines are usually found in
bundles, or "flux tubes." These strongly
interact with the charged particles in the nuclear fluid
(protons and electrons, which gyrate around the field
lines), but not with the abundant, electrically-neutral
neutrons, which hold about 90% of the ultra-dense fluid's
mass. Magnetic forces gradually drag the flux tubes and
their entrained charged particles through the background
neutron fluid, causing the field to evolve via a process
know as "ambipolar diffusion." This is
limited by friction, due to collisions between charged
particles and neutrons, and by the limited rate at which
neutrino-producing processes create and destroy the
charged particles in the fluid and so adjust their
numbers. (The simplest such processes are
We studied the implications of this for magnetars in 1996 (Thompson & Duncan, Astrophysical Journal 473, 322). The diffusing field in the liquid interior pulls on the crust from below, stressing it and sometimes driving significant changes in its structure. (This picture is complicated by magnetic diffusion within the crust, perhaps accompanied by many small-scale shifts in the crust structure.) Because the friction of ambipolar diffusion dissipates magnetic energy, which heats the star, which in turn speeds the diffusion, there is a feedback effect which means that the dissipation of magnetic energy shuts down rapidly once the star cools below a (rough) threshold temperature. This could happen after several tens of thousands of years.
|This may explain why magnetic activity ceases when
magnetars age past the SGR/AXP phase. A magnetar's field
probably does not dramatically decay away during this
time, but only diminishes by a modest factor and grows
simpler in structure. Subsequently, the field is trapped
within the cool star, and changes only very slowly.
Thus, both observations and theory suggest the existence of dead magnetars: magnetars which are no longer significantly powered by ongoing magnetic dissipation. Many millions of such stars, perhaps hundreds of millions, probably exist in the Galaxy around us. These stars are X-ray dark and burst/flare quiet, and therefore quite difficult to detect.
Where are the dead magnetars located? This depends upon the recoil velocities which they acquire at birth. If most magnetars are born fast, with speeds of thousands of kilometers per second (as suggested by the large displacements of the March 5th and August 27th flare sources from the centers of supernova remnants in which they seem to have formed), then they will drift into the remote halo of our Galaxy and eventually escape from the Galaxy altogether. Such distant stars would be especially hard to detect.
However, two AXPs lie at positions near the centers of putative associated SNRs. This means that they are moving more slowly than a few hundred kilometers per second, unless their velocities happen to be pointed almost directly toward or away from us. Such slow stars will remain forever gravitationally-trapped in (or near) the Galaxy's disk. If most magnetars are born this slow, then the nearest dead magnetar might lie only a few tens to one hundred lightyears from Earth. Ongoing studies of magnetar velocities, such as Hurley's, are clearly important for narrowing the possibilities.
Theory suggests that dead magnetars remain strongly-magnetized. This would drive them to spin down to ever-slower rotation rates. Dead magnetars would then be seen as dim, very-slowly-pulsating X-ray sources, most easily detected when they lie near Earth.
In principle, strong magnetism could make these stars easier to find. Their magnetic fields could sweep up diffuse gas from interstellar space, helping to make dead magnetars glow. This gas-sweeping process is most effective when the star is moving quickly, since more gas is swept up faster. Unfortunately, a fast-moving star soon escapes from the galactic disk, and there is little gas outside it. Taking this factor into account, there is little hope that magnetic gas-sweeping greatly improves the prospects for finding dead magnetars.
There is, however, another hope. After the star turns cold and ambipolar diffusion (mostly) shuts down, slow diffusive processes might continue to operate, gradually driving the star toward new magnetic instabilities. Thus a long-dead magnetar might briefly come alive in an episode of magnetic activity. Even if such episodes are very rare, among many millions of dead magnetars they might be spotted.
For now this is speculation, waiting for hard data. Millions of dark, slowly-reeling stars seem to be out there, drifting through space, a challenge for astronomers to find.