an isolated neutron star. credit: NASA photo gallery.
 
    Neutron stars are born with intense magnetic fields around 1012 Gauss.  Earth’s magnetic field is only 0.5 Gauss and an average refrigerator magnet is about 100 Gauss (Duncan, 1998).  They have the greatest magnetic fields of any known object in the universe.  All stars have some magnetic field, though for many stars it is very small (like Earth’s).  The field is embedded in the star’s surface. This intense magnetic field is a result of the dramatic decrease in size when the star’s core collapses.  During collapse, the radius may decrease by a factor of 1010.  The field strength remains constant, however, and so the field on the surface of a neutron star is at least 1010 times more intense than it was as a main-sequence star (Kaufmann, 1998).
    Neutron stars are also born with a small rotational period between 0.033-1.0 seconds.  In this case, during the collapse of a star, the decrease of rotational period is consistent with the Law of Conservation of Momentum.  This states that the angular momentum is the product of angular velocity and moment of inertia:  L=I?.  The moment of inertia of a solid sphere with constant density (not an exact model, but close) is dependent on the mass and the square of the radius.  When a star collapses and its radius decreases dramatically, the mass remains high, though not constant.  So, in order for angular momentum to be conserved, the angular velocity—spin speed--will have to increase dramatically. (Kaufmann, 1998).  This results in a rapidly spinning dense object only 10km in diameter.  Most objects of this nature are detected as radio pulsars.

an artist's model of a pulsar.  credit Imagine the Universe!, HEASARC, LHEA, NASA.

    Other neutron stars are detected as millisecond pulsars, pulsating x-ray sources, x-ray bursters, x-ray binaries, and radio quiet neutron stars (Frail, 1998).  But there are still other strange things, like soft gamma-ray repeaters (SGRs), detected in space.  Could they also be the signatures of neutron stars?  SGRs exhibit long periods (5-8 seconds) and are believed to be the sources of most gamma-ray bursts in our galaxy.  Maybe they could be neutron stars with unusually intense magnetic fields (on the order of 1013 or 1014 Gauss) which, though born with short periods, are rapidly ‘spun down’ to periods of greater than one second.  These stars would fondly be referred to as magnetars.
 

an artist's rendition of a magnetar.  credit: Robert Mallozzi (UAH, MSFC).
 
    Within the first several seconds of infancy, dynamo action ceases, leaving a rapidly rotating neutron star with field strength roughly 1014-1015 Gauss (Thompson and Duncan, 1992).  This is 100 times larger than that of a pulsar.  Its huge magnetic field billows out around it like a parachute causing the star to quickly “spin down” to a period of more than one second (slow, compared to other neutron stars).  By the time the period slows to 10 seconds, the magnetar is about 10,000 years old (Frail, 1998).  Because of their relatively young age, magnetars are expected to be found near supernova remnants.  Unlike pulsars, whose energy comes mostly from its rotation, magnetars have very little rotational energy and get most of their energy from their magnetic fields.  Field decay is therefore a source of an enormous amount of energy.  “This involves both internal creating and seismic activity that shakes the magnetosphere and accelerated particles.  This gradual release of energy is punctuated by intense outbursts that are most plausibly triggered by a sudden fracture of the neutron star’s crust” (Kulkarni and Thompson, 1998).  A neutron star’s interior is mostly superfluid neutrons, which move with no friction, and conduct the electricity of the few loose electrons and protons with no resistance.  This fluid (containing nuclei, protons, electrons, and neutrons) is moving, turning and swirling.  The crust is a rigid, iron-coated plate, about 1km thick, through which the magnetic field drifts (Ruderman, 1998).  The drifting field puts tremendous stress on the solid, inflexible crust.  Eventually, the crust fractures, relieving the stress, and releasing energy.  These cracks and crunches resemble seismic activity on Earth (Cheng, et al., 1995), but “starquakes” (Southwell, 1998) are so intense that bursts of gamma rays are released, and they often emit steady, low-level x-ray radiation. Field strength of magnetars is measured in two ways.  “One can either ask what field is required to drive the present, measured spin-down rate…or how strong the field must have been to spin down the star to period P in the age of the associated supernova remnant”(Thompson and Duncan, 1996).
    There are strange phenomena detected in space.  The exciting thing about the magnetar model is that it offers an explanation for one of them. Soft gamma-ray repeaters, SGRs, are the source of non-periodic gamma-ray bursts that seem to come from within or near our galaxy, and are found in the dust and clouds of supernova remnants (SNRs).  “Soft” gamma rays are in the range of frequency between gamma rays and x-rays.  They could conceivably have been named “hard” x-ray repeaters. SGRs are non-periodic, but they do repeat and are sources of steady, low-level x-ray emissions.  No explanation of their nature has fit well… until now.  The magnetar model explains the long periods of 7-8 seconds that have been measured for SGRs.  The intense heat and turmoil in magnetars cause temperatures high enough to result in steady, low-level x-ray emissions, explaining those found in SGRs.  The quake activity predicted by the magnetar theory, in which blasts of radiation are released, accounts for SGR bursts.
    What happened after Thompson and Duncan developed their magnetar idea?  They published a letter in the Astrophysical Journal in June of 1992.  They introduced their ideas and predicted that magnetars would be “relatively difficult to detect because they drop below the radio death line faster than ordinary pulsars”(Thompson and Duncan, 1992).  They also asserted that there is “evidence that the soft gamma repeaters are young magnetars”(Thompson and Duncan, 1992).  In a later paper (1996), they argued that a magnetar’s magnetic field and its decay could explain all the features of SGRs.  “...Once a strong magnetic field is invoked to explain the various extreme properties of SGR bursts, the decay of the magnetic field itself can plausibly account for the quiescent x-ray and particle emission from these sources.  Magnetars are self-triggering burst sources, and no external impact or mass accretion is required [to explain bursting]”(Thompson and Duncan, 1996).
 

supernova remnant in the Large Magellanic Cloud.  credit: NASA photo gallery
 
    In Sagittarius, within the remnants of a supernova, lies SGR 1806-20 (Duncan, 1998).  From October 1996 to November 1997, it produced over 40 bursts, providing excellent opportunities for observation.  Chryssa Kouveliotou and her associates determined a period of 7.47seconds.  After calculating a spin-down rate of 2.6x10-3s/year, they were able to estimate the age of the star at 1,500 years, and the magnetic field at 8x1014 Gauss.  The team argued that their data confirmed magnetar theory (Kouveliotou, et al., 1998).
  Inside our galaxy, near the edge of a young supernova remnant, SGR 1900+14 has been keeping astronomers busy this year.  Though only six bursts were detected from it before this year (1998), over 50 were detected near the end of May.  The SGR was found to have a period of 5.16 seconds, and an estimated magnetic field of 5x1014 Gauss (Duncan, 1998).  On August 27, another enormous surge of gamma-ray and x-ray radiation “from a cataclysmic magnetic flare”(Wilford, 1998) swamped the Earth’s atmosphere as it blew past, from its source 20,000 light years away, in the constellation Aquila.  It was SGR 1900+14 again.  Duncan says, “‘Magnetar is the leading theory to explain this, but in science the theory has to keep on passing the tests’” (Wilford, 1998).

credit Edward Wright (UCLA), COBE project, courtesy MSFC, NASA.
 
    In the last two years, two other gamma-ray sources have been detected.  The first, discovered in 1997, is referred to as SGR 1815-13 (though it isn’t agreed to be an SGR) and burst only three times.  The other, found in June of 1998, named SGR 1627-41, put on a great show, with 26 bursts that were detected by four separate space observatories.  Researchers expressed confidence that it is an SGR, and probably a magnetar.  Information about its field strength and period have yet to be determined (Duncan, 1998).
    Though all of the previous evidence strengthens magnetar theory, many researchers say that there is lively debate over the existence of magnetars, and are careful to acknowledge that their data may be insufficient to conclusively support the magnetar model.  However, we have not been able to find any published articles that disagree with the model or the idea that soft gamma-ray repeaters are magnetars.  Magnetar theory, like all scientific theories, cannot be proven.  It can only become more widely accepted as supporting evidence accumulates. As exciting as this time is for the emergence of the magnetar model, soft gamma-ray repeaters burst on their own schedules, not on those of the researchers.  Therefore, scientists interested in further confirming this theory are confined to waiting, working on other projects, until another starquake sends some radiation their way.

Anonymous, 1998, Astronomy, 26, 26.  Introductory overview of magnetars and ‘starquakes.’

Cheng, B., Epstein, R.I., Guyer, R.A., and Young, A.C., 1995, Nature, 382, 518.  Comparison of “earthquake-like behaviour” in SGRs and seismic activity on Earth.

Cowen, R., 1998, Science News, 154, 164.  Reports of August gamma-ray burst and begin explanation of how this provides to magnetars.

Duncan, R.C., and Thompson, C., 1992, ApJ., 392, L9.  Thompson and Duncan propose the existence of a special kind of neutron star (magnetar), and explain the process of its formation.

Duncan, R.C., http://solomon.as.utexas/edu/~duncan/magnetar.html, University of Texas at Austin.  Excellent history and overview from one author of the original magnetar article.

Frail, D.A., Vasisht, G., and Kulkarni, S.R., 1997, ApJ., 480, L129.  This article is a report of new observations of SGR 1806-20, and shows how the authors find them to be evidence in support of magnetar theory.

Frail, D.A., 1998, in The Many Faces of Neutron Stars, Eds. Buccheri, R., van Paradijs, J., and Alpar, M.A., (Dordrecht: Kluwer Academic Publishers) p. 179.  Author discusses the objects other than simple pulsars that result from supernovae, including the magnetar model, and his opinion of its validity.

Gursky, H., Ruffini, R., 1975, Neutron Stars, Black Holes and Binary X-Ray Sources, (Boston: Dordrecht-Holland).

Hayden, 1998, Newsweek, 132, 73.  Tells of August gamma-ray burst and discusses properties of magnetars.

Heyl, J.S., 1998, Implications of Intense Magnetic Fields on Neutron-Star Physics, Ph.D. Dissertation, University of California at Santa Cruz.  Coverage of affect of magnetic fields on structure and thermal evolution of neutron stars.  Defines magnetic fields strong enough to ‘dominate’ atomic structure as IMFs—intense magnetic fields.

Hurley, K., Kommers, J., Smith, I., Frail, D., and Murakami, T., 1998, Nature, 393, 235.  This articles argues that SGR 1806-20 is indeed a magnetar, and that SGR bursts are caused by ‘quakes’

Imagine! Team, http://imagine.gsfc.nasa.gov, a service of High Energy Astrophysics Science Archive Research Center of Laboratory for High Energy Astrophysics, at NASA/GSFC.  Great source of information on topics in astronomy and astrophysics, with dictionary.  Great pictures.

Kaufmann, W.J., Freedman, R.A., 1998, Universe, 5th ed.  (New York: W.H. Freedman and Company) ch. 23.  General history and structure of neutron stars.  Valuable overview.

Kouveliotou, C., Dieters, S., Strohmayer, T., van Paradijs, J., Fishman, G.J., Meegan, C.A.,

Kulkarni, S.R., and Thompson, C., 1998, Nature, 393, 215.  A very brief overview of neutron stars, a basic history of magnetars, and a summary of the evidence to date.

Pacini, F., 1998, in The Many Faces of Neutron Stars, Eds. Buccheri, R., van Paradijs, J., and Alpar, M.A., (Dordrecht: Kluwer Academic Publishers) p. 3.  Brief history of scientific activity and theory, and a list of the author’s unanswered questions relating to neutron stars.

Ruderman, M., 1998, in The Many Faces of Neutron Stars, Eds. Buccheri, R., van Paradijs, J., and Alpar, M.A., (Dordrecht: Kluwer Academic Publishers) p. 77.  Anatomy of neutron stars, evolution of magnetic fields, and relation of crustal activity to changes in magnetic fields.

Shapiro, S. L., and Teukolsky, S.A., 1983, Black Holes, White Dwarfs, and Neutron Stars: the physics of compact objects, (Wiley- Interscience).

Southwell, K., 1998, New Scientist, Aug. 15,  26.  Connects the ideas and observational evidence of SGRs with the magnetar theory of Duncan and Thompson.

Space Sciences Lab, http://science.msfc.nasa.gov, Space Science News, NASA’s Marshall Space Flight Center.  Daily-weekly science news updates, with search feature for past stories.  Great pictures.

Spotts, P. N., 1998, Christian Science Monitor, 90, 3.  Reports the August gamma-ray burst and describes what a magnetar is.

Thompson, C. and Duncan, R.C., 1992, ApJ., 392, L-9.  Seminal article advocating the existence of magnetars that could be a source of soft gamma-ray repeaters.

Thompson, C. and Duncan, R.C., 1996, ApJ., 473, 332.  This extensive, detailed account of magnetar theory, contains calculations of the x-ray, neutrino and Alfven wave emissions, comparisons of these with data from x-ray pulsars, discussion of magnetar crust activity and magnetar core-field decay rate.  Offers evidence against other theories.

Wilford, J.N., 1998, New York Times, late ed. 30 Sept.1998, (New York: New York Times Company).  Report of huge radiation blast from SGR 1900+14. Many scientists quoted referred to the source as a magnetar.

http://www.magnetars.com

Robert Duncan's website:  http://solomon.as.utexas.edu/~duncan/magnetar.html