Magnetars: Special Stars With That Attractive Charm
An artist's concept depicts the magnetic field lines rising from the surface of a magnetar, and the plasma clouds around the star. Credit: Dr. Robert Mallozzi, University of Alabama in Huntsville.
Magnetars are the superheroes of the star world, endowed with a colossal
magnetic field strength that has baffled scientists for years. They are
well over a trillion times more magnetic that the Sun and Earth. In
the past ten years magnetars have graduated from
the realm of speculation to near certainty.
Only about a dozen magnetars have been discovered so far. These appear
to be a special breed of neutron stars, which themselves are well known
for their strong magnetic fields.
How common are magnetars, and how did they gain their super magnetic
strength? These are the key questions facing astronomers today. But
finding evidence of their existence over the years has been nothing short
of clever detective work.
The story begins in 1979, in retrospect the year of the magnetar.
Scientists discovered a strange object in January of that year that
flashed lower-energy gamma rays and then grew dim again. Two months later,
on March 5, one of the brightest gamma-ray events in the sky took place.
The initial burst lasted only 0.2 seconds, but had as much energy as
the Sun emits in 1000 years. In addition, it's intensity continued
for 100 seconds, and was modulated with an 8-second period. A third
such type object (though not as bright) appeared soon thereafter. At the
time there was no way to distinguish these flashes from yet another cosmic mystery,
gamma-ray bursts. These three new objects would flash brilliantly several times for
up to a week, then shut off for months or years. It wasn't until about
1986 that scientists realized they had a unique kind of object on their
hands. The outbursts were random, but unlike gamma-ray bursts, these
Being the type not to mince words, scientists called the objects Soft
Gamma-ray Repeaters, or SGRs, where "soft" refers to lower-energy gamma
rays, as opposed to the higher-energy "hard" kind. Uncovering the nature
of these objects wasn't easy. They were dim if not undetectable most of
the time between random outbursts. When they did light up, though, they
were far brighter than an ordinary star. This is when they revealed a
A New Spin on an Old Star...
A diagram of a pulsar showing its rotation axis, its magnetic axis, and its magnetic field.
A smart bet was that SGRs were neutron stars because of the sheer energy
released from such a compact region. The neutron star scenario made
sense. During outbursts scientists could measure how fast the objects
were spinning (the spin rate) and how fast they were slowing down (the
spin-down rate). The ratio of the two rates serves as an estimate of
magnetic field strength. This is because magnetic fields are thought to
act as brakes, slowing the neutron star spin. A higher magnetic field
implies a faster spin-down rate.
Estimates from the March 5, 1979 event indicated that SGRs had
magnetic fields ranging from 100 trillion to a quadrillion
(1015) gauss. Theorists began to speculate on what
forces or conditions were needed to produce such a strong magnetic
field. Two theorists, Robert Duncan (University of Texas, Austin) and
Christopher Thompson (University of Toronto), proposed that newborn
neutron stars could possess magnetic fields greater than a quadrillion
gauss. This was the maximum efficiency for a neutron star dynamo. Most
were not this efficient, but SGRs resulted from the explosion of a
rapidly rotating star. Duncan and Thompson
coined the term magnetar in a 1992 paper.
Observations by the Rossi X-ray Timing Explorer in the mid- and
late 1990s confirmed this
scenario, firmly identifying SGRs with neutron stars and making measurements
of the spin and spin-down rate.
It was a mere cosmic coincidence that the first three SGRs were
discovered only months apart in 1979. A fourth SGR wasn't discovered
until 1998. Scientists had a working theory, but SGR magnetars were
still a major mystery.
A Family Reunion...
Meanwhile, through the 1980s and 1990s, another strange class of neutron
stars were appearing, dubbed Anomalous X-ray Pulsars (AXPs) because of
their X-ray intensity was greater than what could be attributed to by
the spin-down energy. It was thought that the X-ray emission was
powered by a strong magnetc field. The hard X-rays were close to the
energy of the soft gamma rays of SGRs. You may have guessed where
this is going. In 2002, Vicky Kapsi (McGill University) and her
colleagues clinched the connection between AXPs and SGRs by finding
SGR-like bursts in several AXPs. They were both
magnetars, and this doubled the known number to ten... or maybe 12,
depending on whom you ask.
Still, the magnetic field strengths were calculated by the spin versus
spin-down method. Later in 2002, scientists using NASA's Rossi X-ray
Timing Explorer captured a key measurement from SGR 1806-20, the fourth
one discovered. Tod Strohmayer and Alaa Ibrahim (NASA Goddard) found a line
in the spectrum of SGR 1806-20 that was at an energy of 5 keV. This
is the consistent with the energy of a proton in a magnetic field of a
This is the first direct measurement of a magnetar's magnetic strength,
but it doesn't clinch the case. Strohmayer and Ibrahim can only
assume that a proton is exhibiting this spectral line. But there is a
small chance this could be
an electron, which would imply a magnetic strength a thousand times
Another Turn On...
In 2003 Ibrahim made a chance discovery of a new magnetar while studying
a known one. Combing through archived RXTE data, Craig Markwardt
(Univ. Md & NASA/GSFC) found that this object
was a very dim neutron star only months before. Vicki Kaspi confirmed
the transient nature while studying a nearby source. The discovery suggests
that neutron stars can go through a brief magnetar stage.
This magnetar must have "turned on" between January and March of 2003.
One theory suggests that about ten percent of neutron stars are endowed
with an ultrahigh magnetic field at birth, perhaps due to its initial
spin, unique conditions in the implosion that created it, or the mass of
the progenitor star. At first, these isolated neutron stars are too dim
to study. In time, the magnetic fields act to slow the star's spin. This
act of slowing releases energy, making the star brighter. Additional
disturbances in the star's magnetic field and crust can make it brighter
yet, causing it to flare, leading to the measurement of its magnetic
field (a measure of its spin and the rate in which it is slowing).
Then the magnetic fields weaken, making the star perpetually dim. The
magnetar stage may be but a brief moment of beauty in the billion-year
existence of special neutron star. This would explain why scientists
have only about a dozen magnetars so far. There could be many more out
there simply in prolonged dim stages.
Like a superhero, only time will reveal their secret identity.
Publication Date: June 2006
More on the March 5 event (http://spacescience.spaceref.com/newhome/headlines/ast05mar99_1.htm)