Computer animation of what a pulsar undergoing a starquake might
look like. The lines surrounding and passing through the pulsar
represent its magnetic field. (632 Kb - no audio).(Credit: NASA)
Scientists have discovered how to predict earthquake-like events in
pulsars. These explosive episodes likely crack a pulsar's
dense crust and momentarily bump up its spin rate.
Using NASA's Rossi X-ray Timing Explorer, the team has tracked about
20 "starquakes" on one particular pulsar over the past eight years
and uncovered a remarkably simple, predictive pattern.
For a pulsar called PSR J0537-6910 the time to the next quake is
proportional to the size of the last quake. With this simple formula,
scientists have been able to turn the Rossi Explorer to the pulsar a
few days before a quake to watch the event unfold.
Dr. John Middleditch of the Los Alamos National Laboratory in Los
Alamos, N.M., led the discovery and presents this finding today at
the American Astronomical Society Meeting in Calgary.
"By monitoring the pulsar spin rate and changes in the spin, we can
pin down a starquake event to within a couple of days," said
Middleditch. "These and other details have helped to simplify what
has, until now, appeared to be a bewildering assemblage of facts
about starquakes in pulsars. If only predicting earthquakes were this
Middleditch's colleagues include Drs. Frank Marshall and Will Zhang
of NASA's Goddard Space Flight Center in Greenbelt, Md.; Dr. Eric
Gotthelf of Columbia University in New York; and Dr. Daniel Wang of
the University of Massachusetts, Amherst.
A pulsar is the dense remains of an exploded star once several times
more massive than our sun. A pulsar contains about a Sun's worth of
mass compacted in a sphere only about 20 miles across. A pulsar is so
dense that a teaspoon of its material would weigh two billion tons on
Earth. The pulsar is so named because from our perspective it pulses
with radiation from its two magnetic poles as it spins, sending two
lighthouse-like beams through space.
PSR J0537-6910 is located in a 4,000-year old supernova remnant
called N157B in the crowded Tarantula Nebula of the Large Magellanic
Cloud, a small satellitegalaxy near our Milky Way galaxy. This is
about 170,000 light years away and visible in the Southern Hemisphere.
Neutron stars periodically erupt for reasons unclear. This may involve a starquake, in which the star surface cracks. (Credit: Darlene McElroy of LANL)
PSR J0537-6910 is known for its frequent quakes, which scientists
call glitches. Pulsars are born spinning but gradually slow down.
During a glitch, the spin rate increases slightly. PSR J0537-6910
spins at a rate of about 62 times per second, or 62 hertz. During a
glitch, this pulsar's spin jumps up as much as one cycle every 25
seconds, a greater gain than what is seen in any other pulsar. Then
the pulsar proceeds to slow down again.
Monitoring of this pulsar began in 1999. After about ten glitches the
scientists saw a pattern. The amount of increase in spin with each
glitch could be translated directly into the number of days until the
next glitch. Larger glitches meant a longer wait until the next one.
The predictive nature of glitches in PSR J0537-6910 is so strong that
it firms up the leading theory on what causes glitches. Pulsars have
a solid crust, about a kilometer thick, with a neutron superfluid in
its lower regions, which also permeates into the rest of the
interior. The crust's spin slows down more quickly than its own
superfluid. At some point, the differential rotation reaches a
tipping point. A glitch occurs when the superfluid transfers its
angular momentum to the crust, bumping up its speed. That maximum
differential rotation between superfluid and crust in PSR J0537-6910
appears to be one cycle every 25 seconds. Almost all other pulsars,
including the famous Crab pulsar, which spins more slowly and has
fewer glitches, do not exhibit much of a predictive pattern.
Predicting glitches can be fruitful because the biggest glitches
likely entail a massive cracking of the pulsar surface and a terrific
outpouring of energy. By studying these larger and rarer events,
scientists can gain key insight into the nature of pulsars. Pulsars
are laboratories of extreme gravity, exhibiting energy and pressure
far beyond the reaches of human-made experiments, and the study of
pulsars might lead to a better understanding of fundamental physics.
"A month ago we were watching the pulsar get the 'jitters' before the
big quake," Middleditch said. "Then, by May 7th, the big one had
happened. We can only predict one glitch at a time."
Middleditch noted that his team also found evidence the pulsar's
magnetic pole is migrating away from its rotation axis by a degree
every two centuries, or a few feet per year. Although a known feature
on Earth, where the magnetic north pole is a moving target many
kilometers from the North Pole, this is the first strong case for
magnetic pole migration on a pulsar.
Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.