April
2007
Cosmic hiccup0
Links to free teaching resources for the magnetar story
Teaching resources (UK US) designed specifically for this story at www.realscience.org.uk
Ask an astrophysicist: questions and answers about neutron stars.
How do stars form and evolve?
Star Count is a NASA education activity that “turns students into astronomers and gives teachers the resources to capitalize on the fun”. It challenges students to research answers to the questions: “Do people everywhere see the same number of stars in the night sky? Why or why not?” The activity encourages students to go outside at night and count the stars in the sky.
Background on the end of stars.
All the stars in the universe are nuclear furnaces fueled by fusion, which creates all the naturally occurring elements heavier than hydrogen and helium. This video segment from NOVA illustrates the critical role that stars play in creating the elements.
“There’s one kind of star that I find fascinating, and that’s neutron stars. These are the densest things … in the universe, and they actually do have solid surfaces, but it’s a solid form of a whole different state of matter than we’ve ever encountered …”
In a lucky observation, scientists say they have discovered a neutron star in the act of changing into a rare class of extremely magnetic objects called magnetars. No such event has been witnessed definitively until now. This discovery marks only the tenth confirmed magnetar ever found…
The discovery early this year of the first magnetar – a highly magnetized star – put the spotlight on a small class of stars called Anomalous X-ray Pulsars, or AXPs.
“Twenty years ago today, a new astrophysical mystery came banging on the door. It was a burst of gamma radiation so strong that it swamped detectors. It was the calling card for a new type of star – the magnetar.
Links to more
NASA online resources on stars
Teaching tools and astronomy basics from Amazing Space.
The story:
Cosmic hiccup
Using data from X-ray satellites, astronomers have caught a magnetar in a giant cosmic hiccup.
When it comes to eerie astrophysical effects, magnetars are hard to beat. The massive remains of exploded stars, magnetars are the size of mountains. But they weigh as much as our sun. This makes them incredibly dense.
Magnetars are a type of neutron star. But they possess magnetic fields hundreds of trillions of times as powerful as Earth’s magnetic field – which turns compass needles north.
The giant cosmic hiccup still has scientists puzzled. The researchers describe it in multiple reports in the Astrophysical Journal and Monthly Notices of the Royal Astronomical Society.
The magnetar is in a star cluster about 15 000 light-years away, in the Ara constellation. This is in the southern hemisphere. The magnetar goes by the official name of CXOU J164710.2-455216. Informally it is known as the Westerlund 1 magnetar.
“We only know of about a dozen magnetars,” says Michael Muno. He is a scientist at the California Institute of Technology’s Space Radiation Laboratory. He is the original discoverer of the magnetar.
“In brief, what we observed was a seismic event on the magnetar, which tells us a lot about the stresses these objects endure.”
In September 2005, about a year after Muno found the magnetar, the object produced a burst of radiation. Luckily this came at a time when it was being observed closely by several satellites. These included the European Space Agency’s X-ray satellite, XMM-Newton, and NASA’s Swift X-ray and gamma-ray observatory.
Just five days before the burst, Muno and his collaborators had been looking at the magnetar with XMM-Newton. They saw it in the relatively calm state in which he had first found it.
As most magnetars do, this one produces a beam of X-rays. These sweep across the heavens once every ten seconds. Earth lies in the path of this beam. So the magnetar’s speed of rotation could be determined very precisely.
The event that produced the burst caused the magnetar to shine 100 times more brightly. It created three separate beams that sweep past Earth where only one had existed before. It sped up its rotation rate by about a thousandth of a second.
Muno says more work is needed to understand what happened. The magnetar is built of matter far denser than anything on Earth. Its composition is a bit of a mystery.
But it is possible to make educated guesses. This can be done by extending theories that explain other neutron stars:
The magnetic fields inside the neutron star are probably wound up, like a twisted spring. As the magnetic fields unwind they create stresses in the outer crust, rather like the stresses created by plate tectonics on Earth.
The crust would resist these stresses for a while. But eventually it would fracture. This would produce a seismic event – a “starquake”. The fractures would cause the magnetar’s surface to shine brightly, from many sources.
In addition, there is reason to think that part of the interior of the neutron star is liquid. This may be rotating faster than the crust. The seismic event could cause this fluid to become attached to the crust. That would make the outer crust speed up slightly. “We think the crust cracked,” Muno says.
The observations are important for two reasons, he adds. “First, we have now seen another way in which these exotic objects dissipate their internal fields as they age.
“Second, this event was only spotted because a team of us were concentrating hard on this newly discovered object. The fact that we saw the event only a year after we discovered the magnetar implies that dozens more could be lurking in our Galaxy.
“If we find many more of these magnetars, we will have to re-evaluate our understanding of what happens when stars die,” says Gianluca Israel. He is an Italian astronomer who is publishing a separate paper on the magnetar with his collaborators in the Astrophysical Journal.
Muno is lead author of a paper appearing this week in Monthly Notices of the Royal Astronomical Society.
