Some neutron stars have such powerful magnetic fields that they rip themselves open due to magnetic forces, a new study confirms.
A neutron star is the dense core left behind when a massive star explodes as a supernova. Made of subatomic particles called neutrons, the stars are so dense that a teaspoonful of their material would weigh 500 million tonnes.
The spins of some neutron stars decrease rapidly, suggesting they boast extremely powerful magnetic fields that radiate electromagnetic energy that slows their rotation. This type of neutron star is called a magnetar.
But it was always possible that the neutron stars might be slowing down for other reasons, for example as a result of spewing charged particles out into space.
Now, new observations of a candidate magnetar have confirmed that it has a magnetic field 600 trillion times the strength of Earth’s field – powerful enough to explain the ‘starquake’ it experienced in 2003. The study team was led by Tolga Guver of Istanbul University in Turkey.
“It’s the first independent confirmation that a magnetar is truly a magnetar,” team member Feryal Ozel of the University of Arizona in Tucson, US, told New Scientist. “These are the most magnetic objects in the universe, by orders of magnitude.”
The researchers used the European Space Agency’s XMM-Newton spacecraft to measure X-rays from a neutron star called XTE J1810-197, which lies about 10,000 light years from Earth in the constellation Sagittarius.
It was discovered in 2003 when it had a major outburst, suddenly becoming more than 100 times brighter than normal in X-rays. The event was similar to magnetic starquakes seen on other candidate magnetars (see Starquake explosion rips neutron star open).
The idea is that the crust of the neutron star buckles and cracks due to the tremendous magnetic forces exerted by the star’s own magnetic field. Watch an animation of a magnetar outburst.
But there were other possible explanations of these outbursts, such as sudden changes to the cloud of charged particles called the magnetosphere that surrounds the stars.
By analysing the spectrum of X-rays coming from XTE J1810-197 since its 2003 outburst, Guver’s team determined the existence of a hot spot about 7 kilometres wide at the neutron star’s surface. The spot was heated to about 5 million degrees by the outburst, and has been cooling since then. This is consistent with the starquake theory, in which the part of the neutron star’s crust that buckles releases tremendous energy and heats up its surroundings.
Jules Halpern of Columbia University in New York City, US, says further studies of such outbursts may reveal why magnetars have such short lives. The known magnetars all appear to be very young – most are less than 10,000 years old.
“It may be the case that hundreds of outbursts over the lifetime of the star are responsible for dissipating most of the magnetic field,” he told New Scientist. “After that . . . they may turn into ordinary radio pulsars or some other type of neutron star.”
Journal reference: The Astrophysical Journal Letters (vol 667, p 73)