If you thought neutron stars and magnetars were exotic, think again. In studies of magnetars that occasionally blink to life, generating an intense blast of X-rays and gamma-rays, astronomers have been at a loss to explain why these objects have such strong magnetic fields. After all, after a supernova, a neutron star remnant conserves the angular momentum and magnetic field of the parent massive star; it is therefore a rapidly spinning, magnetically dominant entity, often observed emitting intense radiation from its poles (a.k.a. a pulsar).
However, magnetars (the most magnetically powerful objects observed in the Universe) do not have such a reasonable explanation for their magnetic field, it is simply too strong.
During the AAS conference last week, one scientist presented his research, possibly indicating another state of matter may be at play. A massive neutron star may pass through a “quark star phase”, kick starting a mechanism known as colour ferromagnetism…
*This image is copyright Mark A. Garlick and has been used with permission. Please do not use this image in any way whatsoever without first contacting the artist.
I attended the white dwarf session at the American Astronomical Society conference last week to find out more about observations of these dusty old stars. However, like many of the 90 minute sessions, fitting 8-9 quick-fire presentations in each, there was one unexpected and surprising subject that outshone all the others. Dr Denis Leahy was the black sheep of this session, breaking the mould with a talk about “Delayed Magnetic Field Amplification in Magnetars.” It was the only magnetar talk and one of two neutron star presentations (the second was awesome too, and I’ll write about that later).
In a nutshell, magnetars are a mystery. They are violent emitters of X-ray and gamma radiation and they have immensely powerful magnetic fields. In fact, their magnetic fields are too powerful to have originated from the pre-supernova parent star. So astrophysicists are working hard to understand why magnetars are so… well, magnetic.
Dr Leahy, who works at the University of Calgary, has one explanation for how a neutron star can amplify its magnetic field, evolving into a magnetar.
In this case, it is believed that this evolutionary step requires a large neutron star to pass through a quark star phase. A quark star is theorized to be a heavy neutron star. Where neutron stars are filled with neutron-degenerate matter (the structure of nuclei cannot be sustained under the powerful gravitational pressures–exceeding the Chandrasekhar limit–breaking down into a “soup” individual neutrons), quark stars are the next mass up, where the individual neutron structure cannot compete with the even larger gravitational pressures. The individual neutrons break down, forming a lump of quark-degenerate matter (exceeding the Tolman-Oppenheimer-Volkoff mass limit). Therefore, a quark star is a giant hadron.
During the change from neutron star to quark star, a mechanism known as colour ferromagnetism may occur. This only happens when the density of quark matter is ultra-high, in a state known as color-flavour locking quark matter. During this density transition, the macroscopic magnetic field is amplified. I am uncertain as to whether a quark star can be considered a “phase” (i.e. neutron star » quark star phase [magnetic amplification] » magnetar), or whether certain conditions require the quark star to be a magnetar, or whether a magnetar relaxes back into a neutron star state (although I’d be at a loss to explain how the internal density would revert back to neutron degenerate matter), but this mechanism certainly goes to some way of explaining how magnetars may get their magnetism.
As for observations, astronomers should look carefully at the neutron star inside supernovae remnants to be on the look-out for a secondary flash of energy as the neutron star structure collapses (between ~days to 1000 years after the supernova), becoming a quark star and boosting its magnetic field like an instantaneous dynamo.
Further reading: Universe Today