Magnetars

I love to be physicist when exposed to brand new observations totally out of imagination. Today is one fo those days. I attended a colloquium by Victoria M. Kaspi on magnetars.

This is a fairly new idea for explaining two exotic astrophysical objects "Soft Gamma Ray Repeaters" and "Anomalous X-ray Pulsars" and may be more. The problem with these objects is basically energy released by their rotational deceleration is not enough to explain observed brightness and occasional bursts. You need another energy source.

Latest fashionable idea is the magnetars. Claim is these objects are isolated neutron stars with extremely high magnetic fields about 10^15 Gauss (about thousand times the usual pulsars and more than anything we know so far!). Observed radiation is due to instabilities in the crust and the release and reorganization of the magnetic flux. It is a gigantic magnetic-neutron-star-quake. Everything seems to fit with many new observations. The problem is how to create these extraordinary magnetic fields at the first place. There is a dynamo idea in the literature but not really well studied so far.

There are skeptics like distinguished Turkish neutron star expert Ali Alpar. They are argue that it is quite unlikely to have such magnetic fields. Competing idea is the accretion disk model. But Prof. Kaspi argued that the disk will not be stable in that environment and whole thing seems to emit light many orders of magnitude more radiation than Eddington limit, which gives the upper limit for accretion radiation.

By the way, I also learned that my high school physics olympiad knowledge about neutron star acceleration is out of date. I thought that sudden increase in velocity of pulsars can be explained by crust crack and the decrease in the moment of inertia. But I learned that it did not fit the data and abandoned many years ago. Ooops! New mechanism is a superfluid core covered by a thin solid crust. Fluid inside rotate faster and occasional interactions between the core and the surface increase the crust velocity. There are gaps in my understanding like why the magnetic field moves with the crust but I’ll take experts’ words for a while.

You can find more about magnetars in the article of Bob Duncan, one of the originators of the idea.