Pedagoguery

In 1978, NASA launched the Einstein X-Ray Observatory, the first probe with the ability to image the x-ray sky. It saw an unfamiliar constellation of x-ray sources – most of which did not correspond to sources in the visible portion of the spectrum. Most of the things we see in the sky that appear bright, are very dim in x-rays.

It didn't take long before most of the objects were characterized. The bulk of them are tight binaries containing a neutron star and an ordinary star. The pair are so close that the neutron star draws off matter from the ordinary star. The matter spirals down onto the neutron star, heating up as it goes until it is hot enough to emit x-rays. Another class of objects was similar – the difference being that instead of being a tight binary containing a neutron star, it is a tight binary containing a black hole. The most famous example is Cygnus X-1. In this case, none of the x-rays are emitted by the black hole itself; only the accretion disk gives off the radiation. Another category turned out to be solitary neutron stars. Very young, fast-spinning neutron stars emit copious x-rays – mainly due to their spins and their strong magnetic fields. The pulsar at the heart of the Crab Nebula is an example.

This left only a hand full of objects that did not fit. The first of these was discovered in 1979 and is known by the unremarkable name of 1E 2259+586 (for its coordinates in the sky and being part of the 1st Einstein catalog). Located in the constellation of Cassiopeia, it looked superficially like a normal x-ray pulsar, but there was a problem – it wasn't spinning fast enough. A fast pulsar like the Crab pulsar (which spins 30 times per second) emits x-rays through a well understood mechanism. The strong magnetic field is anchored to the surface of the star, so it spins along with the star at 30 times per second. A moving magnetic field produces an electric field, the strength of which depends on the rate of movement and the strength of the magnetic field. This is part of the principal behind electric generators. Given you have a magnetic field that is truly immense and which is moving at a phenomenal rate of speed, the electric fields that are generated are equally immense. Strong electric fields are very good at accelerating charged particles, of which there are many in the high-energy environment of a neutron star. This creates an outflow of charged particles, which is the source of the x-rays. The energy all comes from the rotation of the neutron star. Scientists have measured the Crab's spin-down rate and compared it to what they would expect from theories, and the agreement is very good. However, 1E 2259+586 could not be generating x-rays the same way – it was only spinning once every 7 seconds. Furthermore, its rate of spin-down was also too low to be the cause of the amount of x-rays it was producing. Something just didn't add up. Lacking an explanation, it was given a name. They became known as Anomalous X-ray Pulsars, or AXPs.

Over time, a few others joined the ranks of the AXPs, but to date there are only 8 of them, and for a long time the mystery remained. In 1995, a potential answer arose: magnetars. A magnetar is a neutron star with a magnetic field 100 to 1000 times stronger than the typical 1012 Gauss. (By comparison, the Earth's magnetic field is about 1 Gauss.) This produces truly ferocious spin-down rates during the early life of the star. Calculations showed that the star would spin down to between once every 6 to 10 seconds after only several thousand years, meaning that the chances of catching one spinning faster would be negligible.

So, if the star's spin wasn't powering the x-rays, what was? The answer could be in the magnetic field itself. As the star spins, magnetic field lines become twisted. This drives electric currents along the field lines. Electrons are driven in one direction and ions are driven in the opposite direction. The ions will typically strike near one of the magnetic poles, heating the surface to the point where it will glow in soft x-rays. The electrons, however, being much less massive, are accelerated to near light-speed. As they travel, they will frequently collide with soft x-ray photons. The collision will kick the photons into higher energy hard x-ray photons. This produces the characteristic dual x-ray spectrum produced by AXPs.

For a while there was an alternative explanation involving relic accretion disks, but closer observation has ruled that out. Right now, it looks like magnetars are the most likely explanation for AXPs.

Next issue I will talk about dark energy.