Issue 4.04

The Astrophysics Spectator

February 21, 2007

The radio pulsars are rapidly spinning, strongly magnetized neutrons stars. As implied by their name, virtually all of these stars emit pulses of radio waves. But radio pulsars are also called spin-powered pulsars, which defines these stars in terms of how they generate energy rather than how we see them. This name underscores an important point: visible light, x-rays, and gamma-rays are emitted by many pulsars, and this electromagnetic radiation carries more energy away from the pulsar do the radio waves.

We see most pulsars as only radio emitters. The small number of spin-powered pulsars that generate x-rays are either strikingly young or strikingly rapid in their rotation. Two of the better known pulsars, the Crab pulsar and the Vela pulsar, are bright x-ray sources. The crab is among the youngest pulsars we observe, with an age of less than 1,000 years. Vela is several tens of thousands of years old, which is still very young for a pulsar—pulsars typically live for several million years. The older pulsars, which spin much more slowly than the young pulsars, are invisible in the x-ray band. The rapidly rotating pulsars that emit x-rays are millisecond pulsars. These are recycled neutron stars with relatively weak magnetic fields. These pulsars live for billions of years. The feature that is common in both types of pulsar is that the pulsars that spin most rapidly generate x-rays. An additional property of a young pulsar that cause it to emit x-rays is that the star is still hot, so hot that the surface can generate x-rays.

Next Issue: The next issue of The Astrophysics Spectator is scheduled for March 7.

Stars

Pulsar Light. Almost all pulsars emit strongly pulsating radio wave, but many pulsars also emit visible light, x-rays, and gamma-rays. The pulsating radio waves support the theory that pulsars are rapidly rotating neutron stars with strong magnetic fields. X-rays are seen from very young pulsars and from the most rapidly rotating millisecond pulsars; in these stars, the power released as x-rays is several orders of magnitude greater than that released in radio waves. Most x-rays emitted by the youngest pulsars comes from the neutron star's hot surface. (continue)