XMM-Newton Special Exhibit
Puzzling Pulsar
Pulsars are rotating neutron stars with strong magnetic fields. They are the remains of massive stars after they have gone supernova, and they appear to turn on and off in radio waves because as they are rotating, they are beaming very energetic charged particles at their poles. The rotation and magnetic axes are not aligned, so they effectively sweep their beam across the sky, like a lighthouse. But while astronomers know why pulsars "pulse," they aren't sure how the particles get stripped from the surface of the star and accelerated to high energies. There are two main competing models that describe these processes, and they depend on whether or not charged particles are allowed to freely escape the electric and magnetic fields at the star's surface. In both cases, X-ray emission should follow radio emission, with the X-ray emission having different characteristics in time and energy depending on exactly which mechanism generated it.
This diagram shows how the pulsar switches on and off. In the top section, it is radio-bright and X-ray-faint. In the lower section, it is X-ray bright with diminished radio brightness. (Image credit: ESA/ATG medialab; ESA/XMM-Newton; ASTRON/LOFAR)
A team of astronomoers observed a pulsar known as PSR B0943+10 simultaneously in X-rays and radio waves, in order to determine which physical processes are happening near the magnetic poles, and see which model was correct. However, they observed something very unexpected: just as the pulsar pulses in radio waves, it also pulses in X-rays, but in reverse – that is, when it's bright in radio waves, it's faint in X-rays, and vice versa. There currently is no model that can explain this, and it has reignited debate about the physical mechanisms that drive emission from pulsars.
Published: September 2016
Text Reviewed: September 2018