How do Pulsars send out pulses of radiation?


The pulsar spins on its axis, just like the Earth spins on its axis. The magnetic field of the pulsar is also along an axis, but the magnetic field axis and the rotation axis are not the same! The same is true on Earth- the magnetic North pole is different to the true North pole.

This misalignment of the rotation and magnetic field axes for a pulsar is what makes it give out pulses of radiation.
Advanced: By giving out radiation, pulsars lose their energy and slow down gradually, leading to increased pulse length. This is where the ‘spin-down’ time comes from.

Wherever we have a rotating (or moving) magnetic field, we also have an electric field. This extends out to a distance which we call the ‘light cylinder’. Inside the light cylinder, we have an ionised ‘magnetosphere’ of high energy plasma (a ‘soup’ of charged particles). In the strong electric field of this light cylinder, electrons are accelerated to extremely high speeds. It is these electrons which emit electromagnetic radiation, often in the form of gamma-rays, which we can observe.

 

Why do we see pulses of radiation?

Because of the misalignment of the rotation and magnetic field axes, the pulsar constantly emits radiation- but we see only short pulses of it. This is because of the rotation of the neutron star.

Imagine a lighthouse- it shines light constantly, but if you watch it, you see only flashes as the beam of light passes over you. We see pulses of radiation from pulsars in exactly the same way.

So we see one pulse every time the pulsar rotates round.

We can study the gamma-rays in these pulses with CTA. What are we hoping to learn about?

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