A radio tuned to one station suddenly starts picking up another because someone invisible is turning the tuning knob. That is how the radio signal of pulsar PSR J2144−5237 behaves: the frequency at which its emission sharply weakens drifts smoothly from 0.8 to 3 gigahertz as the star orbits its companion. Astronomers had previously noticed that the pulsar's spectrum changes at different points in the orbit, but tracking this shift continuously had not been possible. Now, thanks to the wideband receiver on the Parkes radio telescope, it has been done.

The pulsar is a rapidly spinning neutron star that sends out radio pulses like a lighthouse. In the binary system, it has a companion surrounded by a cloud of charged particles: plasma. As the pulsar moves along its orbit, its beam passes through different parts of this cloud. In dense, magnetized regions, radio waves are absorbed more strongly, and the break frequency shifts upward. Both pulse components of the pulsar, which correspond to different magnetic poles, change in the same way. This means the cause is not inside the star but outside, in the common plasma cloud.

Modeling showed that the main absorption mechanism is cyclotron resonance: plasma particles spiraling around magnetic field lines steal energy from waves of a certain frequency. Simple absorption in ionized gas cannot account for it. This method can be applied to other double pulsars right now. From changes in the break frequency, one can estimate the electron density and magnetic field strength in the plasma surrounding the companion. In this way, the invisible hand turning the tuning knob becomes visible.