Pulsars

The neutron star from the side, its spin axis upright (dashed), its magnetic axis tilted by α and turning about it, a beam of radio along each pole as wide as the open field lines where it comes from. The line of sight to the Earth lights up as a beam sweeps across it. The turn is strobed down so the eye can follow it: the real star turns at the note, hundreds of times a second.

One turn of the pulse: the beam as our sightline cuts it (dashed), and what the telescope makes of it in the upper half of its band (left channel, blue) and the lower half (right channel, amber), after the plasma's smear and scattering tail. Narrow pulses make a buzzy tone with many harmonics; wide or smeared ones a mellow one.

Each channel across the band (top: its highest frequency) with its pulses folded on the turn, two turns shown. With no dedispersion the pulse arrives later down the band, along the dispersion curve (dashed), and wraps round the turn when the delay is longer than a turn; a filterbank or coherent dedispersion stands it upright. Scattering drags a tail out to the right, longest at the bottom of the band.

A pulse train is every harmonic of the spin at once. Move the DM (the mod wheel) and watch the upper harmonics fade as the pulses smear; with no dedispersion a new note's lower half of the band comes in later.