A pulsar is an astronomical object that cycles very regularly between more and less EM radiation on a time scale of a few seconds or less, the fastest having a cycle of a few milliseconds. The EMR is generally radio but X-ray pulses have also been detected. Their cycles are very precise, like clockwork. They are unresolved point sources, like stars. The name pulsar is derived from pulsating star, but the term pulsating star is used for more conventional stars that generally pulse on a much larger timescale. The first pulsar observation was in 1967, being nicknamed LGM-1 ("little green men 1" because the clockwork frequency of pulses suggested an artificial source), later officially named CP 1919, then PSR B1919+21. Today there are over two thousand known pulsars.
Pulsars are assumed to be rotating, sending a beam of EMR that sweeps a circle, hitting Earth, like a rotating beacon or lighthouse light. They are further assumed to be neutron stars and their beam caused by the effects of a rotating magnetic field. A rationale is that such a small rotation period implies a small object able to produce a sufficiently-strong EMR source, neutron stars having the potential for this. The remarkably consistent cycles and the high density of a neutron star offer astrophysicists unique opportunities for testing and observing physical phenomena such as the influence of gravitational waves. The radio emission is presumed to be synchrotron radiation due to accretion of charged particles from a from a binary companion. An accretion disk is truncated due to the strong magnetic field, presumably near the point where it is sufficiently strong to dominate over the material's kinetic energy, a distance from the star called the Alfvén radius. The accretion is channeled along magnetic field lines to the pulsar's magnetic poles, where the radio emission occurs. The beam is powered by the pulsar's rotation. Millisecond pulsars are presumed to have had their rotation rate increased by the accretion.
Pulsars were first seen in radio (radio sources). In general, neutron stars are very hot, thus bright at high frequencies (black-body radiation) but are too small for us to observe that. An X-ray pulsar (i.e., binary X-ray pulsar) is a neutron star with an exceptionally strong magnetic field accreting matter from its companion star (which is not a neutron star), whose magnetic poles are misaligned with its rotation. The material is channeled to a circular-moving magnetic pole, and the shock heating produces X-rays directed along the axis of the magnetic poles.
The interstellar medium has an effect on the arrival of the pulses as seen on Earth, effecting a delay which depends upon wavelength, specifically, proportional to the inverse of the square of the wavelength. Simultaneous timing of multiple wavelengths yields a dispersion measure (DM), which is this proportionality constant scaled as the column density of electrons along the photons' path that would create the delays. Thus the timing offers information on the intervening ISM. This effect on timing must also be taken into account when combining timing data observed at different wavelengths.