A pulsar is an astronomical object that cycles between more and less radiation on a time scale of a few seconds or less, the fastest having a cycle of a few milliseconds. The 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.
They are assumed to be rotating, sending a beam of electromagnetic radiation 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 a rotation of hundreds of times a second implies a small object able to affect a sufficiently-strong EMR source, e.g., a neutron star.
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 a thousand known pulsars.
The remarkably consistent cycles and the high density of the neutron stars offer astrophysicists unique opportunities for testing and observing physical phenomena such as the influence of gravitational waves.
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 spin. 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.