The first clue is the behavior of a pulsar—a tiny but massive object that rapidly rotates, sending flashes of light toward Earth.
A rapidly spinning neutron star that emits radiation, usually radio waves, in narrow beams focused by the star's powerful magnetic field and streaming outward from its magnetic poles. Because the pulsar's magnetic poles do not align with the poles of its rotational axis, the beams of radiation sweep around like the beacon of a lighthouse and are thus observed on Earth as short, regular pulses, with periods anywhere between 1 millisecond and 4 seconds.
A rapidly rotating neutron star. The radiation from such a star appears to come in a series of regular pulses (one per revolution), which explains the name.