Scientific clues from observations in the 1970's...

Observations with optical telescopes reveal the presence of a "normal" star coincident with the X-ray source.

Some of these stars exhibit doppler motions characteristic of orbital motion in a binary system.

Energy emitted in X-rays and the X-ray spectrum suggest the source of X-rays is accretion onto a neutron star.

Periodic variability on the timescale of seconds can only be explained by a spinning neutron star.