Normally such a star will emit either X-rays or radio waves - but this one does both, switching back and forth between the two.
Pulsars are incredibly dense and magnetised neutron stars.
They pack half a million times Earth's mass into a ball no larger than a city. One teaspoonful weighs as much as a mountain.
They also spin incredibly quickly. The fastest one known whirls at 43,000 revolutions per minute.
Gas from other nearby normal stars feed gas into the pulsar which coats the neutron star in a layer of hydrogen and helium fuel.
When this layer reaches a certain depth, the fuel undergoes a runaway thermonuclear reaction and explodes, creating intense X-ray bursts which heats to millions of degrees giving off X-rays.
This process also speeds up the rotation of a pulsar. When the gas from the nearby star is expended - this takes around a billion years - radio waves are produced due to the increased spin and intense magnetic field.
The newly observed pulsar represents an intermediate phase in this process
Sergio Campana, an astronomer at Brera Observatory in Merate, Italy, and a co-author of a paper on the object that appears in the Sept. 26 issue of the journal Nature, said: "This transitional object took us decades to find, and it provides us with a unique opportunity to observe a pulsar's intense magnetic field in action.
Astronomers used NASA's Chandra observatory to locate the pulsar which is about 18,000 light-years away toward the constellation Sagittarius.
It is spinning at around 15,000 rpm.