NASA's Fermi space telescope has detected the first gamma-ray pulsar ever seen outside our galaxy. This object is a rapidly spinning neutron star – the remnant of a supernova – that emits large amounts of gamma-ray photons, whereas most other pulsars emit lots of X-rays.

This pulsar, PSR J0540-6919, is located in the outer regions of the Tarantula Nebula in the Large Magellanic Cloud, about 163,000 light-years from Earth. The nebula itself is known to be a bright source of gamma rays, but incredibly this one pulsar may be responsible for half of the brightness. 

“That is a genuine surprise,” said lead astrophysicist Pierrick Martin from the National Center for Scientific Research (CNRS) and the Research Institute in Astrophysics and Planetology in Toulouse, France, in a statement.

J0540 is only about 1,700 years old, and it spins just under 20 times a second. It is mostly young pulsars like this that produce these large amounts of gamma rays, with older pulsars emitting large amounts of X-rays instead. Younger pulsars can have magnetic fields that are 10 times stronger than their older counterparts, which also plays a role in the light that is emitted.

^{Above is a NASA video explanation of the discovery. NASA/GSFC}

This particular pulsar is extremely bright, though. It is more than 20 times brighter than the previous record holder – the Crab Pulsar in the Crab Nebula. It was found by Fermi’s Large Area Telescope (LAT), but six years of observations were required to track it down. Since Fermi launched in 2008, more than 160 gamma-ray pulsars have been found. Before then, only seven were known.

Along with J0540, the scientists also studied the pulsar PSR J0537-6910 (J0537), which spins at around 62 times per second, the fastest rotation of a young pulsar.

The intense magnetic field of pulsars results in their radiation being emitted as beams from the poles, and as the small star – the size of a city but with the mass of half a million Earths – rotates, these beams can be spotted when they swoop in our direction, allowing the pulsar to be identified.

The findings are published in the journal Science.