Scientists thought they knew what was going inside the Tarantula Nebula. But they were wrong. Located in the Large Magellanic Cloud, this nebula is one of the most active star-forming regions near the Milky Way Galaxy. It’s also where most of the gamma-rays we see from the Large Magellanic Cloud come from. Until now, scientists thought the gamma-rays were a result of star formation.

Turns out, most of the gamma-rays come from a single gamma-ray pulsar. “It’s now clear that a single pulsar, PSR J0540-6919, is responsible for roughly half of the gamma-ray brightness we originally thought came from the nebula,” said lead scientist Pierrick Martin, an astrophysicist at the National Center for Scientific Research (CNRS). “That is a genuine surprise.”

Scientists used NASA’s Fermi Gamma-ray Space Telescope to spot PSR J0540-6919. It’s also the first gamma-ray pulsar ever discovered outside our galaxy.

How does PSR J0540 stack up against other discovered gamma-ray pulsars? The most luminous gamma-ray pulsar in the Milky Way sits in the famous Crab Nebula. PSR J0540’s gamma-ray pulses are about 20 times more intense than the one in the Crab Nebula.

Another surprise awaited scientists. While the intensity of gamma-rays was off the charts, both pulsars have similar levels of radio, optical and X-ray emission. “Accounting for these differences will guide us to a better understanding of the extreme physics at work in young pulsars,” said co-author Lucas Guillemot.

The hunt for gamma-rays in J0540

NASA’s Einstein and Rossi X-ray Timing Explorer satellites helped discover PSR J0540. But, the journey to detecting gamma-rays from J0540 was a long one. Researchers used Fermi’s Large Area Telescope (LAT) to observe J0540 over six years. They also did a complete reanalysis of all LAT data to detect gamma-ray pulses from the pulsar.

The cool thing about Fermi is that it gets better as time goes on. It collects more gamma-rays and Fermi scientists get better at using the telescope.

Here’s what the area around J0540 looked like near the start of Fermi’s mission.

Fermi Pulsar

And here’s what it looks like now. Fermi also detected J0537, but not its pulses.

Fermi two pulsar

That’s why Fermi scientists go back and reanalyze data. By going back over the data, Fermi scientists were able to detect gamma-ray pulses from J0540. The discovery means astronomers may need to take another look at how cosmic rays are produced. And how they interact with nebulas such as the Tarantula Nebula.

The Fermi Space Telescope

In 2013, NASA’s Fermi Gamma-ray Space Telescope completed its five-year primary mission. But Fermi’s still got some life. Fermi entered an extended phase of its mission to keep observing until 2018.

When Fermi first launched in 2008, we only knew about seven gamma-ray pulsars. Today, more than 160 gamma-ray pulsars have been discovered. And that number will only climb as Fermi keeps exploring and scientists revisit old data.

Check out the video below for an overview of Fermi’s first five years in space.

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