Nature

A ‘galaxy’ is unmasked as a pulsar — the brightest outside the Milky Way

Image of the Tarantula Nebula within the Large Magellanic Cloud by the The ASKAP radio telescope.

The brightest extra-galactic pulsar has been identified in the Large Magellanic Cloud (pictured).Credit: Pennock et al.

Astronomers have confirmed that an object they thought was a distant galaxy is actually the brightest extra-galactic pulsar ever seen. The team made the discovery using a technique that blocks a particular type of polarized light, similar to polarized sunglasses, which could be used to spy more ‘hidden’ pulsars.

Pulsars are highly magnetized spinning neutron stars that form from the collapsed remnants of exploded stars. As pulsars spin, they release a stream of radio waves from their poles — a ‘pulse’ that can be detected using radio telescopes. Astronomers use pulsars to test theories of gravity and to look for evidence of gravitational waves.

The new pulsar, called PSR J0523−7125, is about 50,000 parsecs from Earth, in the Large Magellanic Cloud (LMC), and is quite different from most known pulsars. Its pulse is very wide — more than twice the size of other known pulsars in the LMC, and it is exceptionally ‘bright’ on the radio spectrum, says Yuanming Wang, an astrophysicist at Australia’s Commonwealth Scientific and Industrial Research Organisation in Canberra.

Wang and the team say the pulsar is ten times brighter than any other pulsar found outside the Milky Way. Their study is published in The Astrophysical Journal today1.

“Because of its unusual properties, this pulsar was missed by previous studies, despite how bright it is,” said co-author, Tara Murphy, a radio astronomer at the University of Sydney in Australia, in a press release.

New technique

Pulsars are typically identified from their faint pulse, flickering periodically. But in the case of PSR J0523−7125, its pulse is so wide and bright, that it didn’t fit the typical profile of a pulsar and was dismissed as a galaxy.

Wang and an international team of astronomers first suspected the object might be a pulsar in data from the Variables and Slow Transients survey, conducted using the Australian Square Kilometre Array Pathfinder (ASKAP) telescope in Western Australia. The survey looks at a large amount of sky for highly variable radio wave sources, and collects circular polarization, among other data.

Emissions from pulsars are often highly polarized, and some of them oscillate in a circular way. Few space objects are polarized like this, which makes them stand out.

Using a computer programme, the team was able to block out wavelengths of light that were not circularly polarized, revealing the rare type of pulsar. Other telescopes, including the MeerKAT radio-astronomy telescope in South Africa, confirmed their finding (see Hidden pulsar).

“We should expect to find more pulsars using this technique. This is the first time we have been able to search for a pulsar’s polarization in a systematic and routine way,” said Murphy.

Yvette Cendes, a radio astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, says that radio astronomy hasn’t been as effective as optical astronomy at finding ‘transient’ objects — space objects like pulsars that come in and out of view. “Surveys like VAST are changing that,” she says.

“But just because you find a transient [object] doesn’t mean it’s easy to figure out what it is,” she says. Polarization data helped to narrow down the source of the object, which suggests the technique has the potential to identify other transients in the future, she says.

Although other telescopes are collecting polarization data, there have only been a few large-scale radio surveys using the circular polarization technique. In March, researchers using data from the Low-Frequency Array (LOFAR) telescope in the Netherlands found two new pulsars using the technique, which they detailed in a preprint posted on arXiv2.

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