Telescopes don sunglasses to look at the brightest pulsar

Astronomers have found the brightest pulsar yet discovered – by making things dimmer.

The researchers put “sunglasses” on the Australian Square Kilometre Array Pathfinder (ASKAP) telescope, at Murchison in Western Australia, allowing only polarised light through. This allowed them to spot the unusual pulsar, which is 10 times brighter than any other detected outside the galaxy.

Pulsars (pulsating radio sources) are neutron stars that rotate quickly, emitting two beams of light from their poles as they go. This light is polarised: the light waves are oriented in one direction.

Traditionally, pulsars are spotted from flickering light observed by telescopes; as the pulsar rotates, the beams of light briefly point towards Earth, like a lighthouse.

Cloud of stars with a close-up of one star shooting light out of both of its poles
Artist’s impression of the new pulsar, which has been called PSR J0523-7125. Credit: Carl Knox, ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav).

But pulsars which rotate too quickly or slowly can be missed by telescopes. Yuanming Wang, a PhD candidate at the University of Sydney and researcher with CSIRO, says that “the ingenuity built within the CSIRO’s ASKAP” allowed her and her colleagues to spot the ill-timed pulsar.

ASKAP’s receiver can be tuned to specific types of polarised light – effectively, putting on sunglasses. This pulsar was spotted by examining circular polarised light specifically.

Image labelled sunglasses off showing dozens of stars and clouds against a dark red background, with one small white dot highlighted by a box
MeerKAT’s picture of the Magellanic Cloud with sunglasses off. Pulsar displayed in the box. Credit: Yuanming Wang

Wang, who is lead author on a paper describing the discovery published in The Astrophysical Journal, says that the signal was first spotted “by accident”, when an undergraduate student noticed an anomaly in some of ASKAP’s observations.

The researchers followed this up by using ASKAP’s sunglasses technique methodically to identify the pulsar, then confirmed the data with the South African Radio Astronomy Observatory’s MeerKAT radio telescope.

“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 polarisation in a systematic and routine way,” adds co-author Professor Tara Murphy, from the Sydney Institute for Astronomy at the University of Sydney.

“Because of its unusual properties, this pulsar was missed by previous studies, despite how bright it is,” says Murphy.

Picture labelled sunglasses on with tiny white dot on dark red field
MeerKAT’s picture of the pulsar with “sunglasses” on – receiving only one type of polarised light.

Wang says the technique will be useful for finding other unusual pulsars.

“For example, if [the pulsar] has a binary system, or it has lots of scattering,” says Wang.

“The other thing is it can find some mystery objects. Several months ago our team, also using circular polarisation, we found a radio transience in the direction of the galactic centre. We still don’t know what the signal is – it’s kind of a mystery.”

Next, the researchers will be following up with more observations to determine the age of the pulsar, and whether it has a companion star.

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