Scientists have detected the brightest pulsar ever observed outside the Milky Way, revealing the true identity of a far-off luminous object previously mistaken for a distant galaxy.
Pulsars are incredibly dense neutron stars that emit beams of electromagnetic radiation from their poles.
Due to the way these objects rapidly rotate, their high-energy emissions appear as short, periodic pulses when observed from elsewhere in space, kind of like a cosmic lighthouse glimpsed intermittently from afar.
Since their discovery in the 1960s by Irish astrophysicist Jocelyn Bell, over 2,000 pulsars have been detected, but the vast majority of these shiny, whirling objects are located inside our own galaxy.
The pulsar as seen through polarized light. (Yuanming Wang)
The newly found pulsar – called PSR J0523−7125 – represents the much rarer discovery of an extra-galactic pulsar, located well beyond the limits of the Milky Way, in this case within the Large Magellanic Cloud.
PSR J0523−7125 was discovered by scientists using the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope array in Australia, and the find was made possible by surveying the sky for polarized pulsar emissions – a technique researchers liken to the astronomical equivalent of wearing polarized sunglasses.
"This was an amazing surprise," explains astrophysicist Yuanming Wang from Australia's CSIRO, a PhD candidate at the University of Sydney, and the first author of a new study on PSR J0523−7125.
"I didn't expect to find a new pulsar, let alone the brightest. But with the new telescopes we now have access to, like ASKAP and its sunglasses, it really is possible."
According to the researchers, PSR J0523−7125 is about 10 times more luminous than any other extra-galactic pulsars previously observed. So why was it missed before now?
PSR J0523−7125 obscured by other light sources in non-polarized data. (Yuanming Wang)
The answer has to do with how pulsars are detected. Traditionally, pulsar search procedures look for periodic pulses – the lighthouse-like flickering effect as the pulsar emits radiation in short, observable bursts.
But astronomers have to turn to other ways to detect more elusive pulsars that demonstrate less predictable periodicity or other ambiguous characteristics in their emitted light.
"Abnormal pulsars such as short orbital period binary systems or strongly scattered objects are more difficult to detect," the researchers write in their new paper.
In instances such as this, one potential workaround is looking for signs of circularly polarized light emitted by the objects.
To date, only a couple of large-scale surveys that can pick up circular polarization emission have been conducted, with one of them being performed by the ASKAP array.
Here, in a survey project called VAST, researchers filtered through ASKAP data looking for variable and transient sources of light-emitting phenomena. They identified PSR J0523−7125 as a pulsar, confirming the discovery with follow-up observations from the South African MeerKAT radio telescope and the CSIRO's Parkes Observatory in Australia.
Dishes in the ASKAP radio telescope array. (Alex Cheney/CSIRO)
"We should expect to find more pulsars using this technique," explains senior author and astrophysicist Tara Murphy from Sydney University.
"This is the first time we have been able to search for a pulsar's polarization in a systematic and routine way."
According to the researchers, PSR J0523−7125 exceeds previous theoretical limits of luminosity for how bright pulsars in the Large Magellanic Cloud can be expected to be, showing their brightness is on par with objects seen within the Milky Way.
While extra-galactic pulsars are still a relative rarity, our ability to discover them – and other kinds of pulsars that are hard to find via traditional methods – should only increase from this point, thanks to the increasing availability of large-scale radio continuum surveys and future telescopes, such as the upcoming Square Kilometre Array project being built in South Africa.
"With improved instruments in the Square Kilometre Array era, instantaneous large fields-of-view and great sensitivity will be even more common, leading to the detection of large numbers of radio sources across the sky," the researchers explain.
"Improved next-generation radio telescopes and increasing number of large-scale multi-wavelength surveys will bring large amounts of data with great sensitivity and resolution, giving us an unprecedented opportunity to identify more pulsar (even for extragalactic pulsars farther than the Magellanic Clouds)."
The findings are reported in The Astronomical Journal.