An extragalactic pulsar discovered with a new technique

The pulsar PSR J0523-7125 in the square seen by MeerKAT (Image courtesy Yuanming Wang)
The pulsar PSR J0523-7125 in the square seen by MeerKAT (Image courtesy Yuanming Wang)

An article published in “The Astrophysical Journal” reports the discovery of a pulsar cataloged as PSR J0523-7125, the brightest pulsar discovered outside the Milky Way. A team of researchers led by Tara Murphy of the Australian University of Sydney used a new technique based on the analysis of the polarization of the light emitted by the pulsar to identify it among the observations conducted with the ASKAP radio telescope. The polarization was crucial in the identification because it made it possible to understand that the light came from a pulsar and not from an object of another type.

Astronomers usually search for pulsars hoping to find traces of the intermittent emission that mark this type of object. It’s a type of neutron star, what remains after the death of a massive star, which emits periodic pulses. However, each pulsar has its own spin period, so it’s not possible to know when one of them will emit one of its “flashes” before identifying it.

Tara Murphy’s team tested a technique designed to identify pulsars before detecting their “flash”. The test was conducted on an object observed during the VAST (Variables and Slow Transients) survey conducted with the ASKAP radio telescope, therefore, it was initially cataloged as VAST J052348.6−712552. The object is in the Large Magellanic Cloud, a satellite dwarf galaxy of the Milky Way, about 160,000 light-years from Earth.

Examining the polarization of the light coming from a possible pulsar allows establishing its nature. A strong circular polarization in radio emissions can only be present in a few types of sources and if that source doesn’t have an optical and infrared counterpart, it’s most likely a pulsar.

The object observed in the Large Magellanic Cloud showed the characteristics of a pulsar in observations conducted with the ASKAP (Australian Square Kilometer Array Pathfinder) radio telescope. The difficulty in its identification was also caused by the fact that it’s an object with a brightness that is unexpected for a pulsar, as it’s about ten times brighter than any other pulsar discovered outside the Milky Way.

Verification of the nature of the object was conducted using the MeerKat radio telescope in South Africa, which has the perfect characteristics to detect even faint pulsar signals. Observations conducted on August 25, 2021, with MeerKat made it possible to detect pulsations every 322 milliseconds. PSR J0523-7125, as the pulsar was cataloged, has a relatively slow rotation which is characteristic of young pulsars. However, more observations will be needed to establish its age.

Yuanming Wang, a Ph.D. candidate at the University of Sydney and the first author of the paper, stated that the discovery of PSR J0523-7125 was an incredible surprise. She used the sunglass metaphor thinking of models with polarized lenses that can offer a sharper vision.

The pulsar PSR J0523-7125 has out-of-the-ordinary characteristics that are probably the reason why it wasn’t identified. It will now be possible to conduct follow-up studies to examine its characteristics in depth. It will also be possible to search for other pulsars with the technique tested by Tara Murphy’s team.

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